Before you leave today, you will know:

• What is air pressure?

• Why does air have pressure?

• What causes differences in air pressure?

What is Air Pressure ?

• Pressure is the force with which something pushes on something else

• AIR pressure is the force with which AIR presses on everything

Why does air have pressure?

The air is made up of molecules.

Air molecules are everywhere.

Air pressure is equal in all directions.

What Causes Differences in Air

Pressure ?• Altitude & Temperature!!!

High altitudes = lower pressure

Low altitudes = higher pressure

As elevation goes up

Barometric pressure goes down.

This is an inverse relationship.

Hot air Cold air

Create area of low pressure

Create area of high pressure

expands & rises condenses & sinks

land

Less DenseMore DenseHotterCooler

Which boy will experience greater air pressure ?

Air moves from areas of low pressure towards areas of high pressure.

Air moves from areas of high pressure towards areas of low pressure.

These arrows show how air pressure pushes on an object, such as a book.

“I can’t feel air pressure, because it isn’t very

strong”

“I can’t feel air pressure, because air presses equally on all

parts of my body.”

High pressure

Lowpressure

Air pressure can be measured with a

thermometer.

Air pressure can be measured with a

barometer.

Low Pressure High PressureLow Density High Density

Air pressure is only outside, where

the air is.

Air pressure is everywhere, because

air is everywhere.

The End !!! Thanks for playing

What is wind,

and what causes it?

What is wind?

High pressurearea

Low pressurearea

Wind is the movement of air, across Earth’s surface, from an area of high pressuretowards an area of low pressure.

What is wind?

High pressurearea

Low pressurearea

Wind is the movement of air, across Earth’s surface, from an area of high pressuretowards an area of low pressure.

H

H

HL

L

L

L

HL

Okay, so I know what wind IS…

But what CAUSES those pressure differences?

Changing PressureA rising barometer = increasing air pressure.

This usually means:

Changing Pressure

A falling barometer = decreasing air pressure.

This usually means:

Where’d the hula dancer

go?

Low Pressure System

If air is given more heat, molecules get more energy and move apart. That means fewer molecules are occupying the space making the air less dense. Then, the air rises, and air pressure decreases. As the air rises, it expands, then cools. Cooler air can’t hold the water, so water vapor molecules condense and form clouds.Therefore, low pressure means clouds and precipitation.

Convection Currents

High Pressure System

When cooler, denser, drier air starts sinking, it causes pressure to increase.

Molecules also starts to warm up. Warm air is able to hold more water vapor. The water vapor molecules that are left are able to evaporate.

Therefore, high pressure means fair weather.

When air pressure increases, precipitation decreases.

Important Wind ConceptsA. The sun heats different areas of the earth

by different amounts. This is called differential heating.

B. Warm air expands and is less dense. Less dense air rises in the atmosphere. This creates areas of low pressure

C. As air rises, the pressure on it decreases, and it cools.

Important Wind ConceptsD. Cool air contracts and is more dense.

More dense air sinks in the atmosphere. This makes areas of HIGH pressure.

E. Differences in pressure between two areas causes air to move. (from high pressure to low pressure, always!!!!)

F. Air moving across Earth’s surface is wind!

Local WindsWhich absorbs and releases solar energy faster, land

or water?Land. Water is SLOW!

How does energy transfer to and from the air in the atmosphere?Conduction between air molecules and land.

What happens to air when it is heated?It expands and becomes less dense.

What happens to air pressure over the land when air warms and expands?The air pressure over the surface decreases. A local

low-pressure area forms.

Land heatsfaster

Water heatsslower

Low Pressure

B

A A

C

High Pressure

D

E

WINDF

SEA BREEZE

Water and the Atmosphere

Water is in the atmosphere in form of liquid(rain, mist, fog, clouds) or solid (hail, sleet, snow).

Water is part of the atmosphere in form of gas (water vapor), which resupplies the water around the globe via the water cycle.

Water vapor can range from almost 0 – 4% of the volume of atmospheric gases with a higher % in the tropics, and a lower % in polar regions.

Water vapor is a greenhouse gas.

Humidity Humidity is the water vapor in the air.Relative humidity is the amount of water vapor

in the air compared to the amount of water vapor needed to saturate the air.

Warmer air can hold more vapor, so it takes a greater amount to saturate.

Colder air holds less water vapor, so it takes a smaller amount to saturate…which is why you see your breath condense on a cold day.

Lots of water vapor in the air leads to condensation and precipitation.

Dew Point Dew point is the temperature at which

air is saturated (100% relative humidity). When the temperature drops just below dew point,

condensation occurs on a surface.Warmer air means the dew point is higher, like in the

tropics. (The temperature only has to drop a few degrees before condensation occurs because there is a large amount of water vapor in the air).

Cooler air means the dew point is lower, like in the polar regions. (The temperature has to drop much lower before condensation occurs because there is very little water vapor in the air).

Low Pressure System

If air is given more heat, molecules get more energy and move apart. That means fewer molecules are occupying the space making the air less dense. Then, the air rises, and air pressure decreases. As the air rises, it expands, then cools. Cooler air can’t hold the water, so water vapor molecules condense and form clouds.Therefore, low pressure means clouds and precipitation.