Mean Sea-Level Pressure

Mean sea-level 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 40-Year Reanalysis CD-ROM
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 40-Year Reanalysis CD-ROM.
4) The hypsometric equation was used in the following form to solve for the mean sea-level pressure:
where Psfc is the surface pressure
g is gravity
dz is the distance in meters from the surface to mean sea-level
R is the dry gas constant
T is the mean layer temperature from the surface to sea-level

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

Units: millibars