Geoscience Reference
In-Depth Information
Northern Hemisphere
Southern Hemisphere
(a) Pressure gradient force:
Out
(a) Pressure gradient force:
Out
In
In
High
Low
High
Low
(b) Coriolis force:
Clockwise
(b) Coriolis force:
Counterclockwise
Counterclockwise
Clockwise
High
Low
High
Low
(c) Combined with frictional force:
Spiral out
(c) Combined with frictional force:
Spiral out
Spiral in
Spiral in
High
Low
High
Low
Figure 6.14 The factors that influence large-scale atmospheric circulation.
(a) The pressure gradient force
causes air to flow perpendicular to isobars. (b) The Coriolis force causes movement of air that is parallel to isobars.
(c) In combination with the pressure gradient and Coriolis forces, frictional forces result in winds that flow some-
where intermediate between 0° and 90° of isobars.
angle relative to isobars at ground level. At higher altitudes,
however, winds follow a geostrophic course that is parallel
to isobars.
In an effort to integrate all the major factors that influ-
ence atmospheric circulatory processes, let's now turn to
Figure 6.14 to review. Recall that the pressure gradient force
causes winds to flow at right angles to isobars in the direc-
tion of the lower pressure (Figure 6.14a). If the influence of
the Coriolis force is also included (Figure 6.14b), winds then
flow parallel to the isobars as geostrophic winds. This pro-
cess occurs because of the balancing effect that the Coriolis
force and pressure gradient force have on one another. In other
words, the Coriolis force keeps wind from flowing across iso-
bars, whereas the pressure gradient force stops winds from
curving up the pressure slope. Finally, when the force of fric-
tion is taken into account (Figure 6.14c), the end result is
winds that follow an intermediate course relative to the iso-
bars, somewhere between perpendicular (due to the pressure
gradient force) and parallel (due to the Coriolis force) to those
lines of equal atmospheric pressure.
KEY CONCEPTS TO REMEMBER ABOUT
THE VARIABLES THAT INFLUENCE
LARGE-SCALE WINDS
1.
Air generally flows from high pressure to low pressure.
2.
Large-scale atmospheric circulation is caused by
the unequal heating of the tropics and poles. The
process of airflow begins at the Equator because of
convection.
3.
Isobars are isolines that connect points of equal atmo-
spheric pressure.
4.
The speed of airflow is determined by the pressure gra-
dient force. The steeper the gradient, the stronger the
winds.
5.
The Coriolis force causes air moving toward the Equator
to be deflected to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
