The angle the sunlight strikes the earth's surface varies, depending on:
Location: sunlight strikes at a more direct angle in areas closer
to the equator as compared with polar regions.
Season: sunlight strikes a area of the earth's surface more directly
during the summer.
Time of Day: sunlight will be most direct when the sun is directly
The more directly the sunlight strikes the ground, the
more it heats the ground. As a result, areas near the equator are tropical,
while polar regions are cold. It is hotter in the summertime than in the wintertime.
And, it generally is coldest in the early morning and hottest in the afternoon.
The sun is heating every portion of the earth unevenly, depending
on these factors.
Air is heated at the surface where it contacts the earth. Since the
surface is heated unevenly, the air is also heated unevenly. Add to this the
fact that hot air rises, while cold air sinks, and circulation of the
uneven heating, air
tend to naturally rise above the equator, travel at altitude
to and sink at the north and south poles, then travel along the surface back to
the equator. This flow is interrupted, however, by coriolis force.
Fluids are not fixed in place to the earth's surface. As a result, all
moving fluids are deflected to the right in the northern hemisphere as the earth
rotates underneath. This deflection is called
coriolis force. For example, if air moves south, the earth slides underneath
as it rotates, which results in the air moving south westerly.
is the greatest at the poles and least near the equator. It is also associated
with the speed of the moving fluid. The faster the fluid flows, the more coriolis
force acts to turn it.
Friction between the wind and the ground often slows the wind near the surface.
Because of this lower wind speed closer to the ground, coriolis force is
reduced. This often produces a difference in wind direction near the
surface versus air aloft.
Cells of the General Circulation
As a result of coriolis force, earth's general circulation is broken up into thirds
in each hemisphere, three cells in the northern hemisphere and three cells in the
south. In the northern hemisphere, the cell closest to the equator, has an
easterly flow. This means the winds blow from the east to the west.
These easterly winds are known as the
northeasterly trade winds. They exist
from the equator to 30 degrees north latitude. The United States resides under
the center cell, which covers the 30 to 60 degrees north latitude range. In
this cell, the prevailing westerly winds act to push weather systems from the west
to the east.
Smaller Scale Circulation
Water is difficult to heat or cool. As a result, something containing a lot
of moisture will heat and cool more slowly when compared to something with less
moisture. For example, small updrafts of air are common over plowed farm
fields during the daytime. This is because the plowed fields contain dry
dirt and heat more easily in the sunlight. The vegetation in areas
adjacent to these fields hold moisture, causing them to heat more slowly.
As a result, the plowed fields heat the air above them. That air has a
lowered density and rises. The same things happen over paved areas, such
as parking lots.
Another example is an area of land adjacent to a body of water, Florida for
example. During the daytime, the land mass of Florida heats more easily,
becoming warmer than the surrounding waters. This sets up a circulation
pattern in which air rises over the land and moves out to sea aloft. The
cooler air over the water is sucked inland, resulting in a sea breeze.
At night, the land mass of Florida cools quickly, while the water remains
relatively warm. The opposite circulation pattern is setup, resulting in a
land breeze. The warmer air over the ocean rises and moves inland aloft,
while the cooler air over the land moves out to sea at low altitude and along