Air Pressure & Wind

What is Air Pressure?

Air pressure is the weight of the atmosphere as it pushes down on the Earth’s surface.

How do we measure air pressure?

Instrument used is called a barometer.

mercury barometer aneroid barometer

What units are used?

INCHES - average (at sea-level) is 30 inches.

MILLIMETERS - average (at sea-level) is 760mm.

MILLIBARS - used by meteorologists

average (at sea-level) 1013 mb.

Factors that Affect Air Pressure

Change in Elevation - the higher the altitude, the lower the air pressure.

Factors that Affect Air Pressure Temperature - as temperature increases

the air pressure decreases (at sea-level).

Why? Air molecules spread out as they heat up.

Factors that Affect Air Pressure

Humidity - The more water vapor the lighter the air.

Why? Water molecules have less mass than nitrogen and oxygen (NO) and pushes the nitrogen and oxygen out of the way.

Dry Air

99% nitrogen & oxygen

Warm Air

97% nitrogen & oxygen

Changes in Air Pressure

Air Pressure changes can be used in forecasting weather (not always accurate).

As air pressure decreases often signals warmer, more humid, along with rain or snow.

As air pressure increase it signals cooler, drier air, and fair weather.

Air Pressure & Wind

Changes in air pressure causes the wind to blow.

What Causes Wind to Blow?

Wind is caused by uneven heating of the Earth’s surface, thus causing changes in air pressure.

Wind moves from an area of high pressure to areas of low pressure.

Winds are named for the direction from which they come.

Analyzing Air Pressure Isobars - points on

a map having the same air pressure.

Close together = fast wind speed.

Far apart = calm wind

High Pressure - Happy

Anti-Cyclone - air moving clockwise away from the pressure center.

Cold air - weighs more Dry air Sunny No precipitation

Symbol indicated on a weather is:

H

Low Pressure - Lousy

Cyclones - air moving counter clockwise toward the pressure center

Warm air Humid/moist air Cloudy Precipitation

Symbol indicated on a weather is:

L

Wind blows from Hi to Lo pressure regions