Air Pressure and Smoke

It is helpful to understand air pressure because storms and stagnant air conditions are described in terms of low pressure and high pressure, respectively. Lines of constant pressure are used to illustrate the state of the atmosphere on weather maps, and pressure influences the expansion and contraction of smoke parcels as they travel through the atmosphere. Air pressure is the force per unit area exerted by the weight of the atmosphere above a point on or above the earth’s surface. More simply it can be thought of as the weight of an overlying column of air. Air pressure is greatest near the ground, where the overlying column of air extends the full height of the atmosphere. Pressure decreases with increasing altitude as the distance to the top of the atmosphere shortens. In a standard atmosphere, which represents the horizontal and time-averaged structure of the atmosphere as a function of height only, pressure decreases approximately exponentially with height. With 1,013 millibars (mb) being the standard atmospheric pressure at sea level, the average height of the 850 mb pressure level typically occurs at about 5,000 feet (~1,500 m), the 700 mb pressure level typically occurs at about 10,000 feet (~3,000 m), and the 500 mb height averages around 20,000 feet (~6,000 m). In the lowest part of the atmosphere (less than about 8,000 feet) pressure decreases by approximately 30 mb per 1000 feet. These are useful values to remember when analyzing meteorological data and maps for smoke management. Actual pressure is nearly always within about 30% of standard pressure.

See: Fire Weather for more information.