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Chris Landsea

Jul 18, 1997, 3:00:00 AM7/18/97

Archive-name: meteorology/storms-faq/part1
Posting-Frequency: monthly


Part I:

By Christopher W. Landsea
NOAA AOML/Hurricane Research Division
4301 Rickenbacker Causeway
Miami, Florida 33149

18 July, 1997

New for this month.....
How do tropical cyclones form? (Subject A10)

What names have been retired in the Atlantic basin? (Subject B3 -

What are the most and least tropical cyclones occurring in the
Atlantic basin and striking the USA? (Subject E9 - Revised -
Table of individual years added)

What refereed articles were written in recent years about tropical
cyclones ? (Subject J4 - Revised)
New for this month.....

This is currently a two-part FAQ (Frequently Asked Questions report) that
is in its second full incarnation (version 2.4). However, there may be some
errors or discrepancies that have not yet been found. If you do see an item
that needs correction, please contact me directly. This file (Part I)
contains various definitions, answers for questions about names, myths,
winds, records, forecasting, climatology and observation of tropical
cyclones. Part II provides sites that you can access both real-time
information about tropical cyclones, what is available on-line for historical
storms, as well as good books to read and various references for tropical
cyclones. Keep in mind that this FAQ is not considered a reviewed paper to
reference. Its main purpose is to provide quick answers for (naturally)
frequently asked questions as well as to be a pointer to various sources of

I'd like to thank various people for helping to put together this FAQ: Sim
Aberson, Jack Beven, Gary Padgett, Tom Berg, Julian Heming, Neal Dorst,
Gary Gray, Stephen Caparotta, Steven Young, D. Walston all provided
substantial bits to this FAQ. Also thanks to the many people who provided
additional questions and information for this FAQ: Ilana Stern, Dave Pace,
Dave Blanchard, Ken Fung, James (I R A Aggie) Stricherz, Mike Dettinger,
Jan Schloerer, Eric Blake, Jeff Kepert, Frank Woodcock, Roger Edson, Bill
Cherepy, Stephen Jascourt, Kelly Dean, Malcolm ???, Jon Gill, Ken Waters,
Derek West, Gert van Dijken, George Gumbert III, Edward Reid, Tim Trice,
Michael Scott, Kerry Emanuel, George Sambataro, James Lewis Free, Sam
Biller, David Faciane, Eric Gross, Jeff Hawkins, Mike Fiorino and Madeleine
Hall. Many thanks also to Jan Null for providing the first .html version of
the FAQ. If I didn't get to all the suggested FAQs, I'll try to include
them in future versions.

Where can I get the latest version of this document?????
ASCII VERSION: An ascii edition of the two portions for this FAQ are
posted monthly on sci.geo.meteorology and on sci.environment usually early
in each month. One can also ftp to retrieve the latest files at:
hrd-type42.nhc.noaa.gov. Login as 'anonymous' and password as your
email address. The files are available at that directory (TCfaqI and
TCfaqII). If you do not have ftp access, you can request copies from me
directly via email.

FANCY VERSION: Neal Dorst has created a much enhanced World Wide Web version
that is starting to include in helpful pictures as well. This user friendly
site is available via your favorite web server at:



A1) What is a hurricane, typhoon, or tropical cyclone?
A2) What are "Cape Verde"-type hurricanes?
A3) What is a super-typhoon?
A4) Where do these easterly waves come from and what causes them?
A5) What is a sub-tropical cyclone?
A6) How are tropical cyclones different from mid-latitude storms?
A7) How are tropical cyclones different from tornadoes?
A8) What does the acronym "CDO" in a discussion of tropical cyclones mean?
A9) What is a TUTT?
A10) How do tropical cyclones form?

B1) Why are tropical cyclones named?
B2) What are the tropical cyclone names through 2001?
B3) What names have been retired in the Atlantic basin?
B4) What is the origin of the name "hurricane"?

C1) Doesn't the low pressure in the tropical cyclone center cause the storm
C2) Doesn't the friction over land kill tropical cyclones?
C3) Aren't big tropical cyclones also intense tropical cyclones?
C4) Why don't we try to destroy tropical cyclones by: pick one or more -
a) seeding them with silver iodide.
b) placing a substance on the ocean surface.
c) nuking them
d) etc. ?
C5) During a hurricane are you supposed to have the windows and doors on
the storm side closed and the windows and doors on the lee side open?

D1) How are Atlantic hurricanes ranked?
D2) How are Australian tropical cyclones ranked?
D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise)
in the Northern (Southern) Hemisphere?
D4) How do I convert from mph to knots (to m/s) and from inches of mercury
to mb (to hPa)?
D5) How does the damage that hurricanes cause increase as a function
of wind speed?

E1) Which is the most intense tropical cyclone on record?
E2) Which tropical cyclone intensified the fastest?
E3) Which tropical cyclone has produced the highest storm surge?
E4) What are the largest rainfalls associated with tropical cyclones?
E5) Which are the largest and smallest tropical cyclones on record?
E6) Which tropical cyclone lasted the longest?
E7) Which tropical cyclones have caused the most deaths and most damage?
E8) What are the average, most, and least tropical cyclones occurring in
each basin?
E9) What are the most and least tropical cyclones occurring in the
Atlantic basin and striking the USA?
E10) For the U.S., what are the 10 most intense, 10 costliest, and
10 highest death toll hurricanes on record?
E11) What tropical storms and hurricanes have moved from the Atlantic to
the Northeast Pacific or vice versa?

F1) What regions around the globe have tropical cyclones and who is
responsible for forecasting there?
F2) What is Prof. Gray's seasonal hurricane forecast for this year and
what are the predictive factors?
F3) How has Dr. Gray done in previous years of forecasting hurricanes?
F4) What are those track and intensity models that the Atlantic
forecasters are talking about in the tropical storm and hurricane

G1) What is the annual cycle of occurrence seen in each basin?
G2) How does El Nino-Southern Oscillation affect tropical cyclone activity
around the globe?
G3) What may happen with tropical cyclone activity in a 2xCO2 world?
G4) Are we getting stronger and more frequent hurricanes, typhoons, and
tropical cyclones in the last several years?
G5) Why do tropical cyclones occur primarily in the summer and autumn?
G6) What determines the movement of tropical cyclones?
G7) Why doesn't the South Atlantic Ocean experience tropical cyclones?
G8) Does an active June and July mean the rest of the season will be busy
G9) Why do hurricanes hit the East coast of the U.S., but never the
West coast?
G10) How much lightning occurs in tropical cyclones?


H1) What is the Dvorak technique and how is it used?
H2) Who are the "Hurricane Hunters" and what are they looking for?


Subject: A1) What is a hurricane, typhoon, or tropical cyclone?

The terms "hurricane" and "typhoon" are regionally specific names for
a strong "tropical cyclone". A tropical cyclone is the generic term for a
non-frontal synoptic scale low-pressure system over tropical or sub-
tropical waters with organized convection (i.e. thunderstorm activity)
and definite cyclonic surface wind circulation (Holland 1993).

Tropical cyclones with maximum sustained surface winds (see note
below) of less than 17 m/s (34 kt) are called "tropical depressions".
(This is not to be confused with the condition mid-latitude people get
during a long, cold and grey winter wishing they could be closer to the
equator ;-) Once the tropical cyclone reaches winds of at least 17 m/s
they are typically called a "tropical storm" and assigned a name. If
winds reach 33 m/s (64 kt), then they are called: a "hurricane" (the
North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or
the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific
Ocean west of the dateline); a "severe tropical cyclone" (the Southwest
Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a
"severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone"
(the Southwest Indian Ocean) (Neumann 1993).

Note that just the definition of "maximum sustained surface winds"
depends upon who is taking the measurements. The World Meteorology
Organization guidelines suggest utilizing a 10 min average to get a
sustained measurement. Most countries utilize this as the standard.
However the National Hurricane Center (NHC) and the Joint Typhoon
Warning Center (JTWC) of the USA use a 1 min averaging period to get
sustained winds. This difference may provide complications in comparing
the statistics from one basin to another as using a smaller averaging
period may slightly raise the number of occurrences (Neumann 1993).


Subject: A2) What are "Cape Verde"-type hurricanes?

Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that
develop into tropical storms fairly close (<1000km or so) of the Cape
Verde Islands and then become hurricanes before reaching the Caribbean.
(That would be my definition, there may be others.) Typically, this may
occur in August and September, but in rare years (like 1995) there may be
some in late July and/or early October. The numbers range from none
up to around five per year - with an average of around 2.


Subject: A3) What is a super-typhoon?

A "super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning
Center in Guam for typhoons that reach maximum sustained 1-minute surface
winds of at least 130 kt (240 km/h). This is the equivalent of a strong
Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or
a category 5 severe tropical cyclone in the Australian basin.


Subject: A4) Where do these easterly waves come from and what causes them?

It has been recognized since at least the 1930s (Dunn 1940) that lower
tropospheric (from the ocean surface to about 5 km with a maximum at 3 km)
westward traveling disturbances often serve as the "seedling" circulations
for a large proportion of tropical cyclones over the North Atlantic Ocean.
Riehl (1945) helped to substantiate that these disturbances, now known as
African easterly waves, had their origins over North Africa. While a variety
of mechanisms for the origins of these waves were proposed in the next few
decades, it was Burpee (1972) who documented that the waves were being
generated by an instability of the African easterly jet. (This instability
- known as baroclinic-barotropic instability - is where the value of the
potential vorticity begins to decrease toward the north.) The jet arises
as a result of the reversed lower-tropospheric temperature gradient over
western and central North Africa due to extremely warm temperatures over the
Saharan Desert in contrast with substantially cooler temperatures along the
Gulf of Guinea coast.

The waves move generally toward the west in the lower tropospheric
tradewind flow across the Atlantic Ocean. They are first seen usually
in April or May and continue until October or November. The waves have
a period of about 3 or 4 days and a wavelength of 2000 to 2500 km,
typically (Burpee 1974). One should keep in mind that the "waves" can be
more correctly thought of as the convectively active troughs along an
extended wave train. On average, about 60 waves are generated over North
Africa each year, but it appears that the number that is formed has no
relationship to how much tropical cyclone activity there is over the Atlantic
each year.

While only about 60% of the Atlantic tropical storms and minor hurricanes
(Saffir-Simpson Scale categories 1 and 2) originate from easterly waves,
nearly 85% of the intense (or major) hurricanes have their origins as
easterly waves (Landsea 1993). It is suggested, though, that nearly all
of the tropical cyclones that occur in the Eastern Pacific Ocean can also
be traced back to Africa (Avila and Pasch 1995).

It is currently completely unknown how easterly waves change from year
to year in both intensity and location and how these might relate to
the activity in the Atlantic (and East Pacific).


Subject: A5) What is a sub-tropical cyclone?

A sub-tropical cyclone is a low-pressure system existing in the
tropical or subtropical latitudes (anywhere from the equator to about
50N) that has characteristics of both tropical cyclones and mid-latitude
(or extratropical) cyclones. Therefore, many of these cyclones exist in
a weak to moderate horizontal temperature gradient region (like mid-latitude
cyclones), but also receive much of their energy from convective clouds (like
tropical cyclones). Often, these storms have a radius of maximum winds which
is farther out (on the order of 60-125 miles [100-200 km] from the center)
than what is observed for purely "tropical" systems. Additionally, the
maximum sustained winds for sub-tropical cyclones have not been observed to
be stronger than about 64 kt (33 m/s).

Many times these subtropical storms transform into true tropical
cyclones. A recent example is the Atlantic basin's Hurricane Florence in
November 1994 which began as a subtropical cyclone before becoming fully
tropical. Note there has been at least one occurrence of tropical cyclones
transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973).

Subtropical cyclones in the Atlantic basin are classified by the maximum
sustained surface winds: less than 34 kt (18 m/s) - "subtropical depression",
greater than or equal to 34 kt (18 m/s) - "subtropical storm". Note that
while these are not given names, they are warned on and forecasted for by
the National Hurricane Center similar to the treatment received by tropical
cyclones in the region.


Subject: A6) How are tropical cyclones different from mid-latitude storms?

The tropical cyclone is a low-pressure system which derives its energy
primarily from evaporation from the sea in the presence of high winds and
lowered surface pressure and the associated condensation in convective
clouds concentrated near its center (Holland 1993). Mid-latitude storms
(low pressure systems with associated cold fronts, warm fronts, and
occluded fronts) primarily get their energy from the horizontal temperature
gradients that exist in the atmosphere.

Structurally, tropical cyclones have their strongest winds near the
earth's surface (a consequence of being "warm-core" in the troposphere),
while mid-latitude storms have their strongest winds near the tropopause
(a consequence of being "warm-core" in the stratosphere and "cold-core"
in the troposphere). "Warm-core" refers to being relatively warmer than
the environment at the same pressure surface ("pressure surfaces" are simply
another way to measure height or altitude).


Subject: A7) How are tropical cyclones different from tornadoes?

While both tropical cyclones and tornadoes are atmospheric vortices,
they have little in common. Tornadoes have diameters on the scale of
100s of meters and are produced from a single convective storm (i.e. a
thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter
on the scale of 100s of *kilometers* and is comprised of several to dozens of
convective storms. Additionally, while tornadoes require substantial
vertical shear of the horizontal winds (i.e. change of wind speed and/or
direction with height) to provide ideal conditions for tornado genesis,
tropical cyclones require very low values (less than 10 m/s or 20 kt) of
tropospheric vertical shear in order to form and grow. These vertical shear
values are indicative of the horizontal temperature fields for each
phenomenon: tornadoes are produced in regions of large temperature gradient,
while tropical cyclones are generated in regions of near zero horizontal
temperature gradient. Tornadoes are primarily an over-land phenomena as
solar heating of the land surface usually contributes toward the development
of the thunderstorm that spawns the vortex (though over-water tornadoes have
occurred). In contrast, tropical cyclones are purely an oceanic phenomena -
they die out over-land due to a loss of a moisture source. Lastly, tropical
cyclones have a lifetime that is measured in days, while tornadoes typically
last on the scale of minutes.

An interesting side note is that tropical cyclones at landfall often
provide the conditions necessary for tornado formation. As the tropical
cyclone makes landfall and begins decaying, the winds at the surface die
off quicker than the winds at, say, 850 mb. This sets up a fairly strong
vertical wind shear that allows for the development of tornadoes, especially
on the tropical cyclone's right side (with respect to the forward motion of
the tropical cyclone). For the southern hemisphere, this would be a concern
on the tropical cyclone's left side - due to the reverse spin of southern
hemisphere storms. (Novlan and Gray 1974)


Subject: A8) What does the acronym "CDO" in a discussion of tropical
cyclones mean?

"CDO" is an acronym that stands for "central dense overcast". This is the
cirrus cloud shield that results from the thunderstorms in the eyewall of a
tropical cyclone and its rainbands. Before the tropical cyclone reaches
hurricane strength (64 kt or 33 m/s), typically the CDO is uniformly showing
the cold cloud tops of the cirrus with no eye apparent. Once the storm
reaches the hurricane strength threshold, usually an eye can be seen in
either the infrared or visible channels of the satellites. Tropical cyclones
that have nearly circular CDO's are indicative of favorable, low vertical
shear environments.


Subject: A9) What is a "TUTT"?

A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT
that has completely cut-off. TUTT lows are more commonly known in the
Western Hemisphere as an "upper cold low". TUTTs are different than mid-
latitude troughs in that they are maintained by subsidence warming near the
tropopause which balances radiational cooling. TUTTs are important for
tropical cyclone forecasting as they can force large amounts of harmful
vertical wind shear over tropical disturbances and tropical cyclones. There
are also suggestions that TUTTs can assist tropical cyclone genesis and
intensification by providing additional forced ascent near the storm center
and/or by allowing for an efficient outflow channel in the upper troposphere.
For a more detailed discussion on TUTTs see the article by Fitzpatrick et al.


Subject: A10) How do tropical cyclones form?

To undergo tropical cyclogenesis, there are several favorable
precursor environmental conditions that must be in place (Gray 1968,

1. Warm ocean waters (of at least 26.5 C [80 F]) throughout a
sufficient depth (unknown how deep, but at least on the order of
50 m [150 ft]). Warm waters are necessary to fuel the heat
engine of the tropical cyclone.

2. An atmosphere which cools fast enough with height such that it
is potentially unstable to moist convection. It is the thunderstorm
activity which allows the heat stored in the ocean waters to be
liberated for the tropical cyclone development.

3. Relatively moist layers near the mid-troposphere (5 km [3 mi]).
Dry mid levels are not conducive for allowing the continuing
development of widespread thunderstorm activity.

4. A minimum distance of at least 500 km [300 mi] from the equator.
For tropical cyclogenesis to occur, there is a requirement for
non-negligible amounts of the Coriolis force to provide for near
gradient wind balance to occur. Without the Coriolis force, the
low pressure of the disturbance cannot be maintained.

5. A pre-existing near-surface disturbance with sufficient vorticity
and convergence. Tropical cyclones cannot be generated spontaneously.
To develop, they require a weakly organized system with sizable spin
and low level inflow.

6. Low values (less than about 10 m/s [20 mph]) of vertical wind
shear between the surface and the upper troposphere. Vertical wind
shear is the magnitude of wind change with height. Large values of
vertical wind shear disrupt the incipient tropical cyclone and can
prevent genesis, or, if a tropical cyclone has already formed, large
vertical shear can weaken or destroy the tropical cyclone by
interfering with the organization of deep convection around the
cyclone center.

Having these conditions met is necessary, but not sufficient
as many disturbances that appear to have favorable conditions do
not develop. Recent work (Velasco and Fritsch 1987, Chen and
Frank 1993, Emanuel 1993) has identified that large thunderstorm
systems (called mesoscale convective complexes [MCC]) often produce
an inertially stable, warm core vortex in the trailing altostratus
decks of the MCC. These mesovortices have a horizontal scale of
approximately 100 to 200 km [75 to 150 mi], are strongest in
the mid-troposphere (5 km [3 mi]) and have no appreciable signature
at the surface. Zehr (1992) hypothesizes that genesis of the
tropical cyclones occurs in two stages: stage 1 occurs when the
MCC produces a mesoscale vortex and stage 2 occurs when a second
blow up of convection at the mesoscale vortex initiates the
intensification process of lowering central pressure and increasing
swirling winds.


Subject: B1) Why are tropical cyclones named?

Tropical cyclones are named to provide ease of communication
between forecasters and the general public regarding forecasts, watches,
and warnings. Since the storms can often last a week or longer and that
more than one can be occurring in the same basin at the same time, names
can reduce the confusion about what storm is being described. According
to Dunn and Miller (1960), the first use of a proper name for a tropical
cyclone was by an Australian forecaster early in this century. He gave
tropical cyclone names "after political figures whom he disliked. By
properly naming a hurricane, the weatherman could publicly describe a
politician (who perhaps was not too generous with weather-bureau
appropriations) as 'causing great distress' or 'wandering aimlessly
about the Pacific.'" (Perhaps this should be brought back into use ;-)

During World War II, tropical cyclones were informally given women's
names by USA Air Force and Navy meteorologists (after their girlfriends
or wives) who were monitoring and forecasting tropical cyclones over the
Pacific. From 1950 to 1952, tropical cyclones of the North Atlantic
Ocean were identified by the phonetic alphabet (Able-Baker-Charlie-etc.),
but in 1953 the USA Weather Bureau switched to women's names. In 1979,
the WMO and the USA National Weather Service (NWS) switched to a list of
names that also included men's names.

The Northeast Pacific basin tropical cyclones were named using
women's names starting in 1959 for storms near Hawaii and in 1960 for the
remainder of the Northeast Pacific basin. In 1978, both men's and women's
names were utilized.

The Northwest Pacific basin tropical cyclones were given women's
names officially starting in 1945 and men's names were also included
beginning in 1979.

The North Indian Ocean region tropical cyclones are not named.

The Southwest Indian Ocean tropical cyclones were first named during
the 1960/1961 season.

The Australian and South Pacific region (east of 90E, south of the
equator) started giving women's names to the storms in 1964 and both men's
and women's names in 1974/1975.


Subject: B2) What are the tropical cyclone names through 2001?

(Courtesy of Gary Padgett, Jack Beven and James Lewis Free)

Atlantic, Gulf of Mexico, Caribbean Sea
1996 1997 1998 1999 2000 2001

1. Arthur Ana Alex Arlene Alberto Allison
2. Bertha Bill Bonnie Bret Beryl Barry
3. Cesar Claudette Charley Cindy Chris Chantal
4. Dolly Danny Danielle Dennis Debby Dean
5. Edouard Erika Earl Emily Ernesto Erin
6. Fran Fabian Frances Floyd Florence Felix
7. Gustav Grace Georges Gert Gordon Gabrielle
8. Hortense Henri Hermine Harvey Helene Humberto
9. Isidore Isabel Ivan Irene Isaac Iris
10. Josephine Juan Jeanne Jose Joyce Jerry
11. Kyle Kate Karl Katrina Keith Karen
12. Lili Larry Lisa Lenny Leslie Lorenzo
13. Marco Mindy Mitch Maria Michael Michelle
14. Nana Nicholas Nicole Nate Nadine Noel
15. Omar Odette Otto Ophelia Oscar Olga
16. Paloma Peter Paula PhilippePatty Pablo
17. Rene Rose Richard Rita Rafael Rebekah
18. Sally Sam Shary Stan Sandy Sebastien
19. Teddy Teresa Tomas Tammy Tony Tanya
20. Vicky Victor Virginie Vince Valerie Van

Eastern North Pacific (east of 140W)
1993 1994 1995 1996 1997 1998

1. Adrian Aletta Adolph Alma Andres Agatha
2. Beatriz Bud Barbara Boris Blanca Blas
3. Calvin Carlotta Cosme Cristina Carlos Celia
4. Dora Daniel Dalila Douglas Dolores Darby
5. Eugene Emilia Erick Elida Enrique Estelle
6. Fernanda Fabio Flossie Fausto Felicia Frank
7. Greg Gilma Gil Genevieve Guillermo Georgette
8. Hilary Hector Henriette Hernan Hilda Howard
9. Irwin Ileana Ismael Iselle Ignacio Isis
10. Jova John Juliette Julio Jimena Javier
11. Kenneth Kristy Kiko Kenna Kevin Kay
12. Lidia Lane Lorena Lowell Linda Lester
13. Max Miriam Manuel Marie Marty Madeline
14. Norma Norman Narda Norbert Nora Newton
15. Otis Olivia Octave Odile Olaf Orlene
16. Pilar Paul Priscilla Polo Pauline Paine
17. Ramon Rosa Raymond Rachel Rick Roslyn
18. Selma Sergio Sonia Simon Sandra Seymour
19. Todd Tara Tico Trudy Terry Tina
20. Veronica Vicente Velma Vance Vivian Virgil
21. Wiley Willa Wallis Winnie Waldo Winifred
22. Xina Xavier Xina Xavier Xina Xavier
23. York Yolanda York Yolanda York Yolanda
24. Zelda Zeke Zelda Zeke Zelda Zeke

(The 1999 names will be identical to the list for 1993.)

Central North Pacific (from the dateline to 140W)

Akoni Aka Alika Ana
Ema Ekeka Ele Ela
Hana Hali Huko Halola
Io Iolana Ioke Iune
Keli Keoni Kika Kimo
Lala Li Lana Loke
Moke Mele Maka Malia
Nele Nona Neki Niala
Oka Oliwa Oleka Oko
Peke Paka Peni Pali
Uleki Upana Ulia Ulika
Wila Wene Wali Walaka

Each year the next name is just the one following the last
from the previous year. Once through a list the next name
will be off of the top of the next list.

Western North Pacific (west of the dateline)

Ann Abel Amber Alex
Bart Beth Bing Babs
Cam Carlo Cass Chip
Dan Dale David Dawn
Eve Ernie Ella Elvis
Frankie Fern Fritz Faith
Gloria Greg Ginger Gil
Herb Hannah Hank Hilda
Ian Isa Ivan Iris
Joy Jimmy Joan Jacob
Kirk Kelly Keith Kate
Lisa Levi Linda Leo
Marty Marie Mort Maggie
Niki Nestor Nichole Neil
Orson Opal Otto Olga
Piper Peter Penny Paul
Rick Rosie Rex Rachel
Sally Scott Stella Sam
Tom Tina Todd Tanya
Violet Victor Vicki Virgil
Willie Winnie Waldo Wendy
Yates Yule Yanni York
Zane Zita Zeb Zia

Each year the next name is just the one following the last
from the previous year. Once through a list the next name
will be off of the top of the next list.

North Indian Ocean
Tropical cyclones in this region are not named.

(Thanks to Julian Heming, Jack Beven, Gary Padgett, Frank Woodcock and
Jon Gill.)

Southwest Indian (west of 90E)
1996-1997 1997-1998 1998-1999 1999-2000


[The other areas have lists which they continually rotate through - i.e.
don't start again from 'A' each year]

Western Australian region (90E to 125E)

Northern Australian region (125E to 137E)

Eastern Australian region (137E to 160E, south of ~10S)

Fiji Area next 10 names (160E to 120W)
Yasi, Zaka, Atu, Beti, Cyril, Drena, Evan, Freda, Gavin, Hina

Papua New Guinea (140E to 160E, north of ~10S)
Adel, Epi, Guba, Ila, Kamo, Tako, Upia


Subject: B3) What names have been retired in the Atlantic basin?

In the Atlantic basin, tropical cyclone names are "retired" (that is, not
to be used again for a new storm) if it is deemed to be quite noteworthy
because of the damage and/or deaths it caused. This is to prevent confusion
with a historically well-known cyclone with a current one in the Atlantic
basin. The following list gives the names that have been retired through
the year 1996 and the year of the storm in question. (Kindly provided by
Gary Padgett, Jack Beven and James Lewis Free).

Agnes 1972, Alicia 1983, Allen 1980, Andrew 1992, Anita 1977, Audrey 1957

Betsy 1965, Beulah 1967, Bob 1991

Camille 1969, Carla 1961, Carmen 1974, Carol 1965, Celia 1970, Cesar 1996,
Cleo 1964, Connie 1955

David 1979, Diana 1990, Diane 1955, Donna 1960, Dora 1964

Edna 1968, Elena 1985, Eloise 1975

Fifi 1974, Flora 1963, Fran 1996, Frederic 1979

Gilbert 1988, Gloria 1985, Gracie 1959

Hattie 1961, Hazel 1954, Hilda 1964, Hortense 1996, Hugo 1989

Inez 1966, Ione 1955

Janet 1955, Joan 1988

Klaus 1990

Luis 1995

Marilyn 1995

Opal 1995

Roxanne 1995


B4) What is the origin of the name "hurricane"?

"HURRICANE...derived from 'hurican', the Carib god of evil...
alternative spellings: foracan, foracane, furacana, furacane, furicane,
furicano, haracana, harauncana, haraucane,
haroucana, harrycain, hauracane, haurachana,
herican, hericane, hericano, herocane, herricao,
herycano, heuricane, hiracano, hirecano, hurac[s]n,
huracano, hurican, hurleblast, hurlecan, hurlecano,
hurlicano, hurrican, hurricano, hyrracano, urycan,
hyrricano, jimmycane, oraucan, uracan, uracano"

From the _Glossary of Meteorology_


Subject: C1) Doesn't the low pressure in the tropical cyclone center
cause the storm surge?

No. Many people assume that the partial vacuum at the center of a
tropical cyclone allows the ocean so rise up in response, thus causing the
destructive storm surges as the cyclone makes landfall. However, this
effect would be, for example, with a 900 mb central pressure tropical
cyclone, only 1.0 m (3 ft). The total storm surge for a tropical cyclone
of this intensity can be from 6 to 10 m (19 to 33 ft), or more. Most
(>85%) of the storm surge is caused by winds pushing the ocean surface
ahead of the storm on the right side of the track (left side of the track
in the Southern Hemisphere).

Since the surface pressure gradient (from the tropical cyclone center
to the environmental conditions) determines the wind strength, the central
pressure indirectly does indicate the height of the storm surge, but not
directly. Note also that individual storm surges are dependent upon the
coastal topography, angle of incidence of landfall, speed of tropical
cyclone motion as well as the wind strength.


Subject: C2) Doesn't the friction over land kill tropical cyclones?

(Parts of this section are written by Sim Aberson.)

No. During landfall, the increased friction over land acts -
somewhat contradictory - to both decrease the sustained winds and also
to increase the gusts felt at the surface (Powell and Houston 1996).
The sustained (1 min or longer average) winds are reduced because of
the dampening effect of larger roughness over land (i.e. bushes, trees
and houses over land versus a relatively smooth ocean). The gusts are
stronger because turbulence increases and acts to bring faster winds
down to the surface in short (a few seconds) bursts.

However, after just a few hours, a tropical cyclone over land will
begin to weaken rapidly - not because of friction - but because the storm
lacks the the moisture and heat sources that the ocean provided. This
depletion of moisture and heat hurts the tropical cyclone's ability to
produce thunderstorms near the storm center. Without this convection,
the storm rapidly fills.

An early numerical simulation (Tuleya and Kurihara 1978) had shown
that a hurricane making landfall over a very moist region (i.e. mainly
swamp) so that surface evaporation is unchanged, intensification may
result. However, a more recent study (Tuleya 1994) that has a more
realistic treatment of surface conditions found that even over a swampy
area a hurricane would weaken because of limited heat sources. Indeed,
nature conducted this experiment during Andrew as the hurricane
traversed the very wet Everglades, Big Cypress and Corkscrew Swamp areas
of southwest Florida. Andrew weakened dramatically: peak winds
decreased about 33% and the sea level pressure in the eye filled 19 mb
(Powell and Houston 1996).


Subject: C3) Aren't big tropical cyclones also intense tropical cyclones?

No. There is very little association between intensity (either
measured by maximum sustained winds or by central pressure) and size
(either measured by radius of 15 m/s [gale force] winds or the radius of
the outer closed isobar) (Weatherford and Gray 1988). Hurricane Andrew is
a good example of a very intense tropical cyclone (922 mb central pressure
and 64 m/s (125 kt) sustained winds at landfall in Florida) that was also
relatively small (15 m/s winds extended out only about 150 km from the
center). Weatherford and Gray (1988) also showed that changes of both
intensity and size are essentially independent of one another.


Subject: C4) Why don't we try to destroy tropical cyclones by: pick one
or more - a) seeding them with silver iodide, b) nuking them,
c) placing a substance on the ocean surface, d) etc. ?

Actually for a couple decades NOAA and its predecessor tried to
weaken hurricanes by dropping silver iodide - a substance that serves as a
effective ice nuclei - into the rainbands of the storms. The idea was that
the silver iodide would enhance the thunderstorms of the rainband by
causing the supercooled water to freeze, thus liberating the latent heat of
fusion and helping the rainband to grow at the expense of the eyewall.
With a weakened convergence to the eyewall, the strong inner core winds
would also weaken quite a bit. Neat idea, but it, in the end, had a fatal
flaw: there just isn't much supercooled water available in hurricane
convection - the buoyancy is fairly small and the updrafts correspondingly
small compared to the type one would observe in mid-latitude continental
super or multicells. The few times that they did seed and saw a reduction
in intensity was undoubtedly due to what is now called "concentric eyewall

Concentric eyewall cycles naturally occur in intense tropical cyclones
(wind > 50 m/s or 100 kt). As tropical cyclones reach this threshold of
intensity, they usually - but not always - have an eyewall and radius of
maximum winds that contracts to a very small size, around 10 to 25 km. At
this point, some of the outer rainbands may organize into an outer ring of
thunderstorms that slowly moves inward and robs the inner eyewall of its
needed moisture and momentum. During this phase, the tropical cyclone
is weakening (i.e. the maximum winds die off a bit and the central
pressure goes up). Eventually the outer eyewall replaces the inner one
completely and the storm can be the same intensity as it was previously
or, in some cases, even stronger. A concentric eyewall cycle occurred
in Hurricane Andrew (1992) before landfall near Miami: a strong intensity
was reached, an outer eyewall formed, this contracted in concert with a
pronounced weakening of the storm, and as the outer eyewall completely
replaced the original one the hurricane reintensified.

Thus nature accomplishes what NOAA had hoped to do artificially. No
wonder that the first few experiments were thought to be successes. To
learn about the STORMFURY project as it was called, read Willoughby et al.
(1985). To learn more about concentric eyewall cycles, read Willoughby et
al. (1982) and Willoughby (1990).

As for the other ideas, there has been some experimental work in
trying to develop a liquid that when placed over the ocean surface would
prevent evaporation from occurring. If this worked in the tropical cyclone
environment, it would probably have a detrimental effect on the intensity
of the storm as it needs huge amounts of oceanic evaporation to continue
to maintain its intensity (Simpson and Simpson 1966). However, finding a
substance that would be able to stay together in the rough seas of a
tropical cyclone proved to be the downfall of this idea.

There was also suggested about 20 years ago (Gray et al. 1976) that
the use of carbon black (or soot) might be a good way to modify tropical
cyclones. The idea was that one could burn a large quantity of a heavy
petroleum to produce vast numbers of carbon black particles that would be
released on the edges of the tropical cyclone in the boundary layer. These
carbon black aerosols would produce a tremendous heat source simply by
absorbing the solar radiation and transferring the heat directly to the
atmosphere. This would provide for the initiation of thunderstorm activity
outside of the tropical cyclone core and, similarly to STORMFURY, weaken the
eyewall convection. This suggestion has never been carried out in real-

Lastly, there always appears ideas during the hurricane season that
one should simply use nuclear weapons to try and destroy the storms. Apart
from the concern that this might not even alter the storm, this approach
neglects the problem that the released radiation would fairly quickly
move with the tradewinds to over land. Needless to say, this is not a
good idea.

< Start Soap Box >

Perhaps the best solution is not to try to alter or destroy the
tropical cyclones, but just learn to co-exist better with them. Since
we know that coastal regions are vulnerable to the storms, enforce building
codes that can have houses stand up to the force of the tropical cyclones.
Also the people that choose to live in these locations should willing to
shoulder a fair portion of the costs in terms of property insurance -
not exorbitant rates, but ones which truly reflect the risk of living in
a vulnerable region.
< End Soap Box >


Subject: C5) During a hurricane are you supposed to have the windows and
doors on the storm side closed and the windows and doors on the lee
side open?

No! All of the doors and windows should be closed (and shuttered)
throughout the duration of the hurricane. The pressure differences between
inside your house and outside in the storm do not build up enough to cause
any damaging explosions. (No house is built airtight.) The winds in a
hurricane are highly turbulent and an open window or door - even if in the
lee side of the house - can be an open target to flying debris. All
exterior windows should be boarded up with either wooden or metal shutters.


Subject: D1) How are Atlantic hurricanes ranked?

The USA utilizes the Saffir-Simpson hurricane intensity scale (Simpson
and Riehl 1981) for the Atlantic and Northeast Pacific basins to give an
estimate of the potential flooding and damage to property given a
hurricane's estimated intensity:

Saffir-Simpson Maximum sustained Minimum surface Storm surge
Category wind speed (m/s,kt) pressure (mb) (m,ft)
-------------- ------------------- --------------- ---------------
1 33-42 m/s [64-83 kt] >= 980mb 1.0-1.7 m [3-5 ft]
2 43-49 [84-96] 979-965 1.8-2.6 [6-8]
3 50-58 [97-113] 964-945 2.7-3.8 [9-12]
4 59-69 [114-135] 944-920 3.9-5.6 [13-18]
5 > 69 [> 135] < 920 > 5.6 [> 18]

1: MINIMAL: Damage primarily to shrubbery, trees, foliage, and unanchored
homes. No real damage to other structures. Some damage to poorly
constructed signs. Low-lying coastal roads inundated, minor pier
damage, some small craft in exposed anchorage torn from moorings.
Example: Hurricane Jerry (1989)

2: MODERATE: Considerable damage to shrubbery and tree foliage; some
trees blown down. Major damage to exposed mobile homes. Extensive
damage to poorly constructed signs. Some damage to roofing materials
of buildings; some window and door damage. No major damage to
buildings. Coast roads and low-lying escape routes inland cut by
rising water 2 to 4 hours before arrival of hurricane center.
Considerable damage to piers. Marinas flooded. Small craft in
unprotected anchorages torn from moorings. Evacuation of some
shoreline residences and low-lying areas required. Example: Hurricane
Bob (1991)

3: EXTENSIVE: Foliage torn from trees; large trees blown down.
Practically all poorly constructed signs blown down. Some damage to
roofing materials of buildings; some wind and door damage. Some
structural damage to small buildings. Mobile homes destroyed. Serious
flooding at coast and many smaller structures near coast destroyed;
larger structures near coast damaged by battering waves and floating
debris. Low-lying escape routes inland cut by rising water 3 to 5
hours before hurricane center arrives. Flat terrain 5 feet of less
above sea level flooded inland 8 miles or more. Evacuation of low-
lying residences within several blocks of shoreline possibly required.
Example: Hurricane Gloria (1985)

4: EXTREME: Shrubs and trees blown down; all signs down. Extensive
damage to roofing materials, windows and doors. Complete failures of
roofs on many small residences. Complete destruction of mobile homes.
Flat terrain 10 feet of less above sea level flooded inland as far as
6 miles. Major damage to lower floors of structures near shore due to
flooding and battering by waves and floating debris. Low-lying escape
routes inland cut by rising water 3 to 5 hours before hurricane center
arrives. Major erosion of beaches. Massive evacuation of all
residences within 500 yards of shore possibly required, and of single-
story residences within 2 miles of shore. Example: Hurricane Andrew

5: CATASTROPHIC: Shrubs and trees blown down; considerable damage to
roofs of buildings; all signs down. Very severe and extensive damage
to windows and doors. Complete failure of roofs on many residences and
industrial buildings. Extensive shattering of glass in windows and
doors. Some complete building failures. Small buildings overturned or
blown away. Complete destruction of mobile homes. Major damage to
lower floors of all structures less than 15 feet above sea level within
500 yards of shore. Low-lying escape routes inland cut by rising water
3 to 5 hours before hurricane center arrives. Massive evacuation of
residential areas on low ground within 5 to 10 miles of shore possibly
required. Example: Hurricane Camille (1969)

Note that tropical storms are not on this scale, but can produce extensive
damage with rainfall-produced flooding. Note also that category 3, 4, and
5 hurricanes are collectively referred to as intense (or major) hurricanes.
These intense hurricanes cause over 70% of the damage in the USA even
though they account for only 20% of tropical cyclone landfalls (Landsea

Note that in comparison with the Australian scale (subject D2), Australian
1 and and most of Australian 2 are within the tropical storm categorization
(i.e. would not be on the Saffir-Simpson scale). An Australian 3 would be
approximately equal to either a Saffir-Simpson category 1 or 2 hurricane.
An Australian 4 would be about the same as a Saffir-Simpson category 3 or 4
hurricane. An Australian 5 would be about the same as a Saffir-Simpson
category 5 hurricane.


Subject: D2) How are Australian tropical cyclones ranked?

The Australian forecasters have developed a scale for tropical
cyclone intensity for storms in their area of responsibility - 90 to 160E
(Holland 1993). Note that the sustained winds are based upon a 10 min
averaging period instead of the USA 1 minute period.

Australian Scale Sustained Winds (km/hr)
1 63-90 km/hr
2 91-125
3 126-165
4 166-225
5 > 225

There are further comments on this scale in subject D1).


Subject: D3) Why do tropical cyclones' winds rotate counter-clockwise
(clockwise) in the Northern (Southern) Hemisphere?

The reason is that the earth's rotation sets up an apparent force (called
the Coriolis force) that pulls the winds to the right in the Northern
Hemisphere (and to the left in the Southern Hemisphere). So when a low
pressure starts to form north of the equator, the surface winds will flow
inward trying to fill in the low and will be deflected to the right and
a counter-clockwise rotation will be initiated. The opposite (a deflection
to the left and a clockwise rotation) will occur south of the equator.

NOTE: This force is too tiny to effect rotation in, for example, water
that is going down the drains of sinks and toilets. The rotation in those
will be determined by the geometry of the container and the original
motion of the water. Thus one can find both clockwise and counter-
clockwise flowing drains no matter what hemisphere you are located. If
you don't believe this, test it out for yourself.


Subject: D4) How do I convert from mph to knots (to m/s) and from inches
of mercury to mb (to hPa)?

For winds: 1 mile per hour (mph) = 0.864 knots (kt)
1 mph = 1.609 kilometers per hour (kph)
1 mph = 0.4470 meters per second (m/s)
1 kt = 1.853 kph
1 kt = 0.5148 m/s
1 m/s = 3.600 kph

For pressures: 1 inch of mercury = 33.86 mb = 33.86 hPa

For distances: 1 ft = 0.3048 m


Subject: 41) How does the damage that hurricanes cause increase as a
function of wind speed?

Or to rephrase the question: Would a minimal 74 mph hurricane cause one
half of the damage that a major hurricane with 148 mph winds? No, the
amount of damage (at least experienced along the U.S. mainland) does not
increase linearly with the wind speed. Instead, the damage produced
increases exponentially with the winds. The 148 mph hurricane (a category
4 on the Saffir-Simpson Scale) may produce - on average - up to 100
times the damage of a minimal category 1 hurricane!

Landsea (1993) analyzed the damage caused by various categories of
tropical storms and hurricanes after normalizing by both the inflation
rate and population changes. Tropical cyclones from 1944 through 1990
were tabulated in terms of 1990 U.S. dollars. The following table
summarizes the findings:

Intensity (cases) Median Damage "Potential Damage"
Tropical/Subtropical Storm (75) <$1,000,000 0
Hurricane Cat. 1 (34) $24,000,000 1
Hurricane Cat. 2 (14) $218,000,000 10
Hurricane Cat. 3 (24) $1,108,000,000 50
Hurricane Cat. 4 (6) $2,274,000,000 100
Hurricane Cat. 5 (1) $5,933,000,000 250

The "Potential Damage" values just provide a reference value if one assigns
the median damage caused by a category 1 hurricane to be "1". The rapid
increase in damage as the categories go up is apparent.

Note that this study was done in mid-1992 (i.e. before Andrew) and thus
the median and potential damage values for the category 4 and 5
hurricanes may be on the conservative side.

Other interesting findings:

* Mean annual damage in mainland US is $1,857,000,000. (Again, this value
is pre-Andrew.)

* The damage is nearly evenly divided between that caused on the US Gulf
Coast (Florida panhandle to Texas) and the US East Coast (Florida
peninsula to Maine).

* Even though the intense hurricanes (the category 3, 4 and 5 storms)
comprise only 20% of all US landfalling tropical cyclones, they account
for 71% of all of the damage. (Again, the figure is pre-Andrew. With
Andrew included, the damage percentage is likely 75 to 80%.)


Subject: E1) Which is the most intense tropical cyclone on record?

Typhoon Tip in the Northwest Pacific Ocean on 12 October 1979 was
measured to have a central pressure of 870 mb and estimated surface
sustained winds of 85 m/s (165 kt) (Dunnavan and Diercks 1980). Typhoon
Nancy on 12 September, 1961 is listed in the best track data for the
Northwest Pacific region as having an estimated maximum sustained winds of
185 kt with a central pressure of 888 mb. However, it is now recognized
(Black 1992) that the maximum sustained winds estimated for typhoons during
the 1940s to 1960s were too strong and that the 185 kt (and numerous 160 kt
to 180 kt reports) is somewhat too high.

Note that Hurricane Gilbert's estimated 888 mb lowest pressure in mid-
September 1988 is the most intense [as measured by lowest sea level pressure]
for the Atlantic basin (Willoughby et al 1989), it is almost 20 mb weaker
(higher) than the above Typhoon Tip of the Northwest Pacific Ocean.

While the central pressures for the Northwest Pacific typhoons are
the lowest globally, the North Atlantic hurricanes have provided sustained
wind speeds possibly comparable to the Northwest Pacific. From the best
track database, both Hurricane Camille (1969) and Hurricane Allen (1980)
have winds that are estimated to be 165 kt. Measurements of such winds
are inherently going to be suspect as instruments often are completely
destroyed or damaged at these speeds.


Subject: E2) Which tropical cyclone intensified the fastest?

Typhoon Forrest in September 1983 in the Northwest Pacific Ocean
deepened by 100 mb (976 to 876 mb) in just under 24 hr (Roger Edson,
personal communication). Estimated surface sustained winds increased a
maximum of 30 kt in 6 hr and 85 kt in one day (from 65 to 150 kt).


Subject: E3) Which tropical cyclone has produced the highest storm surge?

The Bathurst Bay Hurricane produced a 13 m (about 42 ft) surge in
Bathurst Bay, Australia in 1899 (Whittingham 1958).


Subject: E4) What are the largest rainfalls associated with tropical

12 hr: 1144 mm (45.0") at Foc-Foc, La Reunion Island in Tropical Cyclone
Denise, 7-8 January, 1966.
24 hr: 1825 mm (71.8") at Foc-Foc, La Reunion Island in Tropical Cyclone
Denise, 7-8 January, 1966.
48 hr: 2467 mm (97.1") at Aurere, La Reunion Island 8-10 April, 1958.
72 hr: 3240 mm (127.6") at Grand-Ilet, La Reunion Island in Tropical
Cyclone Hyacinthe, 24-27 January, 1980.
10 d: 5678 mm (223.5") at Commerson, La Reunion Island in Tropical
Cyclone Hyacinthe, 18-27 January, 1980.
(Holland 1993)


Subject: E5) Which are the largest and smallest tropical cyclones on

Typhoon Tip had gale force winds (15 m/s) which extended out for 1100
km in radius in the Northwest Pacific on 12 October, 1979 (Dunnavan and
Diercks 1980). Tropical Cyclone Tracy had gale force winds that only
extended 50 km radius when it struck Darwin, Australia, on 24 December,
1974 (Bureau of Meteorology 1977).


Subject: E6) Which tropical cyclone lasted the longest?

Hurricane/Typhoon John lasted 31 days as it traveled both the
Northeast and Northwest Pacific basins during August and September 1994.
(It formed in the Northeast Pacific, reached hurricane force there, moved
across the dateline and was renamed Typhoon John, and then finally
recurved back across the dateline and renamed Hurricane John again.)
Hurricane Ginger was a tropical cyclone for 28 days in the North Atlantic
Ocean back in 1971.


Subject: E7) Which tropical cyclones have caused the most deaths and most

"The death toll in the infamous Bangladesh Cyclone of 1970 has had
several estimates, some wildly speculative, but it seems certain that at
least 300,000 people died from the associated storm tide [surge] in the
low-lying deltas." (Holland 1993)

The largest damage caused by a tropical cyclone as estimated by
monetary amounts has been Hurricane Andrew (1992) as it struck the Bahamas,
Florida and Louisiana, USA: US $30 *Billion* (R. Sheets - personal
communication 1996). Most of this figure was due to destruction in
southeast Florida.


Subject: E8) What are the average, most, and least tropical cyclones
occurring in each basin?

Based on data from 1968-1989 (1968/69 to 1989/90 for the Southern

Tropical Storm or stronger Hurricane/Typhoon/Severe Tropical Cyclone
(>17 m/s sustained winds) (>33 m/s sustained winds)
Basin Most/Least Average Most/Least Average

Atlantic 18/4 9.7 12/2 5.4
NE Pacific 23/8 16.5 14/4 8.9
NW Pacific 35/19 25.7 24/11 16.0
N Indian 10/1 5.4 6/0 2.5
SW Indian 15/6 10.4 10/0 4.4
SE Indian/Aus 11/1 6.9 7/0 3.4
Aus/SW Pacific 16/2 9.0 11/2 4.3

Globally 103/75 83.7 65/34 44.9

Note that the data includes subtropical storms in the Atlantic basin
numbers. (Neumann 1993)

Starting in 1944, systematic aircraft reconnaissance was commenced for
monitoring both tropical cyclones and disturbances that had the potential
to develop into tropica cyclones. This is why both Neumann et al. (1993)
and Landsea (1993) recommend utilizing data since 1944 for computing
climatological statistics. However, for tropical cyclones striking the
USA East and Gulf coasts - because of highly populated coast lines,
data with good reliability extends back to around 1899. Thus, the
following records hold for the entire Atlantic basin (from 1944-1995) and
for the USA coastline (1899-1995):

Maximum Minimum
Tropical storms/hurricanes: 19*(1995) 4 (1983)
Hurricanes: 12 (1969) 2 (1982)
Intense Hurricanes: 7 (1950) 0 (many times,1994 last)
USA landfalling storms/hurricanes: 8 (1916) 1 (many,1991)
USA landfalling hurricanes: 6 (1916,1985) 0 (many,1994)
USA landfalling intense hurricanes: 3 (1909,33,54) 0 (many,1994)

(*) As a footnote, 1933 is recorded as being the most active of any
Atlantic basin season on record (reliable or otherwise) with 21 tropical
storms and hurricanes.

For the Northeast Pacific, the records stand at maximums of 27 tropical
storms/hurricanes in 1992 and 16 hurricanes in 1990. Reliable records go
back in this basin to around 1966 when geostationary satellite coverage

For the Northwest Pacific, the peak year stands at 1964 with 39 tropical
storms, 26 of which became typhoons. Reliable records for this basin begin
around 1960.


Subject: E9) What are the most and least tropical cyclones occurring in
the Atlantic basin and striking the USA?

Starting in 1944, systematic aircraft reconnaissance was commenced for
monitoring both tropical cyclones and disturbances that had the potential
to develop into tropical cyclones. This is why both Neumann et al. (1993)
and Landsea (1993) recommend utilizing data since 1944 for computing
climatological statistics. However, for tropical cyclones striking the
USA East and Gulf coasts - because of highly populated coast lines,
data with good reliability extends back to around 1899. Thus, the
following records hold for the entire Atlantic basin (from 1944-1996) and
for the USA coastline (1899-1996):

Maximum Minimum
Tropical storms/hurricanes: 19*(1995) 4 (1983)
Hurricanes: 12 (1969) 2 (1982)
Intense Hurricanes: 7 (1950) 0 (many times,1994 last)
USA landfalling storms/hurricanes: 8 (1916) 1 (many,1991)
USA landfalling hurricanes: 6 (1916,1985) 0 (many,1994)
USA landfalling intense hurricanes: 3 (1909,33,54) 0 (many,1994)

(*) As a footnote, 1933 is recorded as being the most active of any
Atlantic basin season on record (reliable or otherwise) with 21 tropical
storms and hurricanes.

Below is a table with individual years for the numbers of named storms
(tropical storms and hurricanes) - NS, named and subtropical storms -
NS&Sub, hurricanes - H, hurricane days - HD, and intense hurricanes - IH:

Atlantic basin tropical cyclone data:

Year NS NS&Sub H HD IH

1944 11.00 11.00 7.00 27.00 3.00
1945 11.00 11.00 5.00 14.00 2.00
1946 6.00 6.00 3.00 6.00 1.00
1947 9.00 9.00 5.00 28.00 2.00
1948 9.00 9.00 6.00 29.00 4.00
1949 13.00 13.00 7.00 22.00 3.00
1950 13.00 13.00 11.00 60.00 7.00
1951 10.00 10.00 8.00 36.00 2.00
1952 7.00 7.00 6.00 23.00 3.00
1953 14.00 14.00 6.00 18.00 3.00
1954 11.00 11.00 8.00 32.00 2.00
1955 12.00 12.00 9.00 47.00 5.00
1956 8.00 8.00 4.00 13.00 2.00
1957 8.00 8.00 3.00 21.00 2.00
1958 10.00 10.00 7.00 30.00 4.00
1959 11.00 11.00 7.00 22.00 2.00
1960 7.00 7.00 4.00 18.00 2.00
1961 11.00 11.00 8.00 48.00 6.00
1962 5.00 5.00 3.00 11.00 0.00
1963 9.00 9.00 7.00 37.00 2.00
1964 12.00 12.00 6.00 43.00 5.00
1965 6.00 6.00 4.00 27.00 1.00
1966 11.00 11.00 7.00 42.00 3.00
1967 8.00 8.00 6.00 36.00 1.00
1968 7.00 8.00 4.00 10.00 0.00
1969 17.00 18.00 12.00 40.00 3.00
1970 10.00 10.00 5.00 7.00 2.00
1971 13.00 13.00 6.00 29.00 1.00
1972 4.00 7.00 3.00 6.00 0.00
1973 7.00 8.00 4.00 10.00 1.00
1974 7.00 11.00 4.00 14.00 2.00
1975 8.00 9.00 6.00 21.00 3.00
1976 8.00 10.00 6.00 26.00 2.00
1977 6.00 6.00 5.00 7.00 1.00
1978 11.00 12.00 5.00 14.00 2.00
1979 8.00 9.00 5.00 22.00 2.00
1980 11.00 11.00 9.00 38.00 2.00
1981 11.00 12.00 7.00 23.00 3.00
1982 5.00 6.00 2.00 6.00 1.00
1983 4.00 4.00 3.00 4.00 1.00
1984 12.00 13.00 5.00 18.00 1.00
1985 11.00 11.00 7.00 21.00 3.00
1986 6.00 6.00 4.00 11.00 0.00
1987 7.00 7.00 3.00 5.00 1.00
1988 12.00 12.00 5.00 21.00 3.00
1989 11.00 11.00 7.00 32.00 2.00
1990 14.00 14.00 8.00 27.00 1.00
1991 8.00 8.00 4.00 8.00 2.00
1992 6.00 7.00 4.00 16.00 1.00
1993 8.00 8.00 4.00 10.00 1.00
1996 13.00 13.00 9.00 45.00 6.00

Mean from 1950 - 1990
9.34 9.78 5.83 23.69 2.17
Standard Deviation
4.24 4.51 3.15 17.37 1.81


Subject: E10) For the U.S., what are the 10 most intense, 10 costliest,
and 10 highest death toll hurricanes on record?

Updated from Hebert et al. (1992):

10 Most Intense USA (continental) hurricanes from 1900-1994:
(at time of landfall with landfall area)

1. "Labor Day" - FL Keys 1935 5 892 mb
2. Camille - LA/MS 1969 5 909
3. Andrew - SE FL 1992 4 922
4. Unnamed - FL Keys/S TX 1919 4 927
5. Unnamed - Lake Okeechobee, FL 1928 4 929
6. DONNA - FL Keys 1960 4 930
7. Unnamed - Galveston, TX 1900 4 931
8. Unnamed - Grand Isle, LA 1909 4 931
9. Unnamed - New Orleans, LA 1915 4 931
10. Carla - C TX 1961 4 931

Note that Hurricane Gilbert's estimated 888 mb lowest pressure in mid-
September 1988 is the most intense [as measured by lowest sea level
pressure] for the Atlantic basin, but it affected the USA only as a
weakening tropical depression (Neumann et al 1993).

10 Costliest USA (continental) hurricanes from 1900-1994:
(adjusted to 1990 dollars - except for Andrew)

1. Andrew - SE FL/LA 1992 4 ~$30,000,000,000
2. Hugo - SC 1989 4 7,155,120,000
3. Betsy - FL/LA 1965 3 6,461,303,000
4. Agnes - NE U.S. 1972 1 6,418,143,000
5. Camille - LA/MS 1969 5 5,242,380,000
6. Diane - NE U.S. 1955 1 4,199,645,000
7. "New England" 1938 3 3,593,853,000
8. Frederic - AL/MS 1979 3 3,502,942,000
9. Alicia - N TX 1983 3 2,391,854,000
10. Carol - NE U.S. 1954 3 2,370,215,000

Note that this does not take into account the massive coastal population
increases and structural buildup that have occurred along the US East and
especially the Gulf coasts during the past few decades. Intense hurricanes
will continue to inflict massive destruction along the USA coastlines, even
with perfect forecasts of their track and intensity.

10 Deadliest USA (continental) hurricanes from 1900-1994:

1. Unnamed - Galveston, TX 1900 4 6000+
2. Unnamed - Lake Okeechobee, FL 1928 4 1836+
3. Unnamed - Fl Keys/S TX 1919 4 600-900
4. "New England" 1938 3 600
5. "Labor Day" - FL Keys 1935 5 408
6. Audrey - SW LA/N TX 1957 4 390
7. Unnamed - NE U.S. 1944 3 390
8. Unnamed - Grand Isle, LA 1909 4 350
9. Unnamed - New Orleans, LA 1915 4 275
10. Unnamed - Galveston, TX 1915 4 275

+ (These values are estimate and may be conservative of the true
numbers of fatalities.)

ADDENDUM: Unnamed - LA - 1893 - 2000
Unnamed - SC/GA - 1893 - 1000-2000
Unnamed - GA/SC - 1881 - 700

One can take some comfort in the fact that even with the massive damage
amounts reported with hurricanes in the last couple decades, none of those
hurricanes caused huge numbers of deaths in the USA. This is because of
the increasingly skillful forecasts of hurricane tracks, the ability to
communicate warnings to the public via radio and television, and the
infrastructure that allows for evacuations to proceed safely for those in
the hurricane's path (Sheets 1990). However, if people chose to ignore
warnings or if evacuations are not able to remove people from danger (because
of too many people overcrowding limited escape routes - the Florida Keys and
US 1 is a good example), then the potential remains for disasters similar to
what was seen decades ago.


Subject: E11) What tropical storms and hurricanes have moved from the
Atlantic to the Northeast Pacific or vice versa?

(Stephen Caparotta, D. Walston, Steven Young and Gary Padgett compiled
this list.)

Here is a list of tropical cyclones that have crossed from the Atlantic
basin to the Northeast Pacific and vice versa. The tropical cyclone must
have been of at least tropical storm strength in both basins (i.e.
sustained winds of at least 34 kt, or 18 m/s). This record only goes
back to 1949. Before the advent of geostationary satellite pictures in
the mid-1960s, the number of Northeast Pacific tropical cyclones was
undercounted by a factor of 2 or 3. Thus the lack of many of these
events during the 1960s and earlier is mainly due to simply missing the
Northeast Pacific TCs.

There has not been a recorded case where the same tropical cyclone
crossed into the Northeast Pacific then crossed back into the Atlantic.

Atlantic Hurricane Cesar (July 1996) became Northeast Pacific Hurricane

Atlantic Tropical Storm Bret (August 1993) became Hurricane Greg
in the Northeast Pacific.

Northeast Pacific Hurricane Cosme became Atlantic Tropical Storm Allison
(June 1989).

Atlantic Hurricane Joan (October 1988) became Northeast Pacific
Hurricane Miriam.

Atlantic Hurricane Greta (September 1978) became Northeast Pacific
Hurricane Olivia.

Atlantic Hurricane Fifi (September 1974) became Northeast Pacific
Tropical Storm Orlene.

Atlantic Hurricane Irene (September 1971) became Northeast Pacific
Tropical Storm Olivia.

Atlantic Hurricane Hattie (October-November 1961) became Northeast
Pacific Tropical Storm Simone.

A Northeast Pacific Tropical Storm (September-October 1949) became an
Atlantic Hurricane (Storm #10) and made landfall in TX.


Subject: F1) What regions around the globe have tropical cyclones and who
is responsible for forecasting there?

There are seven tropical cyclone "basins" where storms occur on a
regular basis:
--- Atlantic basin (including the North Atlantic Ocean, the Gulf of
Mexico, and the Caribbean Sea)
--- Northeast Pacific basin (from Mexico to about the dateline)
--- Northwest Pacific basin (from the dateline to Asia including the
South China Sea)
--- North Indian basin (including the Bay of Bengal and the Arabian
--- Southwest Indian basin (from Africa to about 100E)
--- Southeast Indian/Australian basin (100E to 142E)
--- Australian/Southwest Pacific basin (142E to about 120W)

The National Hurricane Center in Miami, Florida, USA has responsibil-
ities for monitoring and forecasting tropical cyclones in the Atlantic
and Northeast Pacific basin east of 140W. The Central Pacific Hurricane
Center has responsibilities for the remainder of the Northeast Pacific
basin to the dateline. The Northwest Pacific basin is shared in
forecasting duties by China, Thailand, Korea, Japan, the Philippines, and
Hong Kong. The North Indian basin tropical cyclones are forecasted by
India, Thailand, Pakistan, Bangladesh, Burma, and Sri Lanka. Reunion
Island, Madagascar, Mozambique, Mauritius, and Kenya provide forecasts for
the Southwest Indian basin. Australia and Indonesia forecast tropical
cyclone activity in the Southeast Indian/Australian basin. Lastly, for the
Australian/Southwest Pacific basin Australia, Papua New Guinea, Fiji, and
New Zealand forecast tropical cyclones. Note also that the USA Joint
Typhoon Warning Center (JTWC) issues warnings for tropical cyclones in the
Northwest Pacific, the North Indian, the Southwest Indian, the Southeast
Indian/Australian, and the Australian/Southwest Pacific basins, though they
are not specifically tasked to do so by the WMO. The USA Naval Western
Oceanography Center in Pearl Harbor, Honolulu does the same for the Pacific
Ocean east of 180E. (Neumann 1993)

Note that on rare occasions, tropical cyclones (or storms that appear
to be similar in structure to tropical cyclones) can develop in the
Mediterranean Sea. These have been noted to occur in September 1947,
September 1969, January 1982, September 1983, and, most recently, during
13 to 17 January, 1995. Some study of these storms has been reported on
by Mayengon (1984) and Ernest and Matson (1983), though it has not been
demonstrated fully that these storms are the same as those found over
tropical waters. It may be that these Mediterranean tropical cyclones are
more similar in nature to polar lows.

The following are the addresses of tropical cyclone centers listed
above that are responsible for issuing advisories and/or warnings on tropical
cyclones (thanks to Jack Beven for these):

National Hurricane Center
Mail: 11691 SW 17th St.
Miami, FL 33165-2149
WWW: http://www.nhc.noaa.gov/index.html

Central Pacific Hurricane Center
Mail: National Weather Service Forecast Office
University of Hawaii at Manoa
Department of Meteorology
2525 Correa Rd. (HIG)
Honolulu, HI 96822

Naval Pacific Meteorological and Oceanographic Center
Box 113
Pearl Harbor, HI 96860

Joint Typhoon Warning Center - Guam
PCS 486, Box 17
FPO AP 96536-0051
WWW: http://www.npmocw.navy.mil/npmocw/prods/jtwc.html

Regional Specialized Meteorological Center Tokyo, Japan - Typhoon Center
Mail: Japanese Meteorological Agency
1-3-4 Ote-machi, Chiyoda-ku

Royal Observatory - Hong Kong
Mail: 134A Nathan Road
Hong Kong

Bangkok Tropical Cyclone Warning Center - Thailand
Mail: Director
Meteorological Department
4353 Sukumvit Rd.
Bangkok 10260

Fiji Tropical Cyclone Warning Center
Mail: Director
Fiji Meteorological Services
Private Mail Bag
Nadi Airport

New Zealand Meteorological Service
Mail: Director
Met Service
PO Box 722
New Zealand

Port Moresby Tropical Cyclone Warning Center
Mail: Director
National Weather Service
PO Box 1240
Boroko, NCD
Paupa New Guinea

Brisbane Tropical Cyclone Warning Center
Mail: Regional Director
Bureau of Meteorology
GPO Box 413
Brisbane 4001

Darwin Tropical Cyclone Warning Center
Mail: Regional Director
Bureau of Meteorology
GPO Box 735
Darwin 5790

Perth Tropical Cyclone Warning Center
Mail: Regional Director
Bureau of Meteorology
GPO Box 6080
Perth 9001

Jakarta, Indonesia
Mail: Director
Analysis and Processing Centre
Jalan Arief Rakhman Hakim 3

Regional Tropical Cyclone Advisory Centre - Reunion
Mail: Director of Meteorological Services
PO Box 4
97490 Sainte Clotilde

Sub-Regional Tropical Cyclone Warning Center - Mauritius
Mail: Director of Meteorological Service

Sub-Regional Tropical Cyclone Warning Center - Madagascar
Mail: Director of Meteorological Service
PO Box 1254
Antananarivo 101

Nairobi, Kenya
Mail: Director of Meteorological Services
PO Box 30259

Maputo, Mozambique
Mail: Director of Meteorology
PO Box 256

The following cities are also mentioned as tropical cyclone warning centers,
though I don't have the addresses for them.

Philippines: Manila

China: Beijing

Korea: Seoul

Vietnam: Hanoi

India: New Delhi

Bangladesh: Dhaka

Burma: Rangoon

Sri Lanka: Colombo

Maldive Islands: Male


Subject: F2) What is Prof. Gray's seasonal hurricane forecast for this
year and what are the predictive factors?

Prof. Bill Gray at Colorado State University in Fort Collins, Colorado
(USA) has issued seasonal hurricane forecasts for the Atlantic basin since
1984. Details of his forecasting technique can be found in Gray (1984a,b)
and Gray et al. (1992, 1993, 1994). Landsea et al. (1994) also provides
verifications of the first 10 years of forecasting. A quick summary of the
components follows:

* El Nino/Southern Oscillation (ENSO) - During El Nino events (ENSO warm
phase), tropospheric vertical shear is increased inhibiting tropical
cyclone genesis and intensification. La Nina events (ENSO cold phase)
enhances activity.

* African West Sahel rainfall - In years of West Sahel drought conditions,
the Atlantic hurricane activity is much reduced - especially the intense
hurricane activity (Landsea and Gray 1992). Wet West Sahel years mean a
higher chance of low-latitude "Cape Verde" type hurricanes. This is also
due to higher tropospheric vertical shear in the drought years, though there
may also be changes in the structure of African easterly waves as well to
make them less likely to go through tropical cyclogenesis.

* Stratospheric quasi-biennial oscillation (QBO) - During the 12 to 15
months when the equatorial stratosphere has the winds blowing from the
east (east phase QBO), Atlantic basin tropical cyclone activity is reduced.
The east phase is followed by 13 to 16 months of westerly winds in the
equatorial stratosphere where the Atlantic activity is increased. It is
believed (but not demonstrated) that the reduced activity in east years
is due to increased lower stratospheric to upper tropospheric vertical
shear which may disrupt the tropical cyclone structure.

* Caribbean sea level pressure anomalies (SLPA) - During seasons of lower
than average surface pressure around the Caribbean Sea, the Atlantic
hurricane activity is enhanced. When it is higher than average, the
tropical cyclone activity is diminished. Higher pressure indicates
either a weaker Inter-tropical Convergence Zone (ITCZ) or a more
equatorward position of the ITCZ or both.

* Caribbean 200 mb zonal wind anomalies (ZWA) - The 200 mb winds around
the Caribbean are often reflective of the ENSO or West Sahelian rainfall
conditions (i.e. westerly ZWA corresponds to El Ninos and West Sahel
drought conditions). However, the winds also provide some independent
measure of the tropospheric vertical shear, especially in years of neutral
ENSO and West Sahel rainfall.

Dr. Gray and his forecast team issues seasonal forecasts in late
November, early June, and early August of each year with a verification of
the forecasts given in late November. To obtain these forecasts, surf
to: http://tropical.atmos.colostate.edu/forecasts/index.html

Also available (via unix machines) a finger command to get a table with
the latest forecast info and what the observations have been of the season
so far. Available via: finger fore...@typhoon.atmos.colostate.edu


Subject: F3) How has Dr. Gray done in previous years of forecasting

Here are the numbers that Dr. Gray has issued for his real-time Atlantic
tropical cyclone seasonal forecasting:

Year Early December Early June Early August Observed
Forecast Forecast Forecast

Named Storms: 1950 to 1990 Mean = 9.3
1984 --- 10 10 12
1985 --- 11 10 11
1986 --- 8 7 6
1987 --- 8 7 7
1988 --- 11 11 12
1989 --- 7 9 11
1990 --- 11 11 14
1991 --- 8 7 8
1992 8 8 8 6
1993 11 11 10 8
1994 10 9 7 7
1995 12 12 16 19
1996 8 10 11 13

Hurricanes: 1950 to 1990 Mean = 5.8
1984 --- 7 7 5
1985 --- 8 7 7
1986 --- 4 4 4
1987 --- 5 4 3
1988 --- 7 7 5
1989 --- 4 4 7
1990 --- 7 6 8
1991 --- 4 3 4
1992 4 4 4 4
1993 6 7 6 4
1994 6 5 4 3
1995 8 8 9 11
1996 5 6 7 9

Intense Hurricanes: 1950 to 1990 Mean = 2.3
1990 --- 3 2 1
1991 --- 1 0 2
1992 1 1 1 1
1993 3 2 2 1
1994 2 1 1 0
1995 3 3 3 5
1996 2 2 3 6


Subject: F4) What are those track and intensity models that the Atlantic
forecasters are talking about in the tropical storm and
hurricane Discussions?

(Track model information contributed by Sim Aberson)

A variety of hurricane track forecast models are run operationally
for the Atlantic hurricane basin:

(1) The basic model that is used as a "no-skill" forecast to compare
other models against is CLIPER (CLImatology and PERsistence), a multiple
regression model that best utilizes the persistence of the motion and
also incorporates climatological track information (Neumann 1972, Merrill
1980). Surprisingly, CLIPER was difficult to beat with numerical model
forecasts until the 1980s.

(2) A statistical-dynamical model, NHC90 (McAdie 1991), uses geopotential
height predictors from the Aviation model to produce a track forecast four
times per day. The primary synoptic time NHC90 forecasts (00 and 12
UTC) are based upon 12 h old Aviation runs. A special version of NHC90,
NHC90-LATE, is run at primary synoptic times with the current Aviation
run, and is available a number of hours after NHC90. Both versions of
NHC90 have been run operationally since 1990.

(3) The Beta and Advection Model, BAM, follows a trajectory in the
pressure-weighted vertically-averaged horizontal wind from the Aviation
model beginning at the current storm location, with a correction that
accounts for the beta effect (Marks 1992). Three versions of this model,
one with a shallow-layer (BAMS), one with a medium-layer (BAMM), and one
with a deep-layer (BAMD), are run. BAMS runs using the 850-700 mb layer,
BAMM with the 850-400 mb layer, and BAMD with the 850-200 mb layer. The
deep-layer version was run operationally for primary synoptic times in
1989; all three versions have been run four times per day since 1990.

(4) A nested barotropic hurricane track forecast model (VICBAR) has been
run four times daily since 1989. The 0000 and 1200 UTC runs are based
upon current NCEP analyses, the others upon six hour old data (Aberson
and DeMaria 1994). Another barotropic model, LBAR, for Limited-Area
Barotropic Model, is also being run operationally every 6 h based upon
six hour old data, so is available for earlier use by the NHC forecasters.

(5) A triply-nested movable mesh primitive equation model developed at
the Geophysical Fluid Dynamics Laboratory (Bender et al 1993), known as the
GFDL model, has provided forecasts since the 1992 hurricane season.

(6) The NCEP Aviation and MRF models (Lord 1993) have been used for
track forecasting since the 1992 hurricane season. These are global

(7) The United Kingdom Meterological Office's global model (UKMET) is
utilized for forecasting the track of tropical cyclones around the
world (Radford 1994). The National Hurricane Center starting receiving
these operationally during 1996.

(8) The United States Navy Operational Global Atmospheric Prediction
Systems (NOGAPS) is also a global numerical model that shows skill in
forecasting tropical cyclone track (Fiorino et al. 1993). This model was
also first received operationally at the National Hurricane Center
during 1996.

Despite the variety of hurricane track forecast models, there are
only a few models that forecast intensity change for the Atlantic

(1) Similar to the CLIPER track model, SHIFOR (Statistical Hurricane
Intensity Forecast model) is used as a "no-skill" intensity change
forecast. It is a multiple regression statistical model that best
utilizes the persistence of the intensity trends and also incorporates
climatological intensity change information (Jarvinen and Neumann 1979).
Surprisingly, no other intensity models provide better forecasts on average
than SHIFOR.

(2) A statistical-synoptic model, SHIPS (Statistical Hurricane
Intensity Prediction Scheme), has been available the National Hurricane
Center since the mid-1990s (DeMaria and Kaplan 1994). It takes current
information on the synoptic scale on the sea surface temperatures,
vertical shear, etc. with an optimal combination of the trends in
the cyclone intensity. For the first time in 1996, SHIPS outperformed
SHIFOR (by having lower absolute wind speed errors) from the 24 hour to
72 hour forecasts, though the differences were small.

(3) The GFDL model, described above in the track forecasting models,
also issues forecasts of intensity change for the National Hurricane
Center. However, to date, these have yet to show any skill (i.e. GFDL
errors are larger than those from SHIFOR).


Subject: G1) What is the annual cycle of occurrence seen in each basin?

While the Atlantic hurricane season is "officially" from 1 June to
30 November, the Atlantic basin shows a very peaked season with 78% of the
tropical storm days, 87% of the minor (Saffir-Simpson Scale categories
1 and 2 - see subject D1) hurricane days, and 96% of the intense (Saffir-
Simpson categories 3, 4 and 5) hurricane days occuring in August through
October (Landsea 1993). Peak activity is in early to mid September. Once
in a few years there may be a tropical cyclone occurring "out of season" -
primarily in May or December.

The Northeast Pacific basin has a broader peak with activity beginning
in late May or early June and going until late October or early November
with a peak in storminess in late August/early September.

The Northwest Pacific basin has tropical cyclones occurring all year
round regularly though there is a distinct minimum in February and the
first half of March. The main season goes from July to November with a
peak in late August/early September.

The North Indian basin has a double peak of activity in May and
November though tropical cyclones are seen from April to December. The
severe cyclonic storms (>33 m/s winds) occur almost exclusively from April
to June and late September to early December.

The Southwest Indian and Australian/Southeast Indian basins have very
similar annual cycles with tropical cyclones beginning in late October/
early November, reaching a double peak in activity - one in mid-January
and one in mid-February to early March, and then ending in May. The
Australian/Southeast Indian basin February lull in activity is a bit more
pronounced than the Southwest Indian basin's lull.

The Australian/Southwest Pacific basin begin with tropical cyclone
activity in late October/early November, reaches a single peak in late
February/early March, and then fades out in early May.

Globally, September is the most active month and May is the least
active month. (Neumann 1993)


Subject: G2) How does El Nino-Southern Oscillation affect tropical cyclone
activity around the globe?

The effect of El Nino-Southern Oscillation (ENSO) on Atlantic tropical
cyclones is described in subject F2).

The Australian/Southwest Pacific shows a pronounced shift back and
forth of tropical cyclone activity with fewer tropical cyclones between
145 and 165E and more from 165E eastward across the South Pacific during
El Nino (warm ENSO) events. There is also a smaller tendency to have the
tropical cyclones originate a bit closer to the equator. The opposite
would be true in La Nina (cold ENSO) events. See papers by Nicholls (1979),
Revell and Goulter (1986), Dong (1988), and Nicholls (1992).

The western portion of the Northeast Pacific basin (140W to the
dateline) has been suggested to experience more tropical cyclone genesis
during the El Nino year and more tropical cyclones tracking into the
sub-region in the year following an El Nino (Schroeder and Yu 1995), but
this has not been completely documented yet.

The Northwest Pacific basin, similar to the Australian/Southwest
Pacific basin, experiences a change in location of tropical cyclones
without a total change in frequency. Pan (1981), Chan (1985), and Lander
(1994) detailed that west of 160E there were reduced numbers of tropical
cyclone genesis with increased formations from 160E to the dateline during
El Nino events. The opposite occurred during La Nina events. Again there
is also the tendency for the tropical cyclones to also form closer to the
equator during El Nino events than average.

The eastern portion of the Northeast Pacific, the Southwest Indian,
the Southeast Indian/Australian, and the North Indian basins have either
shown little or a conflicting ENSO relationship and/or have not been looked
at yet in sufficient detail.


Subject: G3) What may happen with tropical cyclone activity in a 2xCO2

Two impacts of anthropogenic climate change due to increasing amounts of
"greenhouse" gases that may occur (Houghton et al., 1990, 1992) are
increased tropical sea surface temperatures (moderate confidence) and
increased tropical rainfall associated with a slightly stronger inter-
tropical convergence zone (ITCZ) (moderate/low confidence). Because of
these possible changes, there have been many suggestions based upon global
circulation and theoretical modeling studies that increases may occur in the
frequency (AMS Council and UCAR Board of Trustees, 1988; Houghton et al.,
1990; Broccoli and Manabe, 1990; Ryan et al., 1992; Haarsma et al., 1993),
area of occurrence (Houghton et al., 1990; Ryan et al., 1992), mean
intensity (AMS Council and UCAR Board of Trustees, 1988; Haarsma et al.,
1993), and maximum intensity (Emanuel, 1987; AMS Council and UCAR Board of
Trustees, 1988; Houghton et al., 1990; Haarsma et al., 1993; Bengtsson et
al., 1994) of tropical cyclones. In contrast, there have been some
conclusions that decreases in frequency may result (Broccoli and Manabe
1990; Bengtsson et al., 1994). One report (Leggett, 1994) has suggested
that increased tropical cyclone incidence and severity have already taken
place, but provided no quantitative evidence.

Any changes in tropical cyclone activity are intrinsically tied in with
large-scale changes in the tropical atmosphere. One key feature that
has been focused upon has been possible changes in sea surface
temperatures (SSTs). But SSTs by themselves cannot be considered without
corresponding information regarding the moisture and stability in the
tropical troposphere. What has been identified in the current climate
as being necessary for genesis and maintenance for tropical cyclones
(e.g. SSTs of at least 80F or 26.5C) might change in a 2xCO2 world
because of possible changes in the moisture and/or stability.

Additionally, besides the thermodynamic variables, changes in the tropical
dynamics will also play a big role in determining changes in tropical
cyclone activity. For example, if the vertical wind shear over the
tropical North Atlantic decreased (increased) during the hurricane season
in a 2xCO2 world, then we would see a significant increase (decrease) in
activity. Another large unknown is how the monsoonal circulations may
change. If the monsoons became more active, then it may be possible
that more tropical cyclones in the oceanic monsoon regions might result.

One last final wild card in all of this is how the El Nino-Southern
Oscillation (ENSO) may change in a 2xCO2 world, as ENSO is the largest
single factor controlling year-to-year variability of tropical cyclones
globally - see sections G2) and F2). If the warm phase of ENSO (the "El
Nino" events) occurred more often and/or with more intensity, then the
inhabitants along the Atlantic basin and Australia would have fewer
tropical cyclones to worry about. But people living in Hawaii and in the
South Central Pacific would have more storms to deal with. The reverse
would be true if the cold phase (or "La Nina") became more prevalent.

Overall, it is difficult to assess globally how changes of tropical cyclone
intensities (both the mean and the maximum), frequencies, and area of
occurrence may change in a 2xCO2 world. It may very well turn out that
changes around the globe may not be consistent, with some regions receiving
more activity while others getting less. Certainly, this is an area of
research that needs to continue until more definitive answers are found.


Subject: G4) Are we getting stronger and more frequent hurricanes,
typhoons, and tropical cyclones in the last several years?

Globally, probably not. For the Atlantic basin, definitely not. In fact,
as documented in Landsea (1993), the number of intense hurricanes (those
hurricanes reaching Saffir-Simpson scale 3, 4, and 5 - defined in subject D1)
has actually gone *down* during the 1970s and the 1980s, both in all basin
intense hurricanes as well as those making landfall along the U.S. coastline.

"With Andrew in 1992 and the busy 1995 hurricane season, have things changed
during the 1990s?" No. Even taking into account Andrew, the period 1991 to
1994 was the *quietest* four years on record - using reliable data going back
to 1944 (Landsea et al. 1996). Of course, with a very active Atlantic
hurricane season (19 tropical storms and hurricanes, 11 hurricanes, and 5
intense hurricanes), it is quite possible that we may be moving to a regime
of more tropical cyclone activity - but one year does not a trend make.
Some more interesting tidbits about Atlantic tropical cyclones (from
Landsea et al. 1996):

* no significant change in total frequency of tropical storms and hurricanes
over 52 years (1944-1995),

* a strong *DECREASE* in numbers of intense hurricanes,

* no change in the strongest hurricanes observed each year,

* A moderate *DECREASE* in the max intensity reached by all
storms over a season,

* no hurricanes have been observed over the Caribbean Sea during
the years 1990-1994 - the longest period of lack of hurricanes in
the area since 1899. This was followed up by 3 hurricanes in
just one year - 1995 - to affect the region,

* 1991-1994 is the quietest (in terms of frequency of total storms
- 7.5 per year, hurricanes - 3.8, and intense hurricanes - 1.0)
four year period on record, since 1944.

As for the other basins, Black (1992) has identified a moderately
severe bias in the Northwest Pacific reported maximum sustained winds
during the 1940s to the 1960s that makes interpretation of trends
difficult for that region.

Nicholls (1992) has shown that the numbers of tropical cyclones
around Australia (105-165E) has decreased rather dramatically since
the mid-1980s. Some of this reduction is undoubtedly due to having more
El Nino events since that time (i.e. 1986-87, 1991-2, 1993, 1994-95).
However, even taking into account the El Nino effect, there is still a
reduction that is unexplained and may be due to changes in tropical
cyclone monitoring.

The other basins have not been examined for trends, partly because
the data will likely not be trustworthy before the advent of the geo-
stationary satellites in the mid-1960s. IMHO, I would suspect though
that the western portion of the Northeast Pacific, the eastern portion of
the Northwest Pacific, and the South Pacific east of 165E would have a
real upward trend of tropical cyclone occurrences because of the more
frequent El Nino events in the last decade or so (see section G2 for more
information on El Nino effects).


Subject: G5) Why do tropical cyclones occur primarily in the summer and

As described in subject G1), the primary time of year for getting tropical
cyclones is during the summer and autumn: July-October for the Northern
Hemisphere and December-March for the Southern Hemisphere (though there
are differences from basin to basin). The peak in summer/autumn is due to
having all of the necessary ingredients become most favorable during this
time of year: warm ocean waters (at least 26C or 80F), a tropical
atmosphere that can quite easily kick off convection (i.e. thunderstorms),
low vertical shear in the troposphere, and a substantial amount of large-
scale spin available (either through the monsoon trough or easterly waves
- see subject A4)). While one would intuitively expect tropical cyclones
to peak right at the time of maximum solar radiation (late June for the
tropical Northern Hemisphere and late December for the tropical Southern
Hemisphere), it takes several more weeks for the oceans to reach their
warmest temperatures. The atmospheric circulation in the tropics also
reaches its most pronounced (and favorable for tropical cyclones) at the
same time. This time lag of the tropical ocean and atmospheric
circulation is analogous to the daily cycle of surface air temperatures -
they are warmest in mid-afternoon, yet the sun's incident radiation peaks
at noon.


Subject: G6) What determines the movement of tropical cyclones?

Tropical cyclones - to a first approximation - can be thought of as
being steered by the surrounding environmental flow throughout the depth
of the troposphere (from the surface to about 12 km or 8 mi). Dr. Neil
Frank, former director of the U.S. National Hurricane Center, used the
analogy that the movement of hurricanes is like a leaf being steered by
the currents in the stream, except that for a hurricane the stream has no
set boundaries.

In the tropical latitudes (typically equatorward of 20-25 N or S),
tropical cyclones usually move toward the west with a slight poleward
component. This is because there exists an axis of high pressure called
the subtropical ridge that extends east-west poleward of the storm. On
the equatorward side of the subtropical ridge, general easterly winds
prevail. However, if the subtropical ridge is weak - oftentimes due to
a trough in the jet stream - the tropical cyclone may turn poleward and
then recurve back toward the east. On the poleward side of the
subtropical ridge, westerly winds prevail thus steering the tropical
cyclone back to the east. These westerly winds are the same ones that
typically bring extratropical cyclones with their cold and warm fronts
from west to east.

Many times it is difficult to tell whether a trough will allow the
tropical cyclone to recurve back out to sea (for those folks on the
eastern edges of continents) or whether the tropical cyclone will
continue straight ahead and make landfall.

For more non-technical information on the movement of tropical cyclones,
see Pielke's _The Hurricane_. For a more detailed, technical summary
on the controls on tropical cyclone motion, see Elsberry's chapter in
_Global Perspectives on Tropical Cyclones_. Both books are detailed in
Part II of the FAQ.


Subject: G7) Why doesn't the South Atlantic Ocean experience tropical

Though many people might speculate that the sea surface temperatures are
too cold, the primary reasons that the South Atlantic Ocean gets no tropical
cyclones are that the tropospheric (near surface to 200mb) vertical wind
shear is much too strong and there is typically no inter-tropical
convergence zone (ITCZ) over the ocean (Gray 1968). Without an ITCZ to
provide synoptic vorticity and convergence (i.e. large scale spin and
thunderstorm activity) as well as having strong wind shear, it becomes very
difficult to nearly impossible to have genesis of tropical cyclones.

However, in rare occasions it may be possible to have tropical cyclones
form in the South Atlantic. In McAdie and Rappaport (1991), the USA
National Hurricane Center documented the occurrence of a strong tropical
depression/weak tropical storm that formed off the coast of Congo in
mid-April 1991. The storm lasted about five days and drifted toward the
west-southwest into the central South Atlantic. So far, there has not
been a systematic study as to the conditions that accompanied this rare


Subject: G8) Does an active June and July mean the rest of the season will
be busy too?

No. The number of named storms (hurricanes) occurring in June and July
correlates at an insignificant r = +0.13 (+0.02) versus the whole season
activity. Actually, there is a slight _negative_ association of early season
storms (hurricanes) versus late season - August through November - r = -0.28
(-0.35). Thus, early season activity, be it very active or quite calm, has
little bearing on the season as a whole. These correlations are based on
the years 1944-1994.


Subject: G9) Why do hurricanes hit the East coast of the U.S.,
but never the West coast?

Hurricanes form both in the Atlantic basin (i.e. the Atlantic
Ocean, Gulf of Mexico and Caribbean Sea) to the east of the
continental U.S. and in the Northeast Pacific basin to the
west of the U.S. However, the ones in the Northeast Pacific
almost never hit the U.S., while the ones in the Atlantic basin
strike the U.S. mainland just less than twice a year on average.
There are two main reasons. The first is that hurricanes tend
to move toward the west-northwest after they form in the tropical
and subtropical latitudes. In the Atlantic, such a motion often
brings the hurricane into the vicinity of the U.S. east coast. In
the Northeast Pacific, a west-northwest track takes those hurricanes
farther off-shore, well away from the U.S. west coast. In addition
to the general track, a second factor is the difference in water
temperatures along the U.S. east and west coasts. Along the U.S.
east coast, the Gulf Stream provides a source of warm (> 80 F or
26.5 C) waters to help maintain the hurricane. However, along the
U.S. west coast, the ocean temperatures rarely get above the lower
70s, even in the midst of summer. Such relatively cool temperatures
are not energetic enough to sustain a hurricane's strength. So
for the occasional Northeast Pacific hurricane that does track
back toward the U.S. west coast, the cooler waters can quickly
reduce the strength of the storm.


Subject: G10) How much lightning occurs in tropical cyclones?

Surprisingly, not much lightning occurs in the inner core (within
about 100 km or 60 mi) of the tropical cyclone center. Only around a
dozen or less cloud-to-ground strikes per hour occur around the eyewall
of the storm, in strong contrast to an overland mid-latitude mesoscale
convective complex which may be observed to have lightning flash rates
of greater than 1000 per hour (!) maintained for several hours.
Hurricane Andrew's eyewall had less than 10 strikes per hour from the
time it was over the Bahamas until after it made landfall along Louisiana,
with several hours with no cloud-to-ground lightning at all (Molinari et
al. 1994). However, lightning can be more common in the outer cores of
the storms (beyond around 100 km or 60 mi) with flash rates on the order
of 100s per hour.

This lack of inner core lightning is due to the relative weak nature
of the eyewall thunderstorms. Because of the lack of surface heating
over the ocean ocean and the "warm core" nature of the tropical cyclones,
there is less buoyancy available to support the updrafts. Weaker updrafts
lack the super-cooled water (e.g. water with a temperature less than 0 C
or 32 F) that is crucial in charging up a thunderstorm by the interaction
of ice crystals in the presence of liquid water (Black and Hallett 1986).
The more common outer core lightning occurs in conjunction with the
presence of convectively-active rainbands (Samsury and Orville 1994).

One of the exciting possibilities that recent lightning studies
have suggested is that changes in the inner core strikes - though the
number of strikes is usually quite low - may provide a useful forecast
tool for intensification of tropical cyclones. Black (1975) suggested
that bursts of inner core convection which are accompanied by increases
in electrical activity may indicate that the tropical cyclone will soon
commence a deepening in intensity. Analyses of Hurricanes Diana (1984),
Florence (1988) and Andrew (1992), as well as an unnamed tropical storm
in 1987 indicate that this is often true (Lyons and Keen 1994 and Molinari
et al. 1994).


Subject: H1) What is the Dvorak technique and how is it used?

The Dvorak technique is a methodology to get estimates of tropical cyclone
intensity from satellite pictures. Vern Dvorak developed the scheme using
a pattern recognition decision tree in the early 1970s (Dvorak 1975, 1984).
Utilizing the current satellite picture of a tropical cyclone, one matches
the image versus a number of possible pattern types: Curved band Pattern,
Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded
Center Pattern or Central Cold Cover Pattern. If infrared satellite
imagery is available for Eye Patterns (generally the pattern seen for
hurricanes, severe tropical cyclones and typhoons), then the scheme
utilizes the difference between the temperature of the warm eye and the
surrounding cold cloud tops. The larger the difference, the more intense
the tropical cyclone is estimated to be. From this one gets a data
"T-number" and a "Current Intensity (CI) Number". CI numbers have been
calibrated against aircraft measurements of tropical cyclones in the
Northwest Pacific and Atlantic basins. On average, the CI numbers
correspond to the following intensities:

CI Maximum Sustained Central Pressure
Number One Minute Winds (mb)
(kt) (Atlantic) (NW Pacific)
0.0 <25 ---- ----
0.5 25 ---- ----
1.0 25 ---- ----
1.5 25 ---- ----
2.0 30 1009 1000
2.5 35 1005 997
3.0 45 1000 991
3.5 55 994 984
4.0 65 987 976
4.5 77 979 966
5.0 90 970 954
5.5 102 960 941
6.0 115 948 927
6.5 127 935 914
7.0 140 921 898
7.5 155 906 879
8.0 170 890 858

Note that this estimation of both maximum winds and central pressure
assumes that the winds and pressures are always consistent. However,
since the winds are really determined by the pressure gradient, small
tropical cyclones (like the Atlantic's Andrew in 1992, for example)
can have stronger winds for a given central pressure than a larger
tropical cyclone with the same central pressure. Thus caution is urged
in not blindly forcing tropical cyclones to "fit" the above pressure-
wind relationships. (The reason that lower pressures are given to
the Northwest Pacific tropical cyclones in comparison to the higher
pressures of the Atlantic basin tropical cyclones is because of the
difference in the background climatology. The Northwest Pacific basin
has a lower background sea level pressure field. Thus to sustain a
given pressure gradient and thus the winds, the central pressure must
accordingly be smaller in this basin.)

The errors for using the above Dvorak technique in comparison to
aircraft measurements taken in the Northwest Pacific average 10 mb with
a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical
cyclone estimates likely have similar errors. Thus an Atlantic hurricane
that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb)
could in reality be anywhere from winds of 60 to 90 kt and pressures of
989 to 969 mb. These would be typical ranges to be expected; errors
could be worse. However, in the absence of other observations, the
Dvorak technique does at least provide a consistent estimate of what the
true intensity is.

While the Dvorak technique was calibrated for the Atlantic and
Northwest Pacific basin because of the aircraft reconnaissance data
ground truth, the technique has also been quite useful in other
basins that have limited observational platforms. However, at some
point it would be preferable to re-derive the Dvorak technique to
calibrate tropical cyclones with available data in the other basins.

Lastly, while the Dvorak technique is primarily designed to provide
estimates of the current intensity of the storm, a 24 h forecast of the
intensity can be obtained also by extrapolating the trend of the
CI number. Whether this methodology provides skillful forecasts is


Subject: H2) Who are the "Hurricane Hunters" and what are they looking for?

(Contributed by Neal Dorst.)

In the Atlantic basin (Atlantic Ocean, Gulf of Mexico, and Caribbean Sea)
hurricane reconnaissance is carried out by two government agencies, the
U.S. Air Force Reserves' 53rd Weather Reconnaissance Squadron and NOAA's
Aircraft Operations Center. The U.S. Navy stopped flying hurricanes in

The 53rd WRS is based at Keesler AFB in Mississippi and maintains
a fleet of ten WC-130 planes. These cargo airframes have been modified to
carry weather instruments to measure wind, pressure, temperature and dew
point as well as drop instrumented sondes and make other observations.

AOC is presently based at MacDill AFB in Tampa, Florida and among
its fleet of planes has two P-3 Orions, originally made as Navy sub hunters,
but modified to include three radars as well as a suite of meteorological
instruments and dropsonde capability. Starting in 1996 AOC has added to
its fleet a Gulfstream IV jet that will be able to make hurricane
observations from much higher altitudes (up to 45,000 feet). It has a
suite of instruments similar to those on the P-3s.

The USAF planes are the workhorses of the hurricane hunting effort.
They are often deployed to a forward base, such as Antigua, and carry out
most of the reconnaissance of developing waves and depressions. Their
mission in these situations is to look for signs of a closed circulation
and any strengthening or organizing that the storm might be showing.
This information is relayed by radio to the National Hurricane Center for
the hurricane specialists to evaluate.

The NOAA planes are more highly instrumented and are generally
reserved for when developed hurricanes are threatening landfall, especially
landfall on U.S. territory. They are also used to conduct scientific
research on storms.

The planes carry between six to fifteen people, both the flight
crew and the meteorologists. Flight crews consist of a pilot, co-pilot,
flight engineer, navigator, and electrical technicians. The weather
crew might consist of a flight meteorologist, lead project scientist,
cloud physicist, radar specialist, and dropsonde operators.

The primary purpose of reconnaissance is to track the center
of circulation, these are the co-ordinates that the National Hurricane
Center issues, and to measure the maximum winds. But the crews are
also evaluating the storm's size, structure, and development and this
information is also relayed to NHC via radio and satellite link. Most of
this data, which is critical in determining the hurricane's threat, cannot
be obtained from satellite.


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