WIND!WIND!

EARTHEARTH

What Wind DOESN’T do…What Wind DOESN’T do… If the Earth were smaller and did If the Earth were smaller and did

not rotate, warm air would rise in not rotate, warm air would rise in the tropics and travel to the poles the tropics and travel to the poles where it would cool and sink. where it would cool and sink.

Because the Earth is not small, the Because the Earth is not small, the air sinks long before it reaches the air sinks long before it reaches the poles.poles.

A Non-Rotating Earth…A Non-Rotating Earth…

But the Earth doesn’t But the Earth doesn’t stand still!stand still!

Over long distances, wind is also Over long distances, wind is also affected by Earth’s rotation. affected by Earth’s rotation.

The Coriolis effect is a change in the The Coriolis effect is a change in the direction of moving air, water, or direction of moving air, water, or any objects on Earth’s surface due to any objects on Earth’s surface due to its rotation.its rotation.

Now we’re talking!Now we’re talking!

Global Wind SystemsGlobal Wind Systems Wind systems are wide zones of prevailing Wind systems are wide zones of prevailing

winds. winds. Prevailing means the usual direction from which Prevailing means the usual direction from which

a wind blows.a wind blows. Earth has three major wind systems: Earth has three major wind systems:

Sailors created the term Sailors created the term “trade winds”“trade winds” to name the to name the dependable winds that helped them transport and dependable winds that helped them transport and trade goods. trade goods.

The prevailing The prevailing westerlieswesterlies in the northern hemisphere in the northern hemisphere are responsible for much of the weather in North are responsible for much of the weather in North America. America.

The The polar easterliespolar easterlies are cold, polar winds. are cold, polar winds.

Jet StreamsJet Streams Jet Streams are ribbons of extremely fast Jet Streams are ribbons of extremely fast

moving air near the top of the moving air near the top of the troposphere. troposphere.

They are caused by contact between cold They are caused by contact between cold and warm air. and warm air.

They are found at the boundaries between They are found at the boundaries between the polar and temperate zones and the polar and temperate zones and between the temperate and tropical zones. between the temperate and tropical zones.

Wind speeds in a jet stream vary from Wind speeds in a jet stream vary from 100km/h to 300km/h.100km/h to 300km/h.

Jet StreamsJet Streams Jet streams are not visible but sometimes Jet streams are not visible but sometimes

they cause water to condense, forming they cause water to condense, forming clouds that reveal their position. clouds that reveal their position.

Jet StreamsJet Streams The polar jet stream, which affects The polar jet stream, which affects

Canada’s weather, forms at the Canada’s weather, forms at the junction of the prevailing westerlies junction of the prevailing westerlies and polar easterlies.and polar easterlies.

Jet StreamsJet Streams In general, the weather north of a jet In general, the weather north of a jet

stream is cold while the weather stream is cold while the weather south of the jet stream is warmer.south of the jet stream is warmer.

Jet StreamsJet Streams Storms often occur just to the south Storms often occur just to the south

of the polar jet stream because the of the polar jet stream because the warm air is rising, creating the right warm air is rising, creating the right conditions.conditions.

Jet StreamsJet Streams

FrontsFronts At any given time, several air masses At any given time, several air masses

over North America affect the over North America affect the weather in different regions. weather in different regions.

These air masses interact as they These air masses interact as they move. move.

FrontsFronts Perhaps on a sunny day you have looked to the Perhaps on a sunny day you have looked to the

west and have seen an approaching wide band west and have seen an approaching wide band of clouds. This band of clouds would have of clouds. This band of clouds would have indicated the boundary between two air indicated the boundary between two air masses, called a front. masses, called a front.

A front may be several hundred kilometres A front may be several hundred kilometres wide as well as thousands of kilometres long.wide as well as thousands of kilometres long.

FrontsFronts Each air mass has its own Each air mass has its own

temperature and pressure. These temperature and pressure. These conditions change at the front. conditions change at the front.

An approaching front means a An approaching front means a change in the weather, and the change in the weather, and the extent of the change depends on the extent of the change depends on the difference between conditions in the difference between conditions in the air masses. air masses.

FrontsFronts Fronts usually bring precipitation. Fronts usually bring precipitation. Warm air at the front is displaced by Warm air at the front is displaced by

denser cold air. The warm, moist air denser cold air. The warm, moist air rises. As it cools, water vapour in the rises. As it cools, water vapour in the air condenses, forming clouds. air condenses, forming clouds.

Under the right conditions, the Under the right conditions, the condensed water vapour will fall to condensed water vapour will fall to Earth’s surface as precipitation.Earth’s surface as precipitation.

Cold FrontCold Front When cold, dense air When cold, dense air

displaces warm air, it displaces warm air, it forces the warm air, forces the warm air, which is less dense, up which is less dense, up along a steep slope.along a steep slope.

Can bring narrow Can bring narrow bands bands of thunderstorms and of thunderstorms and 

severe weather severe weather

Warm FrontWarm Front Advancing warm air Advancing warm air

displaces cold air along displaces cold air along a warm front, which a warm front, which develops a gradual develops a gradual boundary slope.  boundary slope.  

Rainfall gradually Rainfall gradually increases as the front increases as the front approaches. Forms approaches. Forms lots of foglots of fog

Stationary FrontStationary Front When two air masses meet When two air masses meet

but neither advances, the but neither advances, the boundary between them boundary between them stalls. The resulting stalls. The resulting stationary front often stationary front often occurs between two occurs between two modified air masses that modified air masses that have small temperature and have small temperature and pressure differences. The pressure differences. The air masses can continue air masses can continue moving parallel to the front.moving parallel to the front.

Usually form clouds and Usually form clouds and prolonged precipitation prolonged precipitation

Occluded FrontOccluded Front Sometimes a cold air Sometimes a cold air

mass moves so fast that mass moves so fast that it overtakes a warm it overtakes a warm front, forcing the warm front, forcing the warm air up. As the warm air air up. As the warm air is lifted, the advancing is lifted, the advancing cold air mass collides cold air mass collides with the cold air mass in with the cold air mass in front of the warm front. front of the warm front.

Occluded means Occluded means blocked or obstructed.blocked or obstructed.

A variety of weather can A variety of weather can be expected.be expected.

HomeworkHomework Check your Understanding questionsCheck your Understanding questions

Page 32: # 10, 11, 12Page 32: # 10, 11, 12 Page 34 # 13, 14, 15, 16Page 34 # 13, 14, 15, 16