Definition of air pressure

The weight of the air above a given level. This

weight produces a force in all directions caused by constantly

moving air molecules bumping into each other and objects in

the atmosphere. The air molecules in the atmosphere are constantly

moving and bumping into each other with each air

molecule averaging a remarkable 10 billion collisions per second

with other air molecules near the Earth’s surface. The

density of air molecules is highest near the surface, decreases

rapidly upward in the lower 62 miles (100 km) of the atmosphere,

then decreases slowly upward to above 310 miles (500

km). Air molecules are pulled toward the Earth by gravity and

are therefore more abundant closer to the surface. Pressure,

including air pressure, is measured as the force divided by the

area over which it acts. The air pressure is greatest near the

Earth’s surface and decreases with height, because there is a

greater number of air molecules near the Earth’s surface (the

air pressure represents the sum of the total mass of air above a

certain point). A one-square-inch column of air extending

from sea level to the top of the atmosphere weighs about 14.7

pounds. The typical air pressure at sea level is therefore 14.7

pounds per square inch. It is commonly measured in units of

millibars (mb) or hectopascals (hPa), and also in inches of

mercury. Standard air pressure in these units equals 1,013.25

mb, 1,013.25 hPa, and 29.92 in of mercury. Air pressure is

equal in all directions, unlike some pressures (such as a weight

on one’s head) that act in one direction. This explains why

objects and people are not crushed or deformed by the pressure

of the overlying atmosphere.

Air pressure also changes in response to temperature and

density, as expressed by the gas law:

Pressure = temperature × density × constant (gas constant,

equal to 2.87 × 106 erg/g K).

From this gas law, it is apparent that at the same temperature,

air at a higher pressure is denser than air at a lower

pressure. Therefore, high-pressure regions of the atmosphere

are characterized by denser air, with more molecules of air

than areas of low pressure. These pressure changes are caused

by wind that moves air molecules into and out of a region.

When more air molecules move into an area than move out,

the area is called an area of net convergence. Conversely, in

areas of low pressure, more air molecules are moving out than

in, and the area is one of divergence. If the air density is constant

and the temperature changes, the gas law states that at a

given atmospheric level, as the temperature increases, the air

pressure decreases. Using these relationships, if either the temperature

or pressure is known, the other can be calculated.

If the air above a location is heated, it will expand and

rise; if air is cooled, it will contract, become denser, and sink

closer to the surface. Therefore, the air pressure decreases

rapidly with height in the cold column of air because the

molecules are packed closely to the surface. In the warm column

of air, the air pressure will be higher at any height than

in the cold column of air, because the air has expanded and

more of the original air molecules are above the specific

height than in the cold column. Therefore, warm air masses

at height are generally associated with high-pressure systems,

whereas cold air aloft is generally associated with low pressure.

Heating and cooling of air above a location causes the

air pressure to change in that location, causing lateral variation

in air pressure across a region. Air will flow from highpressure

areas to low-pressure areas, forming winds.

The daily heating and cooling of air masses by the Sun

can in some situations cause the opposite effect, if not overwhelmed

by effects of the heating and cooling of the upper

atmosphere. Over large continental areas, such as the southwestern

United States, the daily heating and cooling cycle is

associated with air pressure fall and rise, as expected from

the gas law. As the temperature rises in these locations the

pressure decreases, then increases again in the night when the

temperature falls. Air must flow in and out of a given vertical

column on a diurnal basis for these pressure changes to occur,

as opposed to having the column rise and fall in response to

the temperature changes.

See also ATMOSPHERE.

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