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Objectives
Explain how the sun heats Earth's surface unevenly.
Describe global patterns of wind, precipitation, and ocean currents.
Distinguish between local climates and microclimates.
Key Terms
tropics
polar zones
temperate zones
current
microclimate
What determines the types of ecosystems found in a certain part of the
world? Usually the climate of the region—particularly the range of
temperature and amount of rainfall—is a major part of the answer. These
abiotic factors influence the types of organisms that live in the region.
Earth's climate patterns are largely produced by the uneven heating of the
planet by the sun.
Uneven Heating of Earth's Surface
Energy from the sun warms Earth's surface. But because of Earth's
spherical shape, different locations on Earth's surface receive different
amounts of solar energy. Near the equator, the sun's rays strike the
surface most directly. At latitudes farther from the equator the rays strike
Earth's surface at lower angles, causing the same amount of solar energy
to be spread over a larger area (Figure 34-6). As a result, regions on
Earth's surface farther from the equator absorb less heat and generally
experience cooler temperatures than regions closer to the equator. These
temperature differences also drive global air and water movements.
Figure 34-6
The sun's rays strike Earth most directly near the
equator. Near the poles, the same amount of solar energy
is spread over a much greater area.
Earth's surface can be divided into different temperature zones based on
lines of latitude. The region that lies between 23.5° N latitude (the Tropic
of Cancer) and 23.5° S latitude (the Tropic of Capricorn), called the
tropics, is the warmest temperature zone. The region north of the Arctic
Circle (66.5° N) and the region south of the Antarctic Circle (66.5° S),
called the polar zones, receive the least amount of direct sunlight yearround. The polar zones are the coldest temperature zones. The latitudes in
between the tropics and the polar zones in each hemisphere are called the
temperate zones. Most of the United States is located within the northern
temperate zone. Seasonal temperature changes can be very large in some
temperate zones. But in general, the temperate zones experience less
extreme heat than the tropics and less extreme cold than the polar zones.
Wind, Precipitation, and Ocean Currents
The uneven heating of Earth's surface by the sun is also a driving force
behind global patterns of winds and precipitation (rain, snow, and sleet).
When air is warmed it can absorb more moisture, and it also tends to rise.
Thus air near the equator, heated by the direct rays of the sun, absorbs
moisture and rises. Higher in the atmosphere the air cools again, forming
clouds that produce rainfall. This pattern means that many areas of Earth
close to the equator tend to have warm temperatures and heavy rainfall
year-round. Most rain forests are found in this part of the world, as you
will read in Concept 34.3.
After losing moisture over the equator, air masses spread away from the
tropics. The dry air descends again and warms at latitudes of about 30° N
and 30° S. Some of the world's largest deserts are found in these regions.
At higher latitudes, the moving air absorbs more moisture and produces
precipitation again.
The rising and falling of air masses, combined with Earth's rotation,
produce predictable wind patterns. These wind patterns combine with the
uneven heating of Earth's surface, the rotation of the Earth, and the shapes
of the continents, producing surface currents. A current is a river-like
flow pattern within a body of water. Notice in Figure 34-7 that some
surface currents move warm water from the tropics toward the polar
zones, while others move cold water from the polar zones back toward the
tropics. Surface currents can greatly affect regional climates. For
instance, the Gulf Stream carries warm water northward from the tropics.
This makes the climate in western Europe warmer than other areas at
similar latitudes.
Figure 34-7
Global wind patterns and Earth's rotation create warm and cold surface
currents in the oceans. These surface currents affect the climate on the
continents.
Local Climate
On an August day, people in Los Angeles' San Fernando Valley might
face temperatures of 38°C (100°F), while just 30 kilometers away, people
at the beach are enjoying the comfortable 24°C (75°F) outdoor air. What
causes this difference within such a small geographic area? Local climate
variations may be influenced by nearby large water bodies. Oceans and
large lakes affect climate because water tends to absorb and release heat
more gradually than most other substances (such as soil and rock).
Because of the heat-absorbing ability of water, areas near the shore tend
to be cooler in the summer than inland areas. In turn, the release of heat
from water to the air generally results in milder winters near the shore
than inland.
Mountains also affect local climate. First, air temperature declines by
about 6°C with every 1000-meter increase in elevation. This is why it
becomes cooler as you hike up a mountain. Second, mountains near a
coast can block the flow of cool, moist air from the ocean, causing
different climates on opposite sides of the mountain range. For example,
in California, Oregon, and Washington, precipitation falls on the western,
coastal side of the mountain ranges. The eastern side of the mountains can
be dry and even desert-like.
Microclimate
Organisms living in the same climate region may be exposed to different
conditions created by shade, snow cover, or windbreaks. For example, a
kangaroo rat avoids the hot, dry daytime conditions on the desert surface
by living underground in a dark, moist burrow. Turn over a fallen log, and
it is teeming with life that thrives under conditions quite different from
those on top of the log. Such small-scale differences in climate result in a
microclimate, the climate in a specific area that varies from the
surrounding climate region. In the next concept, you'll return to the global
scale to explore how climate patterns determine the distribution of Earth's
major ecosystems.
Concept Check 34.2
1. Explain how the uneven heating of Earth's surface creates different
temperature zones.
2. How do surface currents affect climate patterns?
3. Why might a town located on a large lake be cooler in the summer than
a town 100 kilometers away from the lake?
4. Give an example of a microclimate.
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