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Wind physics
How are winds created
Global wind changes
Seasonal wind changes
How are winds created
• The wind is the flow of gases on a large scale.
On the surface of the Earth, wind consists of
the bulk movement of air.
• The wind happens due to difference in air
pressure between two neighboring areas .
• What causes the difference in air pressure ?
Different solar energy absorption rates
at different climate zones
How does the air flow ?
• The air doesn’t flow in straight line from high
pressure area to low pressure area , it actually
influenced by the earth circulation around it
self which generates a coriolis force which
makes circulates making vortices .
• Except for air pressure disturbances that accur
when low air pressure and high air pressure
are close such as thunder storms .
Wind flow pattern
Wind strength
• The closer the high and low pressure areas are
together, the stronger the "pressure gradient",
and the stronger the winds.
• The higher difference in pressure the stronger the
wind.
• . On weather maps, lines of constant pressure are
drawn (as in the example, above) which are called
"isobars". These isobars are usually labeled with
their pressure value in millibars (mb). The closer
these lines are together, the stronger the wind.
Wind speed
• The curvature of the isobars is also important
to the wind speed. Given the same pressure
gradient (isobar spacing), if the isobars are
curved anticyclonically (around the high
pressure in the above example) the wind will
be stronger. If the isobars are curved
cyclonically (around the low pressure in the
example above) the wind will be weaker
Wind speed
• Near the surface of the Earth, friction from the
ground slows the wind down. During the day,
when convective mixing is stirring up the
lower atmosphere, this effect is minimized. At
night, however, when convective mixing has
stopped, the surface wind can slow
considerably, or even stop altogether .
• Convective mixing is the vertical movement of
air .
Fastest ever recorded
• SPEED The highest wind speed ever recorded
on the surface of the Earth was 231 mph on
April 12 1934, atop Mt. Washington, New
Hampshire.
Global wind patterns
• Polar Easterlies: At about the latitude of Norway
and northward (60-90 degrees) the Polar
easterliese blow irregularly from the east and
north.
• Westerlies: At about the latitude of Western
Europe and the United States (30-60 degrees)
the Westerliese blow from the west, tending
somewhat toward the north. This causes most
weather in the United States to move from west
to east.
Global wind patterns
• Trade Winds: South of about 30 degrees the
northern or northeast trade winds blow
mostly from the northeast toward the
equator. These were the sailor's favorite winds
since the weather was warm and the winds
usually blew steadily in an advantageous
direction. Columbus used these to sail to the
Caribbean
Global wind patterns
• Southern hemisphere: In the southern
hemisphere the belts are reversed. The southeast
trade winds blow from the southeast toward the
equator. The southern equivalent of the horse
latitudes (or Variables of Cancer) is called the
Variables of Capricorn. The southern westerlies
start somewhat south of South Africa. They tend
to be stronger than the northern westerlies
because they are mostly over water (roaring
forties). The southern polar easterlies are mostly
over Antarctica.
Global wind patterns
The six major wind belts .
Wind patterns with time
• Seasonal shifts: All of the belts move north
during the northern summer and south during
the northern winter. Because global heating and
cooling lags behind the position of the sun they
reach their northernmost latitude at or after the
end of the northern summer. This brought the
trade winds within reach of the Spain and
Portugal and determined the sailing time of
the Spanish treasure fleet. The northernmost
position of the wind belts corresponds to the
Atlantic hurricane season.
Wind patterns with time
• Land and sea breezes: Land gains and loses
heat more rapidly than water. During the day
the land warms more rapidly than the water.
The air above land warms, becomes thinner
and rises drawing cooler air landward from
the sea. At night the process reverses and cool
heavy air from the land flows out to sea.
These land and sea breezes are important
along the coast.
Wind patterns with time
• Monsoon: The annual equivalent of the daily land and
sea breezes is the yearly monsoon. During summer the
continents heat more rapidly than the oceans. Air over
the continents warms, thins and rises drawing cooler
moist ocean air landward, producing a wet season.
During winter the process reverses and cold, dry heavy
air flows outward from the continents, producing a dry
season. The monsoon is most striking in south Asia
because of the size of the Eurasian landmass and
because the Himalayas tend to bottle up the air above
the continent. Approximations of the Indian monsoon
exist in other places but they are poorly developed.
Coriolis force and Haldley cell
• The Coriolis force :
In the northern hemisphere the Coriolis
effect causes wind and water currents to bend to
the right (clockwise). Cold heavy air flows south
from the north pole and is bent west, forming the
polar easterlies. Warm air rises at the equator
drawing air from the north which bends to the
west, contributing to the trade winds. The
Corioliseffect bends the westerlies and trade
winds slightly clockwise in the northern
hemisphere.
Coriolis force and Haldley cell
• Hot air rises at the doldrums. As it rises it
cools producing thunderstorms. The dry air
flows north at a high altitude and descends at
the horse latitudes and flows back to the
equator with the trade winds. This is called
the Hadley cell. There is also a Ferrel cell over
the westerlies and a polar cell over the pole.
There are other complexities, not all of which
are properly understood.