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Geography Lecture 3 (2. Factors affecting Air Movement (a) Pressure…
Geography Lecture 3
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5. Surface Wind Belts
3 main types:
- Trade Winds
- Mid-latitudinal Westerlies
- Polar Easterlies
They provide most of the climatic variation and consistence in the world
Trade Winds
They lie between the equatorial troughs and sub-tropical highs, covering half of the globe
Oceans have more trade winds than continents as movement of ITCZ is less.
Monsoons are more important for continents
In July, ITCZ is further North. Thus trade winds in NH are restricted, especially overland
In January, ITCZ is most southerly and trade winds extend to the equator
In SH, less marked variations occur as Predominance of oceans means southern limit for ITCZ remains close to equator
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Monsoon Winds
Monsoon usually occurs when a humid wind blows from the ocean toward the land in the summer shifts to a dry, cooler wind blowing seaward off the land in the winter and it involves a full reversal of wind systems from both the hemispheres due to pressure gradient in NH and SH during summer and winter months. These winds carry a large amount of rain as they carry moisture from the oceans
June/July:
Winds are induced by Asia, which pulls the southern trade winds from SH to cross the equator into NH. This is due to Asia having summer, thus lower pressure.
Due to Coriolis effect, the South-East trade winds are deflected to the right in NH and become South-West winds which are SW monsoon winds
Areas largely affected by SW monsoon winds include South Asia, Africa and SEA
December/January:
NE monsoon winds are generated as Asia experiences winter. Pressures are higher and thus air moves from Asia towards the equator
Areas affected by NE monsoon include Africa, SEA and Austrialia
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In winter, surface temperature in Asia may be as low as -20 degrees. The surrounding oceans have a temperature of about 20 degrees
In summer, the land heats up quickly and may reach temperatures of 40 degrees, while the sea remains at about 27 degrees
This causes a land sea breeze blowing from a cooler area to the warmer area
In addition, the Himalayan Plateau disrupts the winds of the upper atmosphere, which forces the winds to the north or south, causing a deflection in surface winds
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3. Tri-cell model
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Formation
Jetstreams
Geostrophic Wind
- No friction
- Forms from a balance of pressure gradient and Coriolis Effect
- Major means of transport for weather systems
- 120-250mph, thousands of miles long and across
- With increasing height at the Tropopause, winds tend to be stronger and faster in the West-East direction
- This is as there is little friction to slow down the wind
- Equator has a surplus of energy, thus warm air rises, causing low pressure and Inter-Continental Convergence Zone (ITCZ)
- The warm air approaches the ceiling and travels pole-wards
- When it reaches mid-latitudes, it cools and falls, forming the Sub-Tropical High Pressure Belt (STHP)
- Descending air diverges and the air that moves back to the equator are called Trade Winds while the rest of the surface air moves pole-wards and are called mid-latitudinal westerlies
- When the subtropical air meets cold polar air at 60 degrees, they converge and form the Sub-Polar Low Pressure Belt and the polar front
- At 90 degrees, air cools and sinks, creating the Polar High Pressure Belt
- The air then travels back to the mid-latitudes as polar easterlies
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Additional Information
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ITCZ
It is a key component of the Global Circulation system and has 200 days of precipitation, meaning that it is always hot and
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Diurnal Evaporation Cycle:
Clouds form in the morning and early afternoon, leading to precipitation in the late afternoon
