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?

What is air pressure?

• Air pressure is exerted (pushes) in all directions

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

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– Temperature – Humidity

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 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

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

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.

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

Factors Affecting Wind Direction

• Coriolis Effect

• Pressure Gradients (differences in pressure HIGH vs LOW)

• Friction (resistance)

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

Review Questions

• How can we measure air pressure?

• 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?