1. Unit 2: Climate Winds and Climate
World Geography
Unit 2: Climate
Winds and Climate

2. The Circulation of Air
The circulation of air follows a general pattern:
The equator receives the most solar energy, therefore is heated the most.
Air above the equator heats rapidly and expands, becoming less dense.
This air rises
As the air rises, it cools and is pushed towards the poles, it is pushed by air still rising from the equator.
This cooling air becomes more dense, and begins to fall back to earth.
As it falls back to earth it forces colder air toward the equator where the cycle continues.
2.3.5 Explain how wind systems and
temperature are related. (k)

3. The Circulation of Air This process is known as a convection cell
Convection Cell: A current in the atmosphere in which warmer, lighter air moves upward and is replaced by colder, heavier air.
The movement of the air depends on the density of the air
Warm air is moist, less dense, and lighter.
These are low pressure areas.
Cool air is dry, dense, and heavy.
These are high pressure areas
When areas of low and high pressure are next to each other the air flows from high to low.
This flow is known as Wind.

4. The Circulation of Air
Air moves vertically in convection cells to equalize temperature.
Air moves horizontally by wind to equalize pressure.

6. Prevailing Wind
Prevailing Wind: The most frequently occurring wind direction at a given location.
The circulation of air from the equator to the poles and back is complicated by two things:
The spinning of the planet
The uneven distribution of water and land on the planet

7. Prevailing Winds
The movement of air from the equator to the poles is broken into three convection cells in each hemisphere:
Trade winds: 0⁰-23.5 ⁰ (equator to tropics)
Westerlies : 23.5 ⁰ ⁰(tropics to arctic circle)
Polar easterlies: 66.5 ⁰-to pole (arctic circle to pole)
These latitudes do change due to two factors:
Variations in Earths Surface: Ocean and Continent
Earth’s relative position to the sun: They will move with the seasons

8. Prevailing Winds

10. Coriolis Effect
The apparent deflection of moving objects (wind, ocean currents, missiles) from traveling in a straight path, in proportion to the speed of Earth’s rotation at different latitudes.
Deflection is:
To the right in the Northern Hemisphere
To the left in the Southern Hemisphere.
Maximum effect at the poles
Minimum effect at the equator

11. Coriolis Effect and Wind Direction

12. The Major Pressure Belts
Wind systems are driven by pressure belts that can be found in the different areas of prevailing wind.
These are areas of high and low pressure.
High pressure:
Downward moving
Pushing outward-
Spins clockwise in the north and counter clockwise in the south
Low pressure:
Rising air
Air that pulls inward
Spins counter clockwise in the north and clockwise in the south

13. High Pressure and Low Pressure

14. Pressure Belts The Pressure systems are Equatorial Low Pressure Trough
Subtropical High-Pressure
Sub Polar Low Pressure
Polar High

15. Pressure Belts Equatorial Low Pressure Trough:
Broken band of clouds that straddle the equator
Constant high sun altitude and consistent day length make enormous amounts of energy available
Lots of light, wet, ascending air.
As air rises it cools, and water in the air condenses.
Home of the intertropical convergence zone,
Shifts between the tropics depending on season.
This area is dominated by the TRADE WINDS!!!

16. Pressure Belts Subtropical High- Pressure Cells: Hot, dry, desert air
Cloudless skies
Air forced downward by compression from low pressure cells, is heated.
THIS AREA IS ASSOCIATED WITH THE WESTERLIES

17. Pressure Belts Subpolar Low-Pressure Cells: Cool, moist air.
These cells DOMINATE in the winter, weakening, or disappearing in the summer.
THESE CELLS FORM BETWEEN THE WESTERLIES AND POLAR EASTERLIES

18. Pressure Belts Polar High-Pressure Cells: Frigid, dry deserts
These are weak systems.
Near the poles there is little energy from the sun to power up weather systems
THIS PRESSURE BELT IS RESPONSIBLE FOR FORMING THE POLAR EASTERLIES

20. Land and Sea Breezes Land-Sea breezes occur on most coastlines.
However they have different characteristics.

21. Sea Breeze During the day the land is heated by the sun.
This causes air to warm and rise forming a low pressure system over the land.
As the air rises it is pushed over the sea where it cools and descends (high pressure)
This high pressure is drawn into the land creating a breeze that comes off of the water.

22. Sea Breeze

23. Land Breeze At night water is often warmer than land
This causes the air above the water to rise, forming a low over the water.
As the air rises it is driven in over the land, cools and descends.
The cool air is drawn back out to sea by the low pressure.
This creates a breeze that comes from the land.

24. Land Breeze

25. Wind Systems and Temperature
As seasons change the temperature difference will cause pressure belts and prevailing winds to shift.
The equatorial low follows the sun between the tropics with the seasons.
This causes the other systems to shift to compensate.

26. Points to Remember: WARM AIR RISES FORMING LOW PRESSURE
AS AIR RISES IT COOLS
COOL AIR DESCENDS FORMING HIGH PRESSURE
COOL AIR CANNOT HOLD MOISTURE AS WELL
CONDENSATION HAPPENS WHEN AIR CANNOT HOLD ITS MOISTURE

27. Precipitation: Orographic
Orographic Precipitation: (relief rainfall) this forms when moisture-laden air masses are forced to rise over high ground. The air is cooled, the water vapour condenses, and precipitation occurs
Windward side: The side of the mountain that the prevailing wind blows against. Where the air rises, and precipitation occurs
Leeward side: The side of the mountain in which the air begins to descend again. This area is dry as the air mass has lost its moisture.
Rainshadow: Area where little rain falls due to the loss of moisture within the air mass due to its rising over a landform.

28. Orographic Precipitation

29. Precipitation: Frontal
Frontal Precipitation: Rain resulting from where two air masses meet.
As air is forced to rise over a low pressure system it cools, and drops its moisture.

30. Precipitation: Frontal

31. Precipitation: Convectional
Convection rainfall happens when the land is heated by the sun. The air above this land is then heated and rises. Moisture from the Earth’s surface evaporates and rises with the warm air. As the air rises it cools so this moisture condenses and falls as rain. The continuous cycle of rising warm air and falling cool air creates a convection current in the atmosphere.
This type of rainfall is generally not accompanied by wind.

32. Precipitation: Convectional

33. The Relationship Between Precipitation and Location
Where does each type of rainfall occur?
Orographic: Where westerly winds meet coastal mountains after crossing bodies of water.
Frontal: where low pressure meets high pressure.
Convectional: Mostly over the tropics, however sometimes in the summer.