Geoscience Reference
In-Depth Information
air pressure decreases with increasing altitude because most air
molecules are held close to the surface by gravity. As a result,
the density of air molecules is greater closer to the Earth's sur-
face, which means that air pressure is relatively high in that
part of the atmosphere. With increased altitude, the density of
air molecules becomes progressively less, resulting in progres-
sively lower air pressure.
In addition to the impact of gravity on the molecular den-
sity of air, atmospheric pressure is also strongly influenced by
air temperature. The most obvious way that air temperature
influences atmospheric pressure occurs when air close to the
Earth's surface is warmed a great deal. Such warming causes
air molecules to scatter and density thus decreases, resulting
in relatively low atmospheric pressure. Consider the analogy
of a hot air balloon. When heat is added to the air within the
balloon, this causes the air to expand and lift within the rela-
tively cooler (more dense) air that surrounds it. Low atmo-
spheric pressure also results when air is forced to rise vigor-
ously. Very cold surface air is usually associated with high
atmospheric pressure because cold air is dense and thus sinks.
In some instances, air from the upper atmosphere descends
vigorously toward the Earth's surface, which also results in
high pressure.
Vacuum
Glass tube
Mercury
Height of
mercury
column
(76 cm or 30 in.)
Atmospheric
pressure
Dish of
mercury
Measuring and Mapping Air Pressure
Air pressure is often measured in units called millibars (mb)
with an instrument called a barometer (Figure 6.3). A common
type of barometer consists of a long glass tube, closed at one
end, which is filled with a liquid (usually mercury) and inverted
into a dish containing the same liquid. The liquid in the tube
drops down slightly, leaving a vacuum at the closed end of the
tube. When the liquid in the tube comes to rest, the force due to
atmospheric pressure, pressing down on the liquid in the dish,
exactly balances the weight of the column of liquid. The tube
can then be calibrated to measure atmospheric pressure in terms
Figure 6.3 Measurement of atmospheric
pressure.
The pressure of the atmosphere
is measured by the height of a column of mer-
cury that can be supported by that pressure.
of inches or millimeters of mercury, which can be converted to
millibars or any other pressure unit.
You can observe how air pressure changes with altitude
by looking at three separate locations at different elevations
(Figure 6.4). At sea level, for example, the average pressure
VISUAL CONCEPT CHECK
6.1
Mount McKinley is the tallest mountain in
North America, rising 6194 m (20,320 ft) in
Alaska's Denali National Park. How do you
suppose air pressure changes from the base
of the mountain to the top—does it increase
or decrease? Why does this change occur?
