Air Pressure and Wind

What is air pressure?

• The force exerted by air molecules

as they collide with a surface• The weight of the atmosphere as it

pushes on Earth’s surface

What is air pressure?

kgkg

• Air has mass• Gravity pulls these

molecules towards Earth giving them weight

• The weight of the molecules is air pressure

What is air pressure?

• At sea level the weight of air pushing down is 14.7 pounds per square inch

• The weight on the roof of a typical house is about 2.1 million kg (4.6 million lbs)

Why don’t all of our houses collapse? Or Why aren’t we all squished?

CHECK THIS OUT!!!Extreme Atmospheric Pressure

What is air pressure?

• Air pressure is exerted (pushes) in all directions

• An equal force pushing down is balanced by air pushing up

What Is Air Pressure?

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1. The same thing as gravity

2. The weight of the atmosphere pushing downward

3. A form of pollutant

Why Don’t We Get Crushed By The Pressure Of The Atmosphere?

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1. The air is not that heavy

2. Air pressure is exerted in all directions, and we are in equilibrium

3. The water in our bodies prevents it

Why Did The Oil Drum Collapse?

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1. The water weakened it

2. The atmosphere suddenly got heavier

3. The cooling vapor took up less space than before and created a drop in pressure inside. The regular air pressure then crushed the drum.

Measuring air pressure

• The barometer is the instrument used to measure air pressure

• 2 types of barometers– Mercury– Aneroid

Mercury Barometer

Measuring air pressure

• As the weight of the air increases, the mercury rises

Measuring air pressure

• As the weight of the air decreases, the mercury lowers

Measuring air pressure

The Aneroid barometer• A capsule with most of

the air removed changes size as the pressure outside of it increases or decreases

• This moves the needle

Measuring air pressure

• Pressure will be measured in– Mm Hg– Inches of Hg– Milibars

• Average Pressure at sea level– 1013.25 mb = 29.92

in. Hg

Why does air pressure change?

• 3 factors that affect pressure:

– Elevation/Altitude (p414)– Temperature (p416)– Humidity (p416)

Changes inAIR PRESSURE

Humidity Temperature Elevation

Complete the graphic organizer by filling in the 3 factors that affect pressure. Then create branches explaining how they cause pressure to change.

Changes in air pressure

• Altitude

(height above sea level)

As the altitude increases pressure decreases

As the altitude decreases pressure increases

Changes in air pressure

Altitude Drawing

High Altitude / Low Pressure

Low Altitude / High Pressure

Changes in air pressureHumidity

(the amount of water vapor in air)

– Water molecules weigh less than O2 or N2

– Water vapor replaces these molecules in humid air

As humidity increases, pressure decreases

As humidity decreases, pressure increasesHUMID AIRDRY AIR

Changes in Air Pressure

Humidity Drawing

Low Humidity / High PressureHigh Humidity / Low Pressure

Changes in air pressure

Temperature– In warm air the molecules are far apart.– In cool air the molecules are closer together

As air temperature increases, pressure decreases

As air temperature decreases, pressure increases

Changes in air pressure

Temperature Drawing

High Temperature / Low Pressure Low Temperature / High Pressure

Let’s SummarizeDecrease Pressure Increase Pressure

Temperature

Altitude

Humidity

Low Pressure

Generally associated with stormy weather.

The air is rising.

High Pressure

Generally associated with clear skies.

Air is sinking.

What does a barometer measure?

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1. Atmospheric pressure

2. The humidity of the air

3. Air quality

4. How much it has rained

What affects air pressure?

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1 2 3 4

1. Altitude

2. Humidity

3. Temperature

4. All of the above

Why is water vapor less dense than air?

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1 2 3

1. There is less of it

2. H2O has less mass than O2 or N2

3. It is hotter, so it has less mass

Low pressure is usually associated with…

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1 2 3 4

1. Sunny skies

2. Deserts

3. Earthquakes

4. Stormy weather and rising air

High pressure is usually associated with…

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0% 0%0%0%

1. Rain

2. Tornadoes

3. Sunny, dry weather with sinking air

4. Pushy car salesmen

Factors Affecting Wind Direction and Speed

What makes the wind blow?

• Air moves from

HIGH PRESSURE to LOW PRESSURE

• This movement of air is WIND

What makes the wind blow?

• Island Example

Factors Affecting Wind Direction

• Coriolis Effect Coriolis-2.mp4

• Pressure Gradients (differences in pressure HIGH vs LOW)

• Friction (resistance)

The Coriolis Effect

• An object moving over Earth’s surface will curve away from its intended path

• This is due to Earth’s rotation– Faster at the equator– Slower at the poles

The Coriolis Effect

• Guidelines (p419):

– objects are deflected to the RIGHT in the Northern Hemisphere and the LEFT in the Southern Hemisphere

– The effect (curve) increases as the speed increases– The effect DOES NOT depend on the objects direction– The effect is greater at the poles and less at the equator– Most noticeable of large distances (Not in TOILETS)

• Coriolis-1.mp4

The Coriolis Effect

• Let’s Draw!

Pressure Gradients

• Pressure gradient:– A difference in

pressure from one place to another

• Remember wind blows from HIGH to LOW

– The stronger the gradient the faster the wind

– The faster the wind the more curve from the Coriolis Effect

Friction

• Friction between the air and the ground slows wind.

• The higher above the surface the faster the wind will blow

• The higher above the surface the more curve due to Coriolis

Friction• Little friction in the upper

troposphere allows a jet stream to from moving air west to east.– The jet stream supplies

energy to storms and directs the path of weather across the United States

– Affects airplanes as they travel coast to coast

Jet Stream

Review Questions

• What are the 3 factors that affect wind direction?• What are the 3 factors that affect air pressure?• How does a pressure gradient affect the speed

of wind? Direction of wind?• Why does the Coriolis Effect cause wind to

deflect?• Can you witness the Coriois Effect in your

bathroom?• How does friction affect wind speed?

What are the 3 factors that affect wind direction?

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1. Altitude, Humidity, and Temperature

2. Altitude, Humidity, and Coriolis Effect

3. Coriolis, Pressure, Gradients, and Friction

What are the 3 factors that affect air pressure?

1 2 3

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1. Altitude, Humidity, and Temperature

2. Coriolis, Pressure, Gradients, and Frction

3. Sunlight, Convection, and Friction

How does a pressure gradient affect the speed of wind?

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1. The stronger the gradient, the weaker the wind

2. The stronger the gradient, the stronger the wind

3. It does not effect the speed

Why does the Coriolis Effect cause wind to deflect?

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1. The wind was already curving; the Coriolis Effect only effects airplanes

2. The spinning of the Earth causes the path to curve

How does friction affect wind speed?

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1. Objects or rough surfaces cause a drag on the wind

2. Friction makes the wind faster

3. Friction creates wind

Global and Local Winds

Global Winds • Global winds show the general direction the air is flowing throughout the globe.

• Refer to your worksheet for these winds

• The winds are named according to where the wind comes from or originates– Ex. What would the name

of this wind be?

Global WindsClimate Zones due to

global winds:• Areas of low pressure

are characterized by rising air, precipitation = Rainforest (0, 60)

• Areas of high pressure are characterized by sinking air, dry air = Deserts (30, 90)

** Add these areas to your worksheet

Continental and Local Winds

• These winds can go opposite the expected global winds due to changes in local conditions:– Continental Winds

• Monsoon

– Local Winds• Sea breeze

• Land breeze

• Santa Ana Winds

Monsoon

• A Monsoon is a seasonal change in winds.

• Ex. India, North America

Sea Breeze: Rising Air, Sinking Air, High Pressure, Low Pressure, Dense Air, Less Dense Air, Cloud Formation

Land Breeze: Rising Air, Sinking Air, High Pressure, Low Pressure, Dense Air, Less Dense Air,

Santa Ana Winds

• Occur in Southern California in the late fall/early winter

• A high pressure forms as the desert regions

• As the air sinks it is compressed and usually warms.

Santa Ana Winds

• This creates an offshore wind (blows towards the ocean)

• The air is very dry and often wildfires are a result of this wind

Where will you find areas of high pressure on Earth?

1 2 3 4

28%

8%

44%

20%

1. North and South Pole Only

2. 0 and 60 Degrees Latitude

3. 30 and 90 Degrees Latitude

4. Over the Ocean

Where will you find areas of low pressure on Earth?

1 2 3 4

5% 5%5%

86%1. North and South

Pole Only

2. 0 and 60 Degrees Latitude

3. 30 and 90 Degrees Latitude

4. Over the Ocean

What Creates Santa Ana Winds?

1 2 3

58%

33%

8%

1. High Pressure Over Land and Low Pressure Over Ocean

2. Low Pressure Over Land and High Pressure Over Ocean

3. Politicians In Santa Ana