1. Chapter 12
Section 6
Wind

2. Essential Questions Ch 12 S6
What causes winds?
How do local winds and global winds differ?
Where are the major global wind belts located?

3. Because air is a fluid, it can move easily from place to place
Because air is a fluid, it can move easily from place to place. Differences in air pressure causes the air to move.
What is Wind?
Wind is the horizontal movement of air from an area of high pressure to an area of lower pressure.

4. What causes wind?
The horizontal movement of air (wind) is
caused by differences in air pressure.
Most differences in air pressure are caused by the unequal heating of the atmosphere. Convection currents form when an area of the Earth's surface is heated by the sun’s rays.

5. Wind blows from areas of high pressure toward areas of low pressure.
Air over a heated surface expands (becomes less dense) and the air pressure decreases. If the air in a nearby area is not heated as much, it is cooler (denser) and the air pressure is higher. The cooler, denser air flows underneath the warmer, less dense air. This forces the warm air to rise.
area of high pressure
area of low pressure
Wind blows from areas of high pressure
toward areas of low pressure.

6. Measuring Wind Winds are described by their direction and speed.
Wind direction is determined with a wind
vane.
The name of a wind tells you where the wind is coming from.
A south wind blows toward the north.
A north wind blows toward the south.
Wind speed can be measured with an
anemometer.
See page 419 figure 19

8. Wind-Chill Factor
A cool breeze on a warm day feels good, but on cold days the same breeze can make you feel colder.
The wind blowing over your skin removes the body heat.
The stronger the wind, the colder you feel.
The increased cooling a wind can cause is called the wind-chill factor.

9. Land Breeze… (night time)
Local Winds…
are winds that blow over short distances.
are caused by the unequal heating of the Earth’s surface within a small area.
form only when large-scale winds are weak.
Sea Breeze… (day time)
or lake breeze is a local wind that blows from an ocean or a lake. (water takes longer to heat and takes longer to lose its heat) See page 420
Land Breeze… (night time)
is the flow of air from land to a body of water. (land heats faster and cools faster) See page 42o

11. Global Winds…
* are winds that blow steadily from specific directions over long distances.
* are created by the unequal heating of the Earth’s surface. (like local winds)
* occur over a
large area.
(unlike local
winds)

12. Global Convection Currents
Temperature differences between the equator and the poles produce giant convection currents in the atmosphere

13. The Coriolis Effect
If the Earth did not rotate, the winds would blow in a straight line from the poles toward the equator.
Earth's rotation causes long distance winds to curve.
The way Earth’s rotation makes winds
curve is called the Coriolis Effect.
causes the winds in the Northern Hemisphere to turn to the right.
causes the winds in the Southern Hemisphere to turn to the left.

15. Global Wind Belts
Global convection currents and other factors combine to produce a pattern of calm areas and wind belts.
The calm areas include the doldrums and horse latitudes.
Major Wind Belts
Trade Winds
Polar Easterlies
Prevailing Westerlies

16. Regions near the equator with little or no wind are called the doldrums.
At about 30 degrees north or south latitudes, the air stops moving toward the poles and forms another belt of calm air. Long ago sailors ran out of food for their horses in this area so they threw the horses overboard. This area is called the horse latitude.

17. Trade winds
Winds in the Northern Hemisphere between 30 degrees north latitude and the equator generally blow from the northeast. In the southern hemisphere the winds between 30 degrees south latitude and the equator blow from the southeast. Long ago sailors relied on these winds to carry cargo from Europe to the West Indies and South America. As a result of this, the steady easterly winds are called the trade winds.

18. Prevailing Westerlies
In the mid-latitudes, between 30 degrees and 60 degrees north and south, winds that blow toward the poles are turned toward the east. Because they blow from the west to the east, they are called prevailing westerlies. They play an important part in the weather of the U.S.

19. Polar Easterlies
Cold air near the poles sinks and flows toward the lower latitudes and the Coriolis effect shifts the polar winds to the west. This produces the polar easterlies. The polar easterlies meet the prevailing westerlies at about 60 degrees north latitude and 6o degrees south latitude, along the polar front. The mixing of the warm and cold air along the polar front has a major effect on weather in the U.S.

20. Jet streams
A wind that flows in the upper troposphere from west to east over vast distances at great speeds.
about 10 kilometers above the surface
hundreds of kilometers wide but only a few kilometers deep
blow at speeds of 200 to 400 kilometers per hour
Wander north and south along a wavy path

22. Quiz ________ are caused by differences in air pressure.
_________are caused by the unequal heating of Earth’s surface within a small area.
_________ are created by the unequal heating of earth’s surface over a large area.
Major global wind belts are the ____________, the ______________, and the _______________.
Word Bank
Global winds prevailing westerlies winds trade winds local winds
polar easterlies

23. Essential Questions Answered Ch 12 S6
What causes winds?
Differences in air pressure, which are caused by unequal heating of the atmosphere.

24. 2. How do local winds and global winds differ?
E Q Answered Ch 12 S6
2. How do local winds and global winds differ?
Local winds involve a small area and develop only when global winds are weak. Global winds happen over a large area.

25. 3. Where are the major global wind belts located?
EQ Answered Ch 12 S6
3. Where are the major global wind belts located?
Trade winds: between the equator and 30° N and S;
prevailing westerlies: between 30 ° and 60 ° N and S;
polar easterlies: 60 ° N and 60 ° S to the poles.