Atmospheric pressure

Atmospheric pressure is the pressure caused by the weight of air above any area in the Earth's atmosphere. Standard atmospheric pressure (atm) is discussed in the next section.

Air masses are affected by the general atmospheric pressure within the mass, creating areas of high and low pressure.

As elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height according to the following relationship (only a first-order approximation):

,

where P is the pressure in pascals and H the height in metres. This shows that the pressure at an altitude of 31 km is about 1% of that at sea level. [Source: US Department of Defense Military Standard 810E]

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. A 1 m² column of air would weigh about 10 tonnes.

Standard atmospheric pressure

Standard atmospheric pressure or "the standard atmosphere" (1 atm) is defined as 101,325 pascals. (see also Standard temperature and pressure)

This can also be stated as:

• 29.92 inches or 760 mm of mercury (760 torr)
• 1013.25 millibars (mb)
• 14.7 psia or 0 psig

This "standard pressure" is a purely arbitrary representative value for pressure at sea level, and real atmospheric pressures vary from place to place and moment to moment everywhere in the world.

In a low atmospheric pressure system the atmospheric pressure of the air mass is lower than that of the surrounding air. Low atmospheric pressure systems are symbolized by an L on a weather map and are associated with areas of storminess and precipitation. Wind movement is cyclonic around a low pressure system and cold fronts and warm fronts are generally connected to them.

In a high atmospheric pressure system the atmospheric pressure of the air mass is higher than that of the surrounding air. High atmospheric pressure systems are symbolized by an H on a weather map and are associated with areas of clear weather.

It is possible to demonstrate atmospheric pressure in a classroom or home environment using the crushing can experiment. See Atmospheric pressure demo

External Links

• An Exercise in Air Pressure. A lesson plan that deals with understanding atmospheric pressure.