Mean sealevel pressure is derived from the standard surface pressure reduction
for surface observations, employed by the National Weather Service:
1)

Surface pressure is provided by the NCEP/NCAR 40Year Reanalysis CDROM

2)

The mean of the 2 meter temperature at the observation time and 12 hours
earlier is determined (to reduce any diurnal effects)

3)

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 40Year Reanalysis CDROM.

4)

The hypsometric equation was used in the following form to solve for the
mean sealevel pressure:
MSLP=P_{sfc}exp^{g*dz/(R*T)
}


where

P_{sfc} is the surface pressure


g is gravity


dz is the distance in meters from the surface to mean sealevel


R is the dry gas constant


T is the mean layer temperature from the surface to sealevel


Note that there is no plateau correction and that in practice, a
pressure reduction value is used by the National Weather Service.
Units: millibars
