Air Pressure & WindAir Pressure & Wind

Weather & WaterWeather & Water

Investigation 8Investigation 8

What is air pressure?What is air pressure?

Atmospheric (Air) Pressure Atmospheric (Air) Pressure – the force – the force applied by the air above youapplied by the air above you

BarometerBarometer – tool used to measure air – tool used to measure air pressurepressure– Common units used are millibars or inchesCommon units used are millibars or inches

Pressure in a Jar LabPressure in a Jar Lab What happened to the water level when you What happened to the water level when you

squeezed the jar?squeezed the jar? The water level went down because as you The water level went down because as you

squeezed the jar, you increased the air squeezed the jar, you increased the air pressure which forced the liquid water down. pressure which forced the liquid water down.

Squeezing the jar increases the pressure, Squeezing the jar increases the pressure, increases the density, decreases the volume, increases the density, decreases the volume, but mass stays the same because air cannot go but mass stays the same because air cannot go in or get out.in or get out.

When you released the jar, the water level went When you released the jar, the water level went up to fill the vacuum that was formed.up to fill the vacuum that was formed.

Relationship Between Altitude & Air Relationship Between Altitude & Air PressurePressure

As you increase in altitude, the air pressure As you increase in altitude, the air pressure decreases.decreases.

The closer you are to Earth’s surface, the The closer you are to Earth’s surface, the more air pressure.more air pressure.

How does temperature affect How does temperature affect pressure?pressure?

Cold air is more dense and sinks, creating Cold air is more dense and sinks, creating areas of high pressure.areas of high pressure.

Warm air is less dense and rises, creating Warm air is less dense and rises, creating areas of low pressure.areas of low pressure.

What causes wind?What causes wind?

1.1. Differential heating and cooling affects Differential heating and cooling affects energy transfer to and from the energy transfer to and from the atmosphere.atmosphere.

2.2. Warm air expands and is less dense. Cool Warm air expands and is less dense. Cool air contracts and is denser.air contracts and is denser.

3.3. Rising warm air and sinking cool air Rising warm air and sinking cool air produce convection currents.produce convection currents.

What causes wind? (cont.)What causes wind? (cont.)

4. Dense air (cold) produces high pressure areas.4. Dense air (cold) produces high pressure areas.

5. Less dense (warm) air produces low pressure 5. Less dense (warm) air produces low pressure areas.areas.

6.6. Differences in atmospheric pressure cause air to Differences in atmospheric pressure cause air to move = wind.move = wind.

WindWind – air moving across Earth’s surface – air moving across Earth’s surface– Air always moves from high to low pressure.Air always moves from high to low pressure.

Local WindsLocal Winds

Sea breezesSea breezes – usually occur during the day. – usually occur during the day. Land heats up and cools faster than water. Land heats up and cools faster than water. The air above the land expands, becoming The air above the land expands, becoming less dense. Cool air over the water moves less dense. Cool air over the water moves in to replace the warm air.in to replace the warm air.

Land breezesLand breezes – usually occur at night when – usually occur at night when air over cooler land flows toward warmer air over cooler land flows toward warmer water, pushing the warm air upward.water, pushing the warm air upward.

Local WindsLocal Winds

Valley breezesValley breezes – occur during the day when – occur during the day when air heats over the mountains and rises. The air heats over the mountains and rises. The air flows up along the mountains, bringing air flows up along the mountains, bringing air from the cooler, denser valley air.air from the cooler, denser valley air.

Mountain breezesMountain breezes – occurs at night. Air – occurs at night. Air high in the mountains cools faster than the high in the mountains cools faster than the valleys, therefore, the cooler, denser, valleys, therefore, the cooler, denser, mountain air sinks toward the warmer, less mountain air sinks toward the warmer, less dense valley air.dense valley air.

Wind Measuring ToolsWind Measuring Tools

AnemometerAnemometer – instrument used to measure – instrument used to measure wind speed.wind speed.

Wind vanesWind vanes – measure wind direction – measure wind direction Wind sockWind sock – can measure both wind – can measure both wind

direction and speed.direction and speed.

Air Pressure on Weather MapsAir Pressure on Weather Maps

IsobarsIsobars – lines on a weather map that – lines on a weather map that connect locations with equal air pressure. connect locations with equal air pressure. Shown in millibars (mb)Shown in millibars (mb)

H = high pressure areaH = high pressure area L = low pressure areaL = low pressure area Winds will flow from the H areas Winds will flow from the H areas L areas L areas