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Weather
Lesson 1: How Does Uneven
Heating of Earth Affect Weather?
California Fast Fact!
• What is unusual about the
photo and the month in
which it was taken?
•What clue do the trees
give you about the height
of the snow?
Mount Lassen, California
This picture was taken in June
•Where and when would
you expect to see other
scenes like this? Why?
California Fast Fact!
• The area usually gets
about 1m (39in) of snow
per month from December
to March.
•Most of this snow melts in
the spring and summer.
•Record snowfalls on the
mountain one winter left
these GIANT snowdrifts.
CA State Standards
 4.a – Students know uneven heating of Earth causes air
movements (convection currents)
 4.e – Students know that the Earth’s atmosphere exerts a
pressure that decreases with distance above Earth’s
surface and that at any point it exerts this pressure equally
in all directions.
Valuable Vocabulary!
 Atmosphere – The blanket of air surrounding Earth.
Valuable Vocabulary!
 Weather – The condition of the atmosphere at a certain
place and time.
Valuable Vocabulary
 Air Pressure – The weight of the atmosphere pushing on a
given square unit area of the Earth’s surface.
Valuable Vocabulary
 Convection Current – The upward and downward
movement of air in the atmosphere.
Valuable Vocabulary
 Local Wind – Wind that results from local changes in
temperature.
Valuable Vocabulary
 Prevailing Wind – A global wind that blows constantly
from the same direction.
The Atmosphere
 A blanket of air called the atmosphere surrounds Earth.
 It is very thin compared to the size of Earth.
 If Earth were a peach, it would be thinner than the fuzz on the
peach!
 The atmosphere is made up of several layers.
 1st layer – closest to the Earth’s surface: troposphere
The Atmosphere
The Atmosphere
 The Troposphere contains about 90% of the atmosphere’s
gases.
 Most of the Earth’s weather happens in the troposphere.
 Weather is the condition of the atmosphere at a certain
place and time.
The Atmosphere
 You can’t see, taste, or touch the air in the atmosphere, but you
can feel it.
 Every time the wind blows, you feel the air.
 You can feel it because air has mass.
 Everything on Earth that has mass is pulled toward Earth’s
center by gravity.
 This pull causes air to have weight.
 Air particles closer to Earth’s surface have more weight than the air
higher up.
 This means that air pressure is greatest at sea level.
The Atmosphere
 Air density is another factor that causes air pressure to be
greater at sea level.
 Gravity pulls most of the air in the atmosphere down to the
surface.
 This causes the air closer to Earth’s surface to become much
denser, or heavier, than the air farther away from the
surface.
Show What You Know
In which layer of the atmosphere
does most weather occur?
Show What You Know
In which layer of the atmosphere
does most weather occur?
The troposphere is the layer of the
atmosphere where most weather
occurs.
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 What is the relationship between
position in the atmosphere and air
pressure?
Show What You Know
 What is the relationship between
position in the atmosphere and air
pressure?
The lower the position, the higher the
air pressure.
Show What You Know
Write your own summary of what
you learned in class today about
the atmosphere.
Summary
The earth is covered with a blanket called the atmosphere.
The atmosphere is very thin, and is made up of several layers.
The layer closest to Earth’s surface is called the troposphere.
This layer contains about 90% of of the atmosphere’s gases, and
it also is where most of Earth’s weather happens.
You can not see, taste or touch air, but you know it is there
ever time the wind blows. You can feel air. This is because air
has mass. Gravity pulls air particles down, and particles closer
to the surface have more weight than the air higher in the
atmosphere.
Air Pressure
 The higher you go into the atmosphere, the less air there
is above you.
 Air does not only push downwards, but it actually pushes
evenly in all directions.
 Gas molecules are constantly moving in all directions.
 When you blow up a balloon, the air pressure inside the balloon
increases, and it expands in all directions.
Air Pressure
 Temperature also affects air pressure.
 Cold air is denser than warm air.
 It sinks towards Earth’s surface.
 The cold air forces warmer, less dense air to rise.
 As the warm air rises, it begins to cool.
 Since cold air is denser, it sinks back down to Earth.
 High Pressure = More dense, cold air sinks.
 Low Pressure = Less dense, warm air rises.
Show What You Know
 What factors affect air pressure?
Show What You Know
 What factors affect air pressure?
Some factors that affect air pressure
are: location in the atmosphere
(altitude), temperature, and density.
Show What You Know
 How does the outside air pressure on
the left and right side of a basketball
compare?
Show What You Know
 How does the outside air pressure on
the left and right side of a basketball
compare?
The air pressure is identical on all sides.
Summarize
 Summarize what you learned in class today about air
pressure.
Uneven Heating and Local Winds
 How did the air feel when you left home
this morning?
 Was it windy, or was it calm?
 If you were to go outside right now, would
the air feel the same?
Uneven Heating and Local Winds
 THE BIG IDEA: Air is always moving and changing.
 When the sun’s energy reaches Earth, some of the energy
bounces off objects, such as clouds.
 Earth absorbs the rest of the energy.
 Different types of surfaces absorb different amounts of
energy.
Uneven Heating and Local Winds
 Soil heats up faster and cools off faster than an equal
amount of water.
 During a hot day, soil is hotter than water.
 It gives off more heat.
 Because of this, the air over soil tends to be hotter too.
 Water is cooler than soil, so the air over water tends to be
cooler.
Uneven Heating and Local Winds
 Cool air is denser than warm air, so it sinks.
 Air that is warm is less dense, so it is pushed upward by
the cooler air, denser air.
 The upward and downward movement of air is called
convection currents.
Uneven Heating and Local Winds
 Air in the troposphere moves horizontally.
 As cool, dense air sinks; it spreads out along the surface.
 Air at the surface moves from areas of higher pressure to
areas of lower pressure.
 This horizontal movement of air is called wind.
Uneven Heating and Local Winds
 Winds can be local, affecting small areas, or global,
affecting large parts of the Earth.
 Sometimes, places in the same general area have slightly
different temperatures.
 This produces local wind, or wind that results from a local
difference in temperature.
Uneven Heating and Local Winds
 Local winds often occur along lakeshores or seashores.
 During the day, the air over the land is warmer than the air
over the water.
 The cooler air over the water has a higher pressure than the
warmer air over the land.
 The result is a wind that blows from the water toward the land.
 This is called a sea breeze.
Sea Breeze Explained
Uneven Heating and Local Winds
 At night, the wind usually blows in the opposite direction.
 Land cools off more quickly than water.
 Once the land becomes cooler than the water, a wind blows
from the land toward the water.
 This is called land breeze.
Show What You Know
 How does the absorption of energy from
the suns rays differ for land and for water?
Show What You Know
 How does the absorption of energy from
the suns rays differ for land and for water?
 The land absorbs the sun’s energy more
quickly than water does.
Show What You Know
 What happens to the temperature of
the land compared to that of water?
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 What happens to the temperature of
the land compared to that of water?
 The land becomes warmer during the
day than water does.
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What occurs when cold air sinks?
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What occurs when cold air sinks?
What is the horizontal movement
of air called?
It forces warm air up. The
horizontal movement is called
wind.
Show What You Know
 What is sea breeze? When Does it
happen? What causes it?
Show What You Know
 What is sea breeze? When Does it
happen? What causes it?
 A sea breeze is a wind from the sea
toward the land. During the day, the
land warms up faster than the sea. The
cooler, denser sea air flows under the
warmer air over the land.
Show What You Know
Why is air always moving?
Show What You Know
 Why is air always moving?
 Earth’s surface is heated unevenly,
causing some air to be hotter than
other air. Hot air is forced upward by
sinking colder air, leading to air
movement.
Summarize
 Summarize what you learned in class today about uneven
heating of the Earth’s surface.
Global Winds
 Global winds are prevailing winds that almost always blow
from the same direction.
 Sailors relied heavily on prevailing winds to carry them across
the oceans.
 Uneven heating of Earth’s surface causes prevailing winds.
 Unlike local winds, prevailing winds result from uneven
heating of large parts of Earth’s surface
Global Winds
 An area’s distance from the equator, also known as
latitude, affects the amount of heat that the area will
receive from the sun.
 Earth’s poles receive indirect sunlight, or no sunlight at all, so
they are always cold.
 Earth’s equator gets direct sunlight all year, so it’s always
warm.
 As a result, cold air above the poles sinks and moves towards
the equator.
Global Winds
 Air does not just move in one big circle.
 Air moves in curved paths, because of Earth’s rotation.
 This rotation causes prevailing winds.
 Winds moving north curve to the east.
 Winds moving south curve to the west.
 Around the equator this curving causes trade winds to blow
from the east.
 In the United States, this curving causes the prevailing
westerlies to blow from the west.
Global Winds
Global Winds
 The prevailing westerlies cause most of the weather
systems in the United States to from west to east.
 This helps forecasters predict the weather, because
weather in San Francisco today often moves toward
Kansas City tomorrow and then toward Baltimore.
Show What You Know
How does the air in the atmosphere
vary with latitude?
Show What You Know
How does the air in the atmosphere
vary with latitude (distance)?
In general, with increases in latitude
come decreases in air temperature.
Show What You Know
In what direction do winds that blow
toward the poles curve?
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In what direction do winds that blow
toward the poles curve?
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In what direction do winds that blow
away from the poles curve?
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In what direction do winds that blow
away from the poles curve?
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In what direction do most of the
prevailing winds in the United States
curve?
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In what direction do most of the
prevailing winds in the United States
curve?
Most of the prevailing winds in the
United States curve east.
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Why do global winds move in curved
paths?
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Why do global winds move in curved
paths?
Earth’s rotation causes winds to curve
as they blow northward or southward.
Show What You Know
What two factors cause prevailing
winds?
Show What You Know
What two factors cause prevailing
winds?
Earth’s rotation and uneven heating of
the atmosphere are two factors that
cause prevailing winds.
Summarize
 Summarize what you learned in class
today about global winds.