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? 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 – mb = in. Hg

Why does air pressure change? 3 factors that affect pressure: –Elevation/Altitude –Temperature –Humidity

Changes in AIR PRESSURE HumidityTemperatureElevation 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 Humidity (the amount of water vapor in air) –Water molecules weigh less than O 2 or N 2 –Water vapor replaces these molecules in humid air As humidity increases, pressure decreases As humidity decreases, pressure increases HUMID AIR DRY 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 Summarize decrease 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?