Sea-Level Pressure (SLP) in METAR

[Geoff Peck]

On Jul 1, 1:44pm, Jon Morrow wrote:
> Subject: Sea-Level Pressure (SLP) in METAR
> Can someone please explain the SLP field in a METAR? Take a look at
> this METAR and its DUATS-provided translation:
>
> METAR KDEN 011253Z 24006KT 10SM FEW250 18/11 A3022 RMK AO2 SLP151
>
> T01780111 $
>
> Denver CO [DEN] hourly observation on the 1st at 8:53am EDT (1253Z),
> wind 240 degrees at 6 knots, visibility 10 miles, 25,000 feet few,
> temperature 18C (64F), dewpoint 11C (52F), altimeter 30.22, automated
> station with precipitation discriminator, sea level pressure 29.98" Hg
> (1015.1 hPa), temperature 17.8C (64.0F), dewpoint 11.1C (52.0F),
> maintenance required.
>
>
> Here's my question: I thought SLP was the field pressure/altimeter
> setting corrected for field elevation. If the altimeter setting is
> 30.22", why is the SLP 29.98" (1015.1 hPa)? Shouldn't the pressure at
> sea level be *higher* than the pressure at the mile-high field?
>
> -jon
>
>
>-- End of excerpt from Jon Morrow

The following, from the Federal Meteorological Handbook No. 1, should
answer your question:

11.3 Pressure Parameters

a. Station pressure. The atmospheric pressure at the designated
station elevation.

b. Altimeter setting. The pressure value to which an aircraft
altimeter scale is set so that it will indicate the altitude above
mean sea level of an aircraft on the ground at the location for which
the value was determined.

c. Sea-level pressure. A pressure value obtained by the theoretical
reduction of barometric pressure to sea level. Where the Earth's
surface is above sea level, it is assumed that the atmosphere
extends to sea level below the station and that the properties of
that hypothetical atmosphere are related to conditions observed at
the station.

11.4 Pressure Observing Standards

11.4.1 Barometer Comparisons.

Each agency shall establish an agency standard barometer traceable to
the standard of the National Institute of Standards and Technology.
Each agency shall also establish a system of routine barometer
comparisons to determine corrections required to keep the station's
pressure sensors within the required accuracy (see Appendix C).

11.4.2 Atmospheric Pressure.

The various pressure parameters shall be determined from the barometric
pressure after appropriate corrections are applied. The method used
shall depend on the type of sensor and the available computational
aids. These aids may be systems that result in a direct readout of the
desired parameter, pressure reduction calculators, or tables.
Designated stations may use constants to convert measured pressure to
the desired pressure parameter.

11.4.3 Station Pressure.

Station pressure shall be determined by adjusting the corrected
barometric pressure to compensate for the difference between the height
of the barometer and the designated station elevation.

11.4.4 Sea-Level Pressure.

At designated stations, sea-level pressure shall be computed by
adjusting the station pressure to compensate for the difference between
the station elevation and sea-level. This adjustment shall be based on
the station elevation and the 12-hour mean temperature at the station.
The 12-hour mean temperature shall be the average of the present
ambient temperature and the ambient temperature 12 hours ago.

Stations within 50 feet of sea-level may be authorized by their agency
to use a constant value to adjust station pressure to sea-level
pressure. Otherwise, stations shall use reduction ratios provided by
their responsible agency to calculate sea-level pressure.

11.4.5 Altimeter Setting.

The altimeter setting shall be determined either directly from an
altimeter setting indicator or computed from the station pressure by
applying a correction for the difference between the station elevation
and field elevation in the standard atmosphere. Where this difference
is 30 feet or less, agencies may authorize the use of a constant
correction.

--Geoff

[David Beal]

ge...@siamese.peck.com (Geoff Peck) wrote:
>On Jul 1, 1:44pm, Jon Morrow wrote:
>> Subject: Sea-Level Pressure (SLP) in METAR
>> Can someone please explain the SLP field in a METAR? Take a look at
>> this METAR and its DUATS-provided translation:
>>
>> METAR KDEN 011253Z 24006KT 10SM FEW250 18/11 A3022 RMK AO2 SLP151
>>
>> T01780111 $
>>
>> Denver CO [DEN] hourly observation on the 1st at 8:53am EDT (1253Z),
>> wind 240 degrees at 6 knots, visibility 10 miles, 25,000 feet few,
>> temperature 18C (64F), dewpoint 11C (52F), altimeter 30.22, automated
>> station with precipitation discriminator, sea level pressure 29.98" Hg
>> (1015.1 hPa), temperature 17.8C (64.0F), dewpoint 11.1C (52.0F),
>> maintenance required.
>>
>>
>> Here's my question: I thought SLP was the field pressure/altimeter
>> setting corrected for field elevation. If the altimeter setting is
>> 30.22", why is the SLP 29.98" (1015.1 hPa)? Shouldn't the pressure at
>> sea level be *higher* than the pressure at the mile-high field?
>>
>> -jon
>>
>>
>>-- End of excerpt from Jon Morrow
>

Geoff's answer (below) is much more complete, but the short version is
that the altimeter setting is already corrected to sea-level. So the
question becomes "why is the altimeter setting different from the
sea-level pressure?" Both are calculated by mentally inserting an
atmosphere below the station and measuring the pressure at the sea-level
base of that atmosphere. The difference between sea-level pressure
and altimeter setting is that in calculating altimeter setting, the
imaginary atmosphere is standard (as defined by the ICAO). In
calculating sea-level pressure, the actual temperature at the station
over the last 12 hours is used.

- Dave Beal
PP-ASEL & AGI
Boulder, CO