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WIND!
EARTH
What Wind DOESN’T do…
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If the Earth were smaller and did not rotate,
warm air would rise in the tropics and travel to
the poles where it would cool and sink.
A Non-Rotating Earth…
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Because the Earth is not small, the air sinks long
before it reaches the poles.
But the Earth doesn’t stand still!
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Over long distances, wind is also affected by
Earth’s rotation.
The Coriolis effect is a change in the direction
of moving air, water, or any objects on Earth’s
surface due to its rotation.
Now we’re talking!
Global Wind Systems
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Wind systems are wide zones of prevailing winds.
Prevailing means the usual direction from which a wind
blows.
Earth has three major wind systems:
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Sailors created the term “trade winds” to name the
dependable winds that helped them transport and trade
goods.
The prevailing westerlies in the northern hemisphere are
responsible for much of the weather in North America.
The polar easterlies are cold, polar winds.
Jet Streams
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Jet Streams are ribbons of extremely fast moving
air near the top of the troposphere.
They are caused by contact between cold and
warm air.
They are found at the boundaries between the
polar and temperate zones and between the
temperate and tropical zones.
Wind speeds in a jet stream vary from 100km/h
to 300km/h.
Jet Streams
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Jet streams are not visible but sometimes they
cause water to condense, forming clouds that
reveal their position.
Jet Streams
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The polar jet stream, which affects Canada’s
weather, forms at the junction of the prevailing
westerlies and polar easterlies.
Jet Streams
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In general, the weather north of a jet stream is
cold while the weather south of the jet stream is
warmer.
Jet Streams
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Storms often occur just to the south of the
polar jet stream because the warm air is rising,
creating the right conditions.
Jet Streams
Fronts
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At any given time, several air masses over North
America affect the weather in different regions.
These air masses interact as they move.
Fronts
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Perhaps on a sunny day you have looked to the west
and have seen an approaching wide band of clouds.
This band of clouds would have indicated the
boundary between two air masses, called a front.
A front may be several hundred kilometres wide as well
as thousands of kilometres long.
Fronts
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Each air mass has its own temperature and
pressure. These conditions change at the front.
An approaching front means a change in the
weather, and the extent of the change depends
on the difference between conditions in the air
masses.
Fronts
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Fronts usually bring precipitation.
Warm air at the front is displaced by denser cold
air. The warm, moist air rises. As it cools, water
vapour in the air condenses, forming clouds.
Under the right conditions, the condensed water
vapour will fall to Earth’s surface as
precipitation.
Cold Front
When cold, dense air
displaces warm air, it forces
the warm air, which is less
dense, up along a steep
slope.
 Can bring narrow bands
of thunderstorms and
severe weather
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Warm Front
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Advancing warm air
displaces cold air along a
warm front, which develops
a gradual boundary slope.
Rainfall gradually increases as
the front approaches. Forms
lots of fog
Stationary Front
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When two air masses meet but
neither advances, the boundary
between them stalls. The
resulting stationary front often
occurs between two modified air
masses that have small
temperature and pressure
differences. The air masses can
continue moving parallel to the
front.
Usually form clouds and
prolonged precipitation
Occluded Front
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Sometimes a cold air mass
moves so fast that it overtakes
a warm front, forcing the
warm air up. As the warm air
is lifted, the advancing cold air
mass collides with the cold air
mass in front of the warm
front.
Occluded means blocked or
obstructed.
A variety of weather can be
expected.
Homework
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Check your Understanding questions
Page 32: # 10, 11, 12
 Page 34 # 13, 14, 15, 16
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