1. Weather & Water Investigation 8
Air Pressure & Wind
Weather & Water
Investigation 8

2. What is air pressure?
Atmospheric (Air) Pressure – the force applied by the air above you
Barometer – tool used to measure air pressure
Common units used are millibars or inches

3. Pressure in a Jar Lab
What happened to the water level when you squeezed the jar?
The water level went down because as you squeezed the jar, you increased the air pressure which forced the liquid water down.
Squeezing the jar increases the pressure, increases the density, decreases the volume, but mass stays the same because air cannot go in or get out.
When you released the jar, the water level went up to fill the vacuum that was formed.

4. Relationship Between Altitude & Air Pressure
As you increase in altitude, the air pressure decreases.
The closer you are to Earth’s surface, the more air pressure.

5. How does temperature affect pressure?
Cold air is more dense and sinks, creating areas of high pressure.
Warm air is less dense and rises, creating areas of low pressure.

6. What causes wind?
Differential heating and cooling affects energy transfer to and from the atmosphere.
Warm air expands and is less dense. Cool air contracts and is denser.
Rising warm air and sinking cool air produce convection currents.

7. What causes wind? (cont.)
4. Dense air (cold) produces high pressure areas.
5. Less dense (warm) air produces low pressure areas.
Differences in atmospheric pressure cause air to move = wind.
Wind – air moving across Earth’s surface
Air always moves from high to low pressure.

8. Local Winds
Sea breezes – usually occur during the day. Land heats up and cools faster than water. The air above the land expands, becoming less dense. Cool air over the water moves in to replace the warm air.
Land breezes – usually occur at night when air over cooler land flows toward warmer water, pushing the warm air upward.

9. Local Winds
Valley breezes – occur during the day when air heats over the mountains and rises. The air flows up along the mountains, bringing air from the cooler, denser valley air.
Mountain breezes – occurs at night. Air high in the mountains cools faster than the valleys, therefore, the cooler, denser, mountain air sinks toward the warmer, less dense valley air.

10. Wind Measuring Tools
Anemometer – instrument used to measure wind speed.
Wind vanes – measure wind direction
Wind sock – can measure both wind direction and speed.

11. Air Pressure on Weather Maps
Isobars – lines on a weather map that connect locations with equal air pressure. Shown in millibars (mb)
H = high pressure area
L = low pressure area
Winds will flow from the H areas  L areas