Need definition of cloud/thunderstorm "levels"
Bill McHugh
"levels", and have been with experienced pilots on the ground who look up at
a cloud formation and describe it as a "level 3", "level 4", etc. However,
I have not been able to find a written reference that defines these levels
(I've checked FAR/AIM, Gleim's "Aviation Weather & Weather Services", etc.).
I assume they have something to do with cloud tops, but I'm not sure. Can
anyone out there help? A referral to written material would be especially
appreciated. (As you can probably tell, I'm not instrument rated...yet).
Brad Crosier
thunderstorm from the ground, unless they just got a briefing or looked at a
radar chart. As the AIM puts it, "There is no useful correlation between the
external visual appearance of thunderstorms and the severity or amount of
turbulence or hail within them (AIM 7-1-25)."
Radar intensity levels are based on measurements taken by the National Weather
Service using their ground based radars. They represent the radar reflectivity
in Decibels (dBZ), which roughly translates to precipitation intensity. The
National Weather Service term for them is VIP Levels (Video Integrator
Processor). They are described in the AIM under the pilot/controller glossary.
A bit more detailed explanation gleaned from a couple of sources follows (NTSB
report on Delta 191 from Archie Trammell's seminar handbook, as well as the
course packet from Atmospheric Science 315 at Purdue University, taught by Dr.
T. Q. Carney. Additionally the AIM references AC0045 which I do not have a copy
of). For good measure, I also threw in the equivalent colors shown on an
airborne radar, again courtesy of Mr. Trammell's booklet.
Not to insult your intelligence, but just to define a couple of terms:
Convective - relating to the vertical movement of air; read - thunderstorm
related
Threshold (Thres.) values - the minimum reflectivity necessary to indicate the
given VIP level
VIP Airborne Radar Echo Estimated Convective Rainfall Thres.
Level Display Intensity Precipitation Rate (Inches/hour) Values
--------------------------------------------------------------------------------------
1 Green Weak Light 0.05 - < 0.2 >30dBZ
2 Yellow Moderate Moderate 0.20 - 1.1 30 dBZ
3 Red Strong Heavy 1.10 - 2.2 41 dBZ
4 Red Very Strong Very Heavy 2.20 - 4.5 46 dBZ
5 Magenta* Intense Intense 4.50 - 7.1 50 dBZ
6 Magenta* Extreme Extreme > 7.1 57 dBZ
* Magenta display not available on all airborne radars, models not so equipped
display these levels as red.
The VIP levels really don't correlate with vertical thunderstorm development.
The VIP level at a particular altitude can tell you something about a storm, but
you can have a relatively low topped storm and still have a highly reflective
storm. Also you have to take radar returns and storm tops in context of where
you are looking at the thunderstorm geographically, i.e.; a 40,000 foot tall VIP
level 4 storm in Florida in the summer and a 40,000' VIP level 4 storm in North
Dakota are two very different animals. Certainly no absolute guarantees, but
the ND storm is much more likely to be severe (at that moment, not considering
what the FL storm might develop into, which is certainly a factor). Would I
care to fly into either of them? No. But I'd plan my avoidance strategy
differently for one vs. the other.
One other item semi worthy of mention is cloud tops vs. radar tops. They are
NOT the same. Radar (except for a few special units) does not detect clouds,
only wet precipitation. Cloud tops are usually several thousand feet higher
than detectable radar tops.
Lastly, a plug for the fellow whose resources I repeatedly refer to and use:
Archie Trammell and his airborne weather radar seminar (AJT Inc.). Certainly if
you do or someday will use radar, and probably even if you don't, it is one of
the best courses I've ever been able to attend on thunderstorms and their
avoidance. I've been to it twice and will go as many more times as I can. I am
in no way affiliated with him, just a very satisfied customer. If you have the
chance to attend - do.
Probably way more information than anyone wanted to know, but there it is . . .
BTC
Tri-guy
wo...@home.com
dry, but it is excellent information. One of the things that I *think*
I got from his course is that the turbulence levels associated with
thunderstorms do not increase in intensity as does the radar
return...in other words, if you've got four levels of shading on your
display, the outermost level (lowest intensity precipitation) does not
necessarily have lower levels of turbulence. You can experience the
most extreme levels of turbulence anywhere within the radar return
area. I'm not explainin this very well...but basically, if you think
you can avoid the worst turbulence by just staying out of the heavy
contoured level 4 areas on your radar, you might be most unpleasantly
surprised.
Interesting stuff though.
Brad
Mike Rapoport
The "level" of a thunderstorm is assigned by a Video Interogator Processor
(VIP levels) and is a function of the reflectivity of the storm. It is NOT
POSSIBLE to determine the level of the storm visually. To presume to be
able to do so demonstrates a complete lack of understanding of the problem
(hardly a unique phenomenon among pilots). The levels are not always
related to height though there is usually some correlation. The other thing
to remember is that the guidence on CB severity is based on US midwest data.
In more arid regions CBs will be more severe at the same reflectivity.
Conversely in the tropics CBs will be less severe at the same reflectivity
than in the US midwest.
There is a reference to VIP levels in the AIM somewhere.
Mike
MU-2
Brad Crosier
wo...@home.com wrote:
> I've watched Archie's video course, and it is excellent. It is very
> dry, but it is excellent information. One of the things that I *think*
> I got from his course is that the turbulence levels associated with
> thunderstorms do not increase in intensity as does the radar
> return...in other words, if you've got four levels of shading on your
> display, the outermost level (lowest intensity precipitation) does not
> necessarily have lower levels of turbulence. You can experience the
> most extreme levels of turbulence anywhere within the radar return
> area. I'm not explainin this very well...but basically, if you think
> you can avoid the worst turbulence by just staying out of the heavy
> contoured level 4 areas on your radar, you might be most unpleasantly
> surprised.
You've got it exactly right. More than that, turbulence and hail can exist clear of ANY
radar return. Dry hail is pretty much invisible to radar, and can be thrown well clear of
a thunderstorm.
Bill McHugh
Amazing that you can occasionally get something truly useful from a news
group!
Bill