blow from the southeast. The trade winds flow out of the subtropical highs, which
represent sinking, warming air forced by air that has risen in the Intertropical
Convergence and spread north and south to sink at latitudes centered 25°away
from the equator. The subtropical highs have light and variable winds and are the
causes of the world’s subtropicaldeserts. On the poleward side of the subtropical
highs are the surface westerlies flowing into the polar front. The polar front is a
stormy zone ranging over considerable latitude and represents theboundary
between polar and tropical air masses. The polar zones are home to cold, stable
areas of high pressure called the polar highs. Out of the polar highs and into the
polar lows of both hemispheres flow the polar easterlies.
The entire global system of wind and pressure belts pulses with theseasons.
For instance, the Intertropical Convergence moves northward in the Northern
Hemisphere summer. Also, the northern Subtropical Highs enlarge and shift a bit
poleward and the Polar Front weakens and shifts poleward. All this has the effect
of making seasonal changes in temperature and precipitation. In the summer
season of the Sahelregionjust south of the Sahara, the Intertropical Convergence
comes over head for a few weeks providing some rain. When the Intertropical
Convergence shifts equatorward, the Sahel becomes arid for the rest of the year.
World Systems Theory
A holistic, analytical approach to international relations developed primarily by
the sociologist Immanuel Wallerstein. Although Wallerstein downplays the impor-
tance of geography in some of his writings, World Systems Theory (WST) is
rooted in spatial concepts and may be viewed as a perspective on thegeography
of economic developmentat the globalscale.Thetheoryhasmuchincommon
with Marxism, especially the notion that capitalism is an exploitative economic
structure that enables certain classes (in this case, clusters of economic institu-
tions) to manipulate the system to take advantage of their superior level of wealth,
technology, and development, thereby collectively consigning lesser-developed
clusters to a position of relative poverty. A second theory that foreshadowed the
world systems concept is dependency theory, promulgated by Andre Gunder Frank
and other scholars in the 1960s. Dependency theory holds that the global eco-
nomic system restricts the ability of developing countries to advance, forcing them
to remain “dependent” on the advanced economies for sophisticated technology
and consumer goods, resulting in acore and peripherymodel of international
economic relations. Frank himself adopted WST in a modified form. World
Systems Theory adds to the binary arrangement of dependency theory a third
World Systems Theory 369