Key to map
Convergent
boundary
Less than 20 in (50 cm)
20–79 in (50–200 cm)
More than 79 in (200 cm)
December is
the southern
summer and
northern winter
Arctic Circle,
where Sun’s rays
are dispersed
Equator, where
Sun’s rays are
concentrated
March is the
northern spring
Sahara Desert
Atacama Desert,
Chile
The Sun
Rising
warm,
moist air
near
Equator
Rainforest,
Borneo
Tropic of
Capricorn
THE SEASONS
Earth spins on a tilted axis, so as it orbits the Sun once
a year the North Pole points towards the Sun in June and
away from it in December. This means that in regions
north of the tropics it is summer in June but winter
in December—and the opposite to the south of
the tropics. Near the Equator it is always warm,
but there are annual wet and dry seasons.
COLD POLE
The tropics are the hottest part of the
planet because the Sun’s rays strike them
directly, concentrating the heat energy.
Near the poles the same amount of heat
energy is spread out over a broader area,
so it does not have as much heating
effect, even in summer. At midwinter,
the entire polar region is in permanent
darkness, so it gets no solar energy at
all and is bitterly cold.
JUNGLE AND DESERT
Concentrated sunlight near the Equator heats
Earth’s surface, warming the air above. The warm
air rises, carrying moisture with it. This forms huge
clouds that spill tropical rain, fueling the growth
of rainforests. The dry, cooling air then flows north
and south and sinks over the subtropics, creating
deserts. Similar air circulation patterns affect the
climate in the far north and south.
ANNUAL RAINFALL
Some parts of the world get far more rain than others.
The wettest regions are mainly rainforest zones, where
year-round rain and warmth promote lush plant growth.
Regions of moderate rainfall are naturally forests and
grasslands, although much of this land is now used for
farming. The driest regions may be too dry for many
plants to grow, creating deserts—but they also include
some northern forest zones and polar tundra.
Descending cool,
dry air over
desert zone
Cold air (in blue)
becomes chilled in
upper atmosphere
Warm air (in red)
heats up near
Earth’s surface
Cool, dry air sinks
over desert zone
Tropic of
Cancer
Earth’s axis
v
Active Planet
South Pole
North Pole
June is the
northern
summer and
southern winter
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