DEFINITION OF HADLEY CELLS

The globe-encircling belts of air that rise

along the equator, dropping moisture as they rise in the

Tropics. As the air moves away from the equator at high

elevations, it cools, becomes drier, and then descends at

15–30°N and S latitude where it either returns to the equator

or moves toward the poles. The locations of the Hadley

cells move north and south annually in response to the

changing apparent seasonal movement of the Sun. Highpressure

systems form where the air descends, characterized

by stable clear skies and intense evaporation, because the air

is so dry. Another pair of major global circulation belts is

formed as air cools at the poles and spreads toward the

equator. Cold polar fronts form where the polar air mass

meets the warmer air that has circulated around the Hadley

Cell from the Tropics. In the belts between the polar front

and the Hadley cells, strong westerly winds develop. The

position of the polar front and extent of the west-moving

wind is controlled by the position of the polar jet stream

(formed in the upper troposphere), which is partly fixed in

place in the Northern Hemisphere by the high Tibetan

Plateau and the Rocky Mountains. Dips and bends in the jet

stream path are known as Rossby Waves, and these partly

determine the location of high- and low-pressure systems.

These Rossby Waves tend to be semi-stable in different seasons

and have predictable patterns for summer and winter.

If the pattern of Rossby Waves in the jet stream changes significantly

for a season or longer, it may cause storm systems

to track to different locations than normal, causing local

droughts or floods. Changes to this global circulation may

also change the locations of regional downwelling of cold

dry air. This can cause long-term drought and desertification.

Such changes may persist for periods of several weeks,

months, or years and may explain several of the severe

droughts that have affected Asia, Africa, North America,

and elsewhere.

See also ATMOSPHERE.

hail See PRECIPITATION.

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