Atmospheric Pressure
Pressure is a measure of the
total weight of the atmosphere above the level at which it is measured.
The conventional unit of
pressure used by meteorologists is the millibar. Average sealevel pressure is about 1000.0 mb. But, typically, pressure can vary from
about 960.0 mb to about 1050.0 mb.
(Sealevel pressure in other units:
14 lbs/in2, 30.00 inches of mercury)
Pressure observations are
coded and plotted around the station circle as described in class.
Although pressure at sealevel
does not vary much from 1000 mb, these variations on a level surface account
for wind.
Rule of Thumb
On a given level, in the absence of other effects, air locally tends to move from regions with higher values of pressure to regions with lower values of pressure and with a speed in proportion to how great the local difference in pressure is (how close the isobars are).