World Geography

WIND

Causes of wind

• Wind is caused by differences in pressure
• It always flows from high pressure to low pressure
• The two major driving factors of large scale atmospheric circulation are
  • Heating difference between the equator and the poles
  • Rotation of the planet, which leads to air being deflected according to the Coriolis effect. Coriolis effect is the apparent deflection of moving objects when viewed from a rotating reference frame
• Near the Earth’s surface, friction causes wind to be slower than it otherwise would be
• Away from the surface, large scale winds tend to approach a state of equilibrium called Geostropic Balance

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Measurement of wind

A rock formation in Bolivia, sculpted by wind erosion

• Wind direction is reported based on the direction from which it originates. Eg: a northerly wind blows from the north to the south
• Wind direction is observed using weather vanes (atop buildings) andwindsocks (at airports)
• Wind speed is measured using anemometers
• Weather balloons and RADAR/LIDAR can also be used for measuring wind speed and direction
• Sustained winds are usually observed 10 m from the surface of the Earth
• Globally, wind speeds are reported over a 10 minute average. India reports winds over a 3 minute average

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Wind categorization on the Beaufort scale

Beaufort scale	Wind speed (knots)	General term	Terminology of IMD (covers north Indian Ocean)
0 – 6	0 – 27	Breeze	Depression
7	28-33	Gale	Deep depression
8 – 9	34 – 47	Strong gale	Cyclonic storm
10 – 11	48 – 63	Storm	Severe cyclonic storm
12 – 16	64 – 120	Hurricane	Very severe cyclonic storm
17	> 120	Hurricane	Super cyclonic storm

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AEOLIAN PROCESSES

Aeolian process refers to the action of wind in shaping the surface of the Earth.

1. Wind erosion
   1. Wind erodes the earth by deflation and abrasion. Deflation is the removal of fine, loosely grained particles while abrasion is the wearing down of surfaces by grinding action
   2. Regions that experience intense and sustained erosion are called deflation zones
   3. Desert rocks that have been exposed to wind for long periods of time exhibit a dark shiny stain called desert varnish
   4. Blowouts are hollows formed by the removal of particles by wind
2. Wind transport

   

   Suspension, saltation and creep
   1. Particles are transported by wind through the processes of suspension, saltation and creep
   2. Suspension is the holding of small particles in the atmosphere due to upward currents in air. Dust and haze are examples of suspension.
   3. Saltation is the movement of particles in jumps and skips by lifting up slightly from the surface. Examples of saltation include sand drift over deserts, soil blowing over fields.
   4. Creep is the slow downward progression of rock and soil down a low grade slope. Creep is responsible for the rounded shape of hillsides
      
   5. Other wind transport phenomena include dust storms and dust devils. Covered in detail below.
3. Wind deposition
   1. Wind-deposited bodes occur as sand sheets, ripples and dunes
   2. Sand sheets are flat, gently undulating surfaces of sand. They form about 40% of Aeolian deposition surfaces.
      
   3. Wind blowing on a sand surface also causes ripples, which form into crests and troughs. In ripples, the coarsest materials collect on the crests
   4. Sand dunes are hills of sand similar to ripples, except that they are larger and have the coarsest materials on the troughs

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ROLE OF WIND IN NATURE

1. Desert dust migration
   1. Dust from deserts is carried across huge distances over to other continents
   2. Example: dust from the Sahara desert blows via the Caribbean to North America
   3. Desert dust migration can affect rainfall patterns
   4. It also causes the sky to change colour from blue to white
2. Effect on plants
   1. The dispersal of seeds through wind is called anemochory
   2. Examples of seeds that disperse through wind: dandelions, maples, weeds
   3. Wind also limits tree growth. The tree line is often lower in coasts and isolated mountains because high winds reduce tree growth
   4. Wind also causes soil erosion leading to uprooting of trees
3. Effect on animals
   1. Cattle and sheep are prone to wind chill when high wind speeds render their protective covering inffective
   2. For penguins, their flippers and feet are susceptible as well
   3. Bird migration and insect return tend to flow with wind patterns

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WIND IN OUTER SPACE

1. Planetary wind
   1. The loss of gas from a planet to outer space is called planetary wind
   2. This happens when light elements such as hydrogen move up to the exobase (limit of atmosphere), and then reach escape velocity to escape to outer space
   3. The planet Venus is said to have its atmosphere due to planetary wind
2. Solar wind
   1. Solar wind is a stream of charged particles (plasma) ejected from the sun
   2. This plasma is ejected from the upper atmosphere of the sun at up to 400 km/s
   3. Solar wind creates the heliosphere, a vast bubble in interstellar medium
   4. Planets require large magnetic fields to reduce the ionization of their upper atmosphere by solar wind. Mars is said to have lost its atmosphere due to solar wind
   5. The surfaces of Venus and the Moon are bombarded directly by solar wind, resulting in high radiation levels

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GEOLOGICAL PHENOMENA CAUSED BY WIND

1. Sand dunes
   1. Sand dunes are hills of sand built by wind
   2. Dunes are usually longer on the windward side and shorter on the leeward side
   3. Sand dunes can form in dry inland regions and also in coastal areas and underwater as well
   4. Dunes can move over tens of metres due to the consistent action of strong wind. Through saltation, wind picks up particles from the windward side and deposits it on the leeward side, gradually moving the dune
   5. The tallest sand dunes in the world are found in the Namib Desert
2. Erg
   1. An Erg is a large area of desert covered with wind-swept sand with little or no vegetative cover
   2. In essence, ergs are large dune fields
   3. Ergs are mainly found in Africa, central and western Asia and central Australia
   4. Ergs have been found on Venus, Mars and Titan as well
3. Loess

   

   The rich fertile soil of the Loess plateau has been the mainstay of Chinese agriculture for centuries
   1. Loess is a sediment formed by the accumulation of wind-blown silt, sand and clay, loosely cemented by calcium carbonate
   2. Loess deposits often occur in very thick layers, sometimes more than 100 m thick. It occurs as a blanket deposit covering areas of hundreds of square km
   3. Loess is highly prone to erosion
   4. Loess can occur from glacial or non-glacial soils. Example of glacial loess: Mississippi Valley, USA. Example of non-glacial loess: Shanxi, China
   5. Loess tend to develop into highly fertile soils
4. Wind waves
   1. Wind waves are surface waves that occur in oceans, lakes etc due to the action of wind
   2. Wind waves can range from ripples to more than 30 m in height
   3. A wind wave system generated by local winds in called wind sea. Wind wave system not generated by local winds is called swell
   4. Factors that influence the formation of waves include: wind speed, distance of open water, time duration and water depth
   5. Tsunamis and tides are specific types of waves caused not by wind, but by geological effects. They have longer wavelength than wind waves
      
   6. Waves can be measured using buoys that record the motion of the water surface
   7. A breaking wave is one whose one can no longer support its top causing it to collapse
   8. There are three main types of waves
      1. Spilling or rolling waves: Safest waves for surfing. Most common type of waves found at shores.
      2. Plunging or dumping waves: preferred by experienced surfers. Found where there is a sudden rise in ocean floor like a sandbar
      3. Surging waves: very dangerous for surfing. Tend to form on steep shorelines, where the depth results in waves not breaking as they approach the shore

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GEOGRAPHICAL FEATURES SHAPED BY WIND

Feature	Location	Wind effect	Notes
Mississippi River Alluvial Valley	South-central USA	Glacial loess
Loess plateau	Shanxi, Northern China	Non-glacial loess	Thickest loess in the world (335 m) Most erodible soil on earth
Selima sand sheet	Egypt, Sudan	Wind deposition	Largest sand sheet
Namib desert sand dunes	Namibia, Angola	Wind deposition	Oldest desert in the world (55 million years)
Paha	Iowa, USA	Sand dunes
Badain Jaran desert	Inner Mongolia (China, Mongolia)	Sand dunes	Tallest stationary sand dunes in the world (500 m)
Great dune of Pilat	France	Sand dune	Largest sand dune in Europe
Merheb	UAE	Sand dune	Used for motor sports
Kelso dunes	USA	Sand dune
Cerro Blanco	Sechura Desert, Peru	Sand dune	Highest sand dune in the world