drier. Low pressure zones often have climates that are warm and rainy.
Figure 17.2: The atmospheric circulation cells and their relationships to air movement on
the ground. ( 34 )
The low pressure area near the equator is located at the boundary between the two Hadley
Cells. In both these cells, air rises up at the equator and then travels away from the equator.
This band of rising air is called theIntertropical Convergence Zone (ITCZ)(Figure
17.3). As the air rises, it cools and condenses to create clouds and rain. Climate along the
ITCZ is therefore warm and wet. In an area where the air is mostly rising, there is not much
wind. Early mariners called this region the doldrums because their ships were often unable
to sail without steady winds.
The ITCZ migrates slightly with the season. Land areas heat more quickly than the oceans.
Since there are more land areas in the Northern Hemisphere, the ITCZ is influenced by the
heating effect of the land. In Northern Hemisphere summer, it is approximately 5onorth of
the equator while in the winter, it shifts back and is approximately at the equator. As the
ITCZ shifts, the major wind belts also shift slightly north in summer and south in winter,
which causes the wet and dry seasons in this area (Figure17.4).
At the high pressure zone where the Hadley cell and Ferrell cells meet, at about 30oN and
30 oS, the air is fairly warm since much of it came from the equator. It is also very dry for
two reasons: (1) The air lost much of its moisture at the ITCZ, and (2) Sinking air is more
likely to cause evaporation than precipitation. Mariners had a very grim reason for naming
this region the horse latitudes. Often the lack of wind would cause their ships to be delayed