Geoscience Reference
In-Depth Information
3.
Low-pressure systems are usually associated with
clouds and precipitation.
4.
High-pressure systems are called anticyclones and
consist of rotating air masses that descend toward the
surface. In the Northern Hemisphere, these systems
rotate clockwise, whereas they rotate counterclock-
wise in the Southern Hemisphere.
5.
High-pressure systems are usually associated with
clear skies.
6.
On weather maps, isobars indicate areas of equal air
pressure.
7.
Winds low from high to low pressure.
The Direction of Airflow
Now that we have discussed the fundamentals of air pres-
sure, let's look more closely at the concept of airflow and the
factors that govern its movement. We will examine several
factors, including unequal heating of land surfaces, the pres-
sure gradient force, the Coriolis force, and various frictional
forces.
Unequal Heating of Land Surfaces
The ultimate cause for all wind patterns on Earth is the unequal
heating of surfaces that results from variations in the amount of
solar radiation received between latitudes. This spatial varia-
tion in surface air temperature means that air density (and thus
pressure) differs from place to place. At a fundamental level,
surface air flows from areas of high pressure to low pressure be-
cause the atmosphere works to balance the difference between
the two areas.
The best example of unequal heating on Earth is the dif-
ference that exists between the tropics and the poles. Recall
from Chapters 4 and 5 that the tropics are much warmer than
the poles because the equatorial regions receive the most direct
insolation throughout the year. If no mechanism existed to bal-
ance this difference, then the tropical and polar regions would
become excessively hot and cold, respectively.
Instead, this process of unequal heating causes motion of
the atmosphere through the process of convection. Recall from
Chapter 4 that convection is the vertical mixing of fluid material
(in this case, air) due to differences in temperature. In contrast,
remember that the term
advection
refers to the horizontal move-
ment of air or water. Figure 6.9 shows how the processes of con-
vection and advection relate to air movement. On a global scale,
air heated near the Equator results in low air pressure as air rises
within the upward-moving part of the convection process. Sub-
sequently, it travels to higher latitudes in both hemispheres by
advection, where the air cools and descends as a high-pressure
system at some point in the downward part of the convection
process. The combined processes of convection and advection
represent the first stage of atmospheric circulation on Earth and
Figure 6.8 Atmospheric pressure systems in Europe.
The
low is indicated by the clouds that cover Great Britain and Ire-
land, whereas the high is the clear sky to the east over France,
Spain, and Germany.
such as the large region in eastern Europe, are places dominat-
ed by high pressure. On the other hand, the cloudy skies over
the British Isles reflect the influence of a low-pressure system.
Remember that air pressure is low in the center of a cyclone and
high in the middle of an anticyclone. Because air rises in the cen-
ter of a low, the resulting lower pressure pulls air into the area
from the adjacent high-pressure system. We feel this exchange
of air as wind. Look at Figure 6.5 again to visualize how this
process works and be sure you understand it thoroughly.
KEY CONCEPTS TO REMEMBER ABOUT
ATMOSPHERIC PRESSURE SYSTEMS
1.
Air pressure refers to the weight of air distributed on the
surface of Earth. It generally decreases with increasing
altitude.
2.
Low-pressure systems are called cyclones and con-
sist of rotating air masses that lift air from the surface.
In the Northern Hemisphere, these systems rotate
counterclockwise, whereas they rotate clockwise in
the Southern Hemisphere.
