Atmospheric pressure
Atmospheric pressure is the amount of force exerted over a surface area, caused by the weight of air molecules above it.
Air masses are affected by the general atmospheric pressure within the mass, creating areas of high and low pressure.
Low atmospheric pressure system- where the atmospheric pressure of the air mass is lower than the surrounding air. Low atmospheric pressure systems are symbolized by an L on a weather map and are associated with areas of stormyness and precipitation. Wind movement is cyclonic around a low pressure system and cold fronts and warm fronts are generally connected to them.
High atmospheric pressure systems- where the atmospheric pressure of the air mass is higher than the surrounding air. High atmospheric pressure systems are symbolized by an H on a weather map and are associated with areas of clear weather.
As elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 lb/in2. The standard value for atmospheric pressure at sea level is:
29.92 inches or 760 mm of mercury
1013.25 millibars (mb) or 101,325 pascals (pa).
See also: atmosphere, standard pressure, weather front