Download Air Pressure & Wind

Air Pressure & Wind
Weather & Water
Investigation 8
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
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.
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.
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.
What causes wind?
1. Differential heating and cooling affects
energy transfer to and from the
atmosphere.
2. Warm air expands and is less dense. Cool
air contracts and is denser.
3. Rising warm air and sinking cool air
produce convection currents.
What causes wind? (cont.)
4. Dense air (cold) produces high pressure areas.
5. Less dense (warm) air produces low pressure
areas.
6. Differences in atmospheric pressure cause air to
move = wind.

Wind – air moving across Earth’s surface
– Air always moves from high to low pressure.
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.
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.
Wind Measuring Tools
 Anemometer – instrument used to measure
wind speed.
 Wind vanes – measure wind direction
 Wind sock – can measure both wind
direction and speed.
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