Geography Reference
In-Depth Information
a 58-degree annual temperature range (the difference between the coldest and warmest month). In
Portland, the coldest month averages 23° F and the warmest month averages 68° F, making its annual
average temperature range 45° F. So winters are harsher and summers are hotter in the mid-continent
city.
Going with the Flow: Ocean Currents
The oceans have warm and cold surface currents that act like a global heating and air-con-
ditioning system. They bring significant warmth to high latitude areas that would otherwise
be much cooler, and significant coolness to low latitude areas that would otherwise be much
warmer.
The currents also play a major role in determining the global geography of precipitation. The sun
can more easily evaporate warm water than cold water, and thereby produce the atmospheric vapor
that results in rain. Therefore, lands that get sideswiped or impacted by warm currents tend to have
abundant precipitation in addition to a comparatively warm climate. Conversely, lands impacted by
cold currents tend to receive very little precipitation in addition to a comparatively cool climate.
Generally, surface currents exhibit circular movements (see Figure 9-7). North of the equator, the
flow is usually clockwise. South of the equator, the flow tends to be counter-clockwise. These move-
ments are principally products of prevailing winds that “push” the ocean's surface. On the map you
can see occasional exceptions to the general rules of circulation. They are the results of deflections
caused by the angle at which a current strikes a land mass or the continental shelf, or by the direction
of prevailing sea level winds at particular latitudes.
Warm currents, cold currents
The warm and cold portions of these circulatory systems have rather predictable geographies. As
ocean currents move westward along the equator, they absorb lots of solar energy, heat up, and be-
come warm currents. As they turn away from the equator, they generally continue to absorb about as
much heat as they dissipate, at least while they remain in the Tropics — that is, the region between
the Tropic of Cancer and the Tropic of Capricorn.
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