Don may have a more accurate answer than I do, but as I understand it:
Atmospheric pressure varies with weather and altitude. NWS never reports the
actual measured station pressure. They report two different reductions to
pressure at sea level for different uses. One reduction converts station
pressure to altimeter setting used in aviation and this is in inches of mercury
in the US. A different algorithm which includes average temperature as well as
elevation is sea level pressure in millibars.
In METAR (a coded weather observation for aviation) altimeter setting is
preceded with A (if in Hg are used as in US) and is in centi-inches (no
decimal), while sea level pressure is in "remarks" preceded by SLP and the
leading 9 or 10 omitted. When converted to a common unit, these are usually
not much different at low elevations, but may be quite noticeable in Denver for
example.
The National Hurricane Center seems to only use millibars.
From: "[email protected]" <[email protected]>
To: USMA List Server <[email protected]>
Sent: Sunday, October 23, 2016 10:38 AM
Subject: [USMA 391] Atmospheric Pressure
This may be a question that Dan Hillger can answer. I notice that most of
the meteorological reporting on television uses hectopaschals (millibars)
for atmospheric pressure, as during hurricane reporting. Only The Weather
Channel seems to use inches of mercury. What is the practice of the
National Weather Service? Is TWC actually translating the NWS values from
hectopaschals?
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