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).