Geography Reference
In-Depth Information
predictable wind for reaching the Americas. Where these two bands meet over the equator, the air is heated
and rises, creating an area of calm known by sailors as the
doldrums.
Another typically calm, windless
band occurs near latitude 30˚, which sailors christened the
horse latitudes
because ships carrying horses
from Spain were often becalmed for weeks. As animal feed ran out, the animals died and were thrown
overboard, littering the sea with the carcasses of horses. After the horse latitudes, there is another reliable
band of winds, the
prevailing westerlies
, which basically flow from west to east around the world between
30˚ and 60˚ latitude, both north and south of the equator. They are not as constant and strong as the trades,
but are comparable to a meandering river of air. The third broad pattern, again repeated in both north and
south, is the
polar easterlies
, cold, dry winds that move from east to west above latitude 60˚.
These winds, the earth's rotation, and the placement of the continents also cause the great ocean cur-
rents. These currents move huge masses of water—sometimes cold, sometimes warm—from region to re-
gion. As they move, ocean currents have a great influence on climate. That's why the United Kingdom,
which is warmed by the North Atlantic drift originating off Florida, has a milder climate than Labrador,
which is on the same latitude but is cooled by a current from the Arctic Ocean.
Another factor of local geography with great impact on climate is the combination of mountains near
the coast. Warm, moist winds rise on the windward side of mountains. As they rise, they expand and cool,
causing the water vapor they carry to condense and fall as rain. The dry, cool air descends on the other
side of the mountains. As it does so, it is compressed and warmed. The resultant dry, warm wind creates a
markedly different climate and vegetation, as in California's Sierra Nevada or in Tibet, a dry region only
hundreds of miles from a very rainy area. On one side of the mountains, the coastal side, rain is usually
plentiful. But on the other side, the land turns arid.
If This Is Aphelion, Why Is It So Hot?
If the earth were truly a sphere—it isn't—and it was smooth—it isn't—and it wasn't tilted to one side—it
is—it would be a much different place in which to live. But all of these peculiarities of the earth contrib-
ute to its uniqueness as a place able to sustain life. Change any one of these factors even slightly and you
would have a very different earth.
Of course, the greatest factor, not only in our weather but in our very existence, is the sun, the explosive
ball of gas located some 93 million miles distant. But that's another of the earth's quirks. We aren't always
93 million miles away from the sun. As the earth makes its annual orbit around the sun, our path is not a
perfect circle. It is actually an ellipse. Sometimes we are closer, sometimes farther away from the sun. The
seemingly obvious conclusion to draw from this quirk is that it is summer when we are closer to the sun
and winter when we are farther away. That seems logical; but it's exactly opposite the truth. At
aphelion
,
the word for the period of a planetary orbit farthest from the sun, it is summertime, at least in the northern
hemisphere. (
Aphelion
comes from the Greek words
apo
, “away from,” and
helios
, “sun.”)
This seemingly illogical fact is due to the tilt of the earth's axis away from a parallel with the sun. The
23.5-degree tilt is the factor that produces our seasons. At all times, the sun's rays hit one half of the earth.
But in the course of a year, when the Northern Hemisphere is tilted toward the sun, the sun's rays hit this
half of the earth more directly. The earth is actually farther away from the sun during this period, which
lasts from the vernal equinox in March—when the sun is directly overhead at the equator, giving the earth
equal amounts of day and night—through the “summer” months of June, July, and August until Septem-
ber's autumnal equinox—when the sun is again directly overhead at the equator.
After the autumnal equinox, the earth has turned in its yearly orbit and the Northern Hemisphere begins
its tilt away from the sun, receiving fewer direct solar rays. This creates the shorter days of the Northern
