Geoscience Reference
In-Depth Information
Global Atmospheric Circulation and Water Vapor Movement
An excellent way to visualize global atmospheric circulation is to
observe the movement of water vapor. In this context, view the
simulation
Global Atmospheric Circulation and Water Vapor
Movement
. This excellent simulation models the movement of
water vapor on Earth over the course of the year as it relates to
the wind patterns discussed so far. After you watch this simu-
lation, be sure to answer the questions at the end to test your
understanding of this concept.
India in July. This is a migration of about 45° latitude, or twice
that which occurs in South America. The primary reason for
this tremendous differential is the huge size or continental-
ity of the Asian continent (as compared to the Americas),
which results in the extreme range of temperature you saw in
Figure 5.11.
How does this large temperature range and seasonal
ITCZ migration create the Asian monsoon? A good place
to examine the monsoon is India (Figure 6.24). In the win-
ter a strong outflow of very dry, continental air moves from
central Asia south across India. This
winter monsoon
oc-
curs because the pressure gradient slopes steeply from the
very strong Siberian high-pressure system (in central Asia)
toward the ITCZ, which is far to the south over the Indian
Ocean (Figure 6.24a).
During the summer, the direction of surface winds reverses,
causing the
summer monsoon
. This reversal occurs because
temperatures over the Asian landmass increase significantly
due to the high Sun angle in summer. As a result, the ITCZ
shifts northward over Asia (see Figure 6.23 again), bringing
low pressure into the region. Given the seasonal shift in the
large pressure systems, a subtropical high is now located over
the Indian Ocean and the pressure gradient slopes steeply to-
ward the continent (Figure 6.24b). The warm winds from this
high-pressure anticyclone pick up moisture as they pass over
the Indian Ocean. As the air moves north along the pressure
gradient, it is lifted initially by the warmth of the Indian land-
mass and then intensively by the high Himalaya Mountains. As
the air rises, it begins the process of cooling and precipitation
occurs. The high precipitation of the monsoonal rainy season
lasts from June to September.
Human Interactions:
Harnessing Wind Energy
People have used flowing air as an energy source for
thousands of years. Ancient Egyptians, for example, used
wind to sail ships on the Nile River. In the late 12th century,
Winter
Summer
High-pressure system
over Himalayas
(Siberian High)
Low-pressure system
over Himalayas
L
H
ITCZ
northward
Cool, dry
northeast winds
Warm, moist
southwest winds
L
L
ITCZ southward
H
H
Low-pressure
system over
Indian Ocean
High-pressure system
over Indian Ocean
(a)
(b)
Figure 6.24 Monsoon flow in India during (a) winter and (b) summer.
The flow of air is distinctly related to the seasonal position
of the ITCZ and the relative temperature of the Asian landmass to the north.
