Geography - World & Indian (Atmospheric Dynamics):

 

Global Wind Patterns
Wind is the rough horizontal movement of air caused by uneven heating of the Earths surface. The two major influences on the atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Wind will always flow from low pressure to high pressure area, although these flows will be modified by the Coriolis effect in the extratropics.

Winds can be classified either by their scale, the kinds of forces which cause them (according to the atmospheric equations of motion), or the geographic regions in which they exist.

Winds can also shape landforms, via a variety of eolian processes.

 

Prevailing winds â€" the general circulation of the atmosphere
Prevailing winds are winds which come about as a consequence of global circulation patterns. These include

• the Trade Winds
• the Westerlies
• the Polar Easterlies
• the jet streams.

 

Trade Wind
The region of Earth receiving the Suns direct rays is the equator. Here, air is heated and rises, leaving low pressure areas behind. Moving to about thirty degrees north and south of the equator, the warm air from the equator begins to cool and sink. Between thirty degrees latitude and the equator,  most of the cooling sinking air moves back to the equator. The rest of the air flows toward the poles. The air movements toward the equator are called trade winds meaning "path" or "track", - warm, steady breezes that blow almost continuously. The Coriolis Effect makes the trade winds appear to be curving to the west, whether they are traveling to the equator from the south or north. The trade winds coming from the south and the north meet near the equator. These converging trade winds produce general upward winds as they are heated, so there are no steady surface winds. This area of calm is called the doldrums. Sinking air creates an area of high area called horse latitudes. Here the winds are weak.

Westerlies
Between thirty and sixty degrees latitude, the winds that move toward the poles appear to curve to the east. Because winds are named from the direction in which they originate, these winds are called prevailing westerlies. Prevailing westerlies in the Northern Hemisphere are responsible for many of the weather movements across the United States and Canada.

 

Easterlies
At about sixty degrees latitude in both hemispheres, the prevailing westerlies join with polar easterlies to reduce upward motion. The polar easterlies form when the atmosphere over the poles cools. This cool air then sinks and spreads over the surface. As the air flows away from the poles, it is turned to the west by the Coriolis effect. Again, because these winds begin in the east, they are called easterlies. Many of these changes in wind direction are hard to visualize. Complete this exercise to see the pattern of the winds.

 

Jet Streams
Narrow belts of high speed winds that blow in the upper troposphere and lower stratosphere. The polar jet stream also marks the presence of Rossby waves, long-scale (4000 - 6000 km in wavelength) harmonic waves which perpetuate around the globe.

 

Seasonal winds
Seasonal winds are winds that only exist during specific seasons, for example, the Indian monsoon.

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).

 

Local winds
Some local winds blow only under certain circumstances, i.e. they require a certain temperature distribution. The following are the examples

• Sea Breeze --> A cool breeze blowing from the sea toward the land.
• land breeze --> A breeze that blows from the land toward open water.
• A katabatic wind --> derived from the Greek word katabatikos meaning "going downhill", is a wind that blows down a topographic incline such as a hill, mountain, or glacier. Such winds, particularly when they occur over a wide area, are sometimes called fall winds.
• Aanabatic wind --> The opposite of a katabatic wind is an anabatic wind, or an upward-moving wind.
• Mountain wind -->A breeze that blows down a mountain slope due to the gravitational flow of cooled air.
• Valley wind --> A gentle wind blowing up a valley or mountain slope in the absence of cyclonic or anticyclonic winds, caused by the warming of the mountainside and valley floor by the sun.

--
with warm regards

Harish Sati
Indira Gandhi National Open University (IGNOU)
Maidan Garhi, New Delhi-110068

(M) + 91 - 9990646343 | (E-mail) Harish.sati@gmail.com


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