air circulation: The circulation of air over the Earth’s surface
is complex. If the Earth were not rotating, things would be much
simpler. The temperature gradient between the equator and the poles
would cause the formation of a large convection cell in each hemisphere;
warm equatorial air would rise and flow toward the poles, become
cold, then sink and flow at the surface back toward the equator.
However, because the Earth is rotating there is a more complex wind
of input of significant solar energy, air warms and rises at the
equator. This creates a low pressure area sometimes called the doldrums.
As the air rises, it cools and sinks back toward the surface 30
degrees north and south of the equator. The descending air produces
a high pressure area. Some of this sinking air will flow back toward
the equator and some will flow toward the pole. The air flowing
toward the equator will be deflected to the west by the Coriolis
force. This air forms the westward-flowing wind belts we call
the trade winds. The air flowing toward the poles will be
deflected to the east. These east-flowing winds are called westerlies
(because they come from the west). As these winds reach 60 degrees
latitude, they meet air flowing from the poles, which causes the
air to rise creating a low pressure area. At the poles, a high pressure
area exists because cold air is sinking and flowing along the surface
toward the equator. Thus, going from the equator towards each pole,
there are three convection cells that describe atmospheric circulation;
these cells are called Hadley cells, Ferrel cells, and polar
cells. Winds that are part of the global air circulation and
result from pressure-gradient flow modified by the Coriolis effect
are often called geostrophic winds.
sites below for information and diagrams regarding global air circulation:
Companies site on global
air circulation. Good, concise explanation of insolation, Hadley,
Ferrel, and Polar cells, and the Coriolis effect.
site from the Okanagan
University College (Canada) on 1) solar insolation, 2) atmospheric
temperatures and thermal convections, 3) global air circulation,
including Hadley, Ferrel, and Polar cells, 4) trade winds and other
wind systems, and 5) some of the effects of air circulation on climate.
Many parts of the chapter should be useful, but definitely click
on the parts that deal with winds and global air circulation: sections
n and p.
are most deserts located on the earth and why?
around low pressure centers: The general term cyclone
is used for an area of low pressure in the atmosphere that displays
circular inward movement of air. In the northern hemisphere, circulation
of a cyclone is counterclockwise, whereas southern hemisphere cyclones
have clockwise wind patterns. Tropical storms or cyclones in the
northern hemisphere usually are called hurricanes, and in
the southern hemisphere they are called typhoons.
||The following site describes
flow around low pressure areas and fronts. There is a good explanation,
with diagrams, as to why cyclones rotate counterclockwise in the
northern hemisphere. John Denker at AT&T Bell Laboratories describes
air circulation around low pressure centers and fronts. The site
is a good overview, written from the perspective of an airplane
pilot. Read only through section 20.2.1.
||The websites below describe
force in more detail and explain how it influences world weather
patterns. This is an interesting phenomenon first described by the
French mathematician Gaspard Coriolis in the early 19th century.
An object that moves in a straight line above the surface of the
Earth (and not parallel to the equator) will appear to curve because
the Earth is turning under it. This Coriolis effect must be taken
into account in determining the trajectory of an object launched
||This website from The
Ohio State University provides a teacher's viewpoint on explaining
the Coriolis force.
Local wind systems: Global winds usually are important in
determining the prevailing winds in a given area, but local conditions
(terrain, etc.) sometimes determine the most common wind directions.
sea and land breeze is typically found along coastlines adjacent
to large bodies of water and results from differences between the
heating or cooling of the water and the adjacent land. Water has
a higher heat capacity (i.e., more heat is required far a given
temperature change) than the materials on the land, and this means
that solar radiation heats the land more than the water. As a result,
the land transfers more heat to its overlying air mass and thus
induces a circulation cell with surface winds moving from the water
toward the land (sea breeze). At night, the land cools more rapidly
than the water. Thus, the cooler landmass causes a circulation cell
with air movement opposite to that during the day. This air flow
from land to water is a land breeze.
winds are caused by the presence of mountains or valleys. Mountain
winds or breezes are the result of differential heating or cooling
along mountain slopes. During the day, heating of sunlit slopes
causes the overlying air to move upslope. At night, as the slopes
cool, the air motion is reversed.
winds (also called chinook winds east of the Rocky Mountains,
and Santa Ana winds in southern California), are induced by adiabatic
temperature changes as air moves up and over a mountain. Adiabatic
temperature changes occur without the external addition or subtraction
of heat when air moves vertically. When air rises, it expands because
pressure is lower. This expansion is accompanied by a reduction
of temperature called adiabatic cooling. When air sinks, it is compressed
and undergoes adiabatic warming.
wind is the local flow of cold, dense air. This can occur when
a cold plateau is adjacent to a relatively warm region at lower
elevation. Air over the cold plateau cools and flows down to the
lower elevations. Two varieties of katabatic wind are well known
in Europe: One is the bora, which blows from the highlands of Croatia,
Bosnia and Hercegovina, and Montenegro to the Adriatic Sea; the
other is the mistral, which blows out of central and southern France
to the Mediterranean.
||The following site from
Okanagan University College (Canada) is on local
and regional wind systems, global air circulation, fronts, etc.
and is optional: