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What causes wind?
Wind is the roughly
horizontal movement of air caused by uneven heating of the Earth's
surface.
Different parts
of the Earth—such as the equator and the poles—receive
different amounts of heat energy from the sun. This energy, along
with the rotation of the Earth, helps form areas of high and
low pressure in an effort to balance out the differences in energy.
Air flows from
areas of high pressure toward areas of low pressure. The greater
the difference in pressure between the high- and low-pressure
areas—known as the pressure gradient force—the stronger
the wind. Other forces that affect the wind at the Earth's surface
include the Coriolis force (a result of the Earth's rotation)
and friction.
You can visualize
wind on a weather map by looking at the isobars, which are the
lines around high- and low-pressure areas that depict equal pressure.
The closer the isobars are to one another, the stronger the wind
will blow.
If you're looking
for locations with the lowest annual average wind speeds,
your best bet is to head inland away from the coasts and choose
a valley location protected by mountains. According to average
wind speed data from the National Climatic Data Center, Oak Ridge,
Tenn., is the least windy city in the USA, with an average wind
speed of just 4.1 mph.
The windiest
location in the USA is Mount Washington, N.H. The average
wind speed there, at an elevation of 6,262 feet, is 35.1 mph.
The winds are strongest in January, with an average speed of
46.1 mph. The strongest gust ever recorded on Mount Washington
was 231 mph on April 12, 1934, which still stands as the world's
all-time highest surface wind speed. |
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Microscale winds
Microscale winds take place over very short durations of time
- seconds to minutes - and spatially over only tens to hundreds
of metres. The turbulence following the passage of an active
front is composed of microscale winds, and it is microscale wind
which produces convective events such as dust devils. Though
small in scope, microscale winds can play a major role in human
affairs. It was the crash of a fully loaded Lockheed L-1011 at
Dallas-Fort Worth International Airport in the summer of 1985,
and the subsequent loss of 133 lives, that introduced the term
"microburst" to many people, and that was a factor
in the installation of doppler radar in airports and weather
installations worldwide.
Mesoscale winds
Synoptic winds occupy the lower boundary of what is considered
to be "forecastable" wind. Winds at the next lowest
level of magnitude typically arise and fade over time periods
too short and over geographic regions too narrow to predict with
any long-range accuracy. These mesoscale winds include such phenomena
as the cold wind outflow from thunderstorms. This wind frequently
advances ahead of more intense thunderstorms and may be sufficiently
energetic to generate local weather of its own.
Synoptic winds
Synoptic winds are winds associated with large-scale events such
as warm and cold fronts, and are part of what makes up everyday
weather. These include the geostrophic wind, the gradient wind,
and the cyclostrophic wind.
As a result of
the Coriolis force, winds in the northern hemisphere always flow
clockwise (when seen from above) around a high pressure area
and counterclockwise around a low pressure area (the reverse
occurs in the southern hemisphere). At the same time, winds always
flow from areas of high pressure to areas of low pressure. These
two forces are opposite but not equal, and the path that results
when the two forces cancel each other runs parallel to the isobars.
Wind following this path is known as geostrophic wind. Winds
are said to be truly geostrophic only when other forces (e.g.
friction) acting on the air are negligible, a situation which
is often a good approximation to the large-scale flow away from
the tropics.
In certain circumstances,
the Coriolis force acting on moving air may be almost or entirely
overwhelmed by the centripetal force. Such a wind is said to
be cyclostrophic, and is characterized by rapid rotation over
a relatively small area. Hurricanes, tornadoes, and typhoons
are examples of this type of wind.
Names for
specific winds in certain regions
Alizé (northeasterly across central Africa and the Caribbean)
Alizé Maritime (a wet, fresh northerly wind across west
central Africa)
Amihan (northeasterly wind across the Philippines)
Bayamo (a violent wind on Cuba's southern coast)
Bora (northeasterly from eastern Europe to Italy)
Chinook (warm dry westerly off the Rocky Mountains)
Etesian (Greek name) or Meltemi (Turkish name) (northerly across
Greece and Turkey)
Föhn (warm dry southerly off the northern side of the Alps
and the North Italy)
Fremantle Doctor (afternoon sea breeze from the Indian Ocean
which cools Perth, Western Australia during summer)
Gilavar (south wind in the Absheron Peninsula)
Gregale (northeasterly from Greece)
Habagat (southwesterly wind across the Philippines)
Harmattan (dry northerly wind across central Africa)
Halny (in northern Carpathians)
Khamsin (southeasterly from north Africa to the eastern Mediterranean)
Khazri (cold north wind in the Absheron Peninsula)
Kosava (strong and cold southeasterly season wind in Serbia)
Levanter (easterly through Strait of Gibraltar)
Libeccio (southwesterly towards Italy)
Marin (south-easterly from Mediterranean to France)
Mistral (cold northerly from central France and the Alps to Mediterranean)
Nor'easter (eastern United States)
Nor'wester (A wind that brings rain to the West Coast of New
Zealand, and warm dry winds (and bad tempers for some) to the
East Coast of New Zealand.)
Santa Ana winds (southern California)
Simoom (strong, dry, desert wind that blows in the Sahara, Palestine,
Jordan, Syria, and the desert of Arabia)
Sirocco (southerly from north Africa to southern Europe)
Southerly Buster (rapidly arriving low pressure cell that dramatically
cools Sydney, Australia during summer)
Tramontane (cold northwesterly from the Pyrenees or northeasterly
from the Alps to the Mediterranean, similar to Mistral)
Vendavel (westerly through Strait of Gibraltar)
Zonda wind (on the eastern slope of the Andes in Argentina)
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