Global circulation and wind systems
Much of the solar energy that the earth receives causes intense heating in the equatorial regions. This intense heat produces powerful convection in these areas.
As the warm, moist air rises, it creates a zone of low pressure, clouds, and precipitation along the equator. As that warm air rises, it eventually reaches the troposphere and can rise no higher. It spreads outwards towards the poles.
As it spreads, it cools and sinks back down to the surface at about 30 degrees north and south of the equator. This sinking air produces areas of higher pressure with drier conditions.
Many of the world's deserts are located in these high pressure areas, around 30o north and south of the equator.
Examples?
Sahara Desert
Great Victoria Desert, Australia
Kalahari Desert
Sonoran Desert
Some of this air, as it sinks, moves back towards the equator. This air flowing back towards the equator produces what is know as the trade winds.
The trade winds obtained their name from the sailing ships used in foreign trade, which were called traders. These winds propelled the ships from Europe to the New World.
The area near the equator where these winds die out is referred to as the doldrums.
These areas where air rises at the equator, sink at 30 degrees north and south latitude, and then flow back to the equator are known as Hadley cells. Although most of the air that sinks at 30 degrees north and south latitude returns to the equator, some of it continues to move poleward.
At approximately 60 degrees north and south, this air meets cold polar air. The areas where these air masses meet form polar fronts. The air moving in from the lower latitudes is generally warmer and will rise. It then moves back towards the equator, sinking at about 30 degrees north and south. This sinking air contributes to the high pressure systems located there.
The circulation cells that form between 30 degrees and 60 degrees north and south are called Ferrel cells.
Some of the air that rises at the polar fronts continues to move poleward, sinking at the poles and then moving back towards 60 degrees north and south.
These Hadley cells are weaker than the tropical ones.
As the air that sinks at these locations flows back along the surface of the earth, it does not flow in a straight north-south path. This flow of air is affected by the Coriolis Effect.
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The Coriolis Effect
Because of the earth's rotation, any freely moving object or fluid will appear to:
Turn to the right of its direction of motion in the Northern Hemisphere and
Turn to the left of its direction of motion in the Southern Hemisphere.
(For example, note the curved wind arrows in the diagram below.)
This causes winds to travel clockwise around high pressure systems in the Northern Hemisphere, and counter-clockwise in the Southern Hemisphere.
Low pressure winds travel in the opposite direction (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere).
Because of the Coriolis Effect, winds traveling along the surface of the earth from 30 - 60 degrees north and south of the equator flow from the west to the east. These winds are referred to as the westerlies.
Idealized global circulation
The air currents moving along the surface of the Earth from the poles (90 degrees) to 60 degrees north and south of the equator, flow from east to west and are referred to as the polar easterlies.
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Jet Stream
The jet streams are the result of the poleward-flowing air at latitudes between 30 - 60 degrees north and south of the equator (westerlies). The speed of this wind may exceed 160 km/hr (or 100 mi/hr).