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.
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