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Mean sea-level pressure is derived from the standard surface pressure reduction
for surface observations, employed by the National Weather Service:
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1)
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Surface pressure is provided by the NCEP/NCAR 40-Year Reanalysis CD-ROM
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2)
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The mean of the 2 meter temperature at the observation time and 12 hours
earlier is determined (to reduce any diurnal effects)
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3)
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The mean temperature from the surface to sea level is calculated using
the 12 hour averaged surface temperature, and an assumed lapse rate of
6.5 K/1000m. The surface elevation is given by a fixed elevation file
provided by the NCEP/NCAR 40-Year Reanalysis CD-ROM.
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4)
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The hypsometric equation was used in the following form to solve for the
mean sea-level pressure:
MSLP=Psfcexpg*dz/(R*T)
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where
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Psfc is the surface pressure
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g is gravity
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dz is the distance in meters from the surface to mean sea-level
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R is the dry gas constant
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T is the mean layer temperature from the surface to sea-level
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Note that there is no plateau correction and that in practice, a
pressure reduction value is used by the National Weather Service.
Units: millibars
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