Visualizing Environmental Science

45°

30°

45°

30°

500 km

4. Rising air cools


5. Cool air
   sinks


6. Cool air
   sinks


7. Warm air rises


8. Warmed ground heats air above it


9. Sun’s energy
   heats ground
   surface

60°N

30°N

60°S

30°S

0°

Polar

easterlies

Westerlies

Trade winds

Polar

easterlies

Westerlies

Trade winds

NASA MODIS Rapid Response Team

a. In atmospheric convection, heating of the ground surface
heats the air, producing an updraft of less dense, warm air.
The convection process ultimately causes air currents that mix
warmer and cooler parts of the atmosphere.

b. Atmospheric circulation transports heat from the equator to

the poles (left side of figure). The greatest solar energy input

occurs at the equator, heating air most strongly in that area. The

air rises, travels toward the poles, and cools in the process so

that much of it descends again at around 30 degrees latitude

in both hemispheres. At higher latitudes, the patterns of air

circulation are more complex.

c. Spring sandstorm in China. This NASA satellite image
shows a massive sandstorm in China. The sand passed over
Japan a few days later.

the surface at about 30 degrees north and south latitudes.

This descending air splits and flows over the surface.

Similar upward movements of warm air and its subse-

quent flow toward the poles also occur at higher latitudes

farther from the equator. At the poles, the air cools, sinks,

and flows back toward the equator, generally beneath the

currents of warm air that simultaneously flow toward the

poles. These constantly moving currents transfer heat

from the equator toward the poles and cool the land over

which they pass on their return. This continuous circula-

tion moderates temperatures over Earth’s surface.

Atmospheric Circulation

The amount of solar energy that reaches different areas
on Earth varies over the course of each year and from
place to place around the globe. This variation creates dif-
ferences in temperature, which then drive the circulation
of the atmosphere. The very warm regions near the equa-
tor heat the air, which expands and rises (Figure 8.4). As
this warm air rises, it cools, spreads, and then sinks again.
Much of it recirculates almost immediately to the same
areas it has left. The remainder of the heated air splits
and flows toward the poles. The air chills enough to sink to

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What direction do winds usually come

from where you live? Is this consistent

with what you would expect based on

your latitude?

GLOBAL

LOCAL