Geoscience Reference
In-Depth Information
6.
Features on the Earth's surface, such as mountains,
forests, and buildings, create a frictional force that acts
opposite to the wind's direction.
Cold air falls, flows
to lower pressure
H
Advection
7.
The combined effect of the pressure gradient force,
Coriolis force, and frictional force causes a spiral motion
of air in both low- and high-pressure systems.
North Pole
Global Pressure and Atmospheric
Circulation
Warm air
rises, flows
to poles
Equator
L
L
In the preceding sections, we looked at the fundamental charac-
teristics of air pressure systems and the variables that influence
the process of airflow in the atmosphere. With these concepts in
mind, let's now examine the general circulation of air around
the globe. This discussion will refer to the flow of air both in
the upper part of the atmosphere and at the surface. The differ-
ence between the flow in these different parts of the atmosphere
is often noticeable on the ground. If you want to see this dif-
ference yourself sometime, look for a partly cloudy day where
two distinct layers of clouds occur, one low and another high.
When these conditions exist, the clouds in the upper part of the
atmosphere are often moving in a slightly different direction or
speed from those closer to the surface.
As discussed previously, the primary driver of global circula-
tion is the unequal heating of the tropics and the poles. Because of
this energy imbalance, the atmosphere works to balance the system
through the process of airflow. If the Earth's surface had a uniform
character (i.e., no distinction between continents and oceans), did
not rotate, and its axis were not tilted, the circulatory system would
be very easy to understand. In this simplistic scenario, which is il-
lustrated in Figure 6.15, low pressure would occur at the Equator
because the air is very warm and high pressure would occur at the
poles because the air is very cold. Very simply, air would rise away
from the surface at the Equator, within a low-pressure system,
and would then flow toward the poles in the upper atmosphere by
advection. Once it reached the poles, the air would descend toward
the surface in a high-pressure system where it would then diverge
and flow back toward the Equator by advection.
As you know, however, Earth does not have a uniform sur-
face, it does rotate, and it is tilted with respect to the plane of
the ecliptic. Given these factors, the global circulatory system is
more complex than the simplified model presented in Figure 6.15.
Figure 6.16 illustrates the general global circulation model as it
truly functions, showing the major wind systems on Earth. Let's
look at the model in more detail in order to explain how these
wind patterns occur. We will begin at the Equator and proceed
systematically toward higher latitudes, focusing on the directions
of winds at both the surface and the upper part of the troposphere.
South Pole
H
Figure 6.15 Global circulation on a nonrotating, untilted
Earth with a consistent surface composition.
In this
model, a few simple convection loops would dominate the sys-
tem, with rising air (low pressure) at the Equator, descending
air (high pressure) at the poles, and horizontal flow of air by
advection in between.
atmospheric circulatory processes are set in motion in this
part of Earth because of the high Sun angles and extremely
warm temperatures there. Although tropical air eventually
finds its way into the midlatitudes, it is useful to think of
the tropical circulatory system as being a convection loop,
with air flowing between the Equator and about 30° N and S
latitude. Each of these loops is referred to as a Hadley cell. In
that context, the discussion of tropical circulation focuses on
the Intertropical Convergence Zone and the Subtropical High
Pressure System.
Intertropical Convergence Zone
Tropical circulatory
processes begin at the Equator, where air is warmed due to
year-round receipt of direct sunlight. This warming helps
create a zone of low pressure, called the
equatorial trough
,
because warm air is less dense and more buoyant. As a re-
sult of the rising air mass, air from higher tropical latitudes in
both hemispheres flows toward the equatorial trough along the
surface by advection, converging in a narrow band known as
the
Intertropical Convergence Zone
or
ITCZ
(Figure 6.16).
Equatorial trough
Core of low-pressure zone associated with
the Intertropical Convergence Zone.
Tropical Circulation
The logical place to begin discussion of global atmospheric
circulation is the tropics. As noted previously in this chapter,
Intertropical Convergence Zone (ITCZ)
Band of low pres-
sure, calm winds, and clouds in tropical latitudes where air
converges from the Southern and Northern Hemispheres.
