Chapter 12 Section 6 Wind

Essential Questions Ch 12 S6

1.What causes winds?2.How do local winds and

global winds differ?3.Where are the major

global wind belts located?

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.

What causes wind? The horizontal movement of air

(wind) iscaused 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.

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.

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

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.

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

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)

Global Convection Currents

• Temperature differences between the equator and the poles produce giant convection currents in the atmosphere

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.

• 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

Global Wind Belts

• 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.

Trade windsWinds 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.

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.

Polar EasterliesCold 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.

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

Quiz1. ________ are caused by differences in air

pressure.2. _________are caused by the unequal

heating of Earth’s surface within a small area.

3. _________ are created by the unequal heating of earth’s surface over a large area.

4. Major global wind belts are the ____________, the ______________, and the _______________.

Word BankGlobal winds prevailing westerlies

winds trade winds local winds

polar easterlies

Essential Questions Answered Ch 12 S6

1.What causes winds?Differences in air

pressure, which are caused by unequal heating of the atmosphere.

E Q Answered Ch 12 S62. 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.

EQ Answered Ch 12 S63. 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.