Air Pressure

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. Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.

What Causes Differences in Air Pressure ? Altitude & Temperature!!!

High altitudes = lower pressure Air pressure can be thought of as the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up. Low altitudes = higher pressure

Barometric pressure goes down. As elevation goes up Barometric pressure goes down. This is an inverse relationship.

Hot air Cold air expands & rises condenses & sinks Create area of low pressure Create area of high pressure land

Which boy will experience greater air pressure ? More Dense Less Dense Cooler Hotter

What's Wrong with Me!

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 Low pressure

Air pressure can be measured with a thermometer.

Air pressure can be measured with a barometer.

Low Pressure High Pressure Low 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's Wrong with Me!

What is wind, and what causes it?

What is wind? Wind is the movement of air, across Earth’s surface, from an area of high pressure towards an area of low pressure. High pressure area Low pressure area

What is wind? Wind is the movement of air, across Earth’s surface, from an area of high pressure towards an area of low pressure. Low pressure area High pressure area

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Okay, so I know what wind IS… But what CAUSES those pressure differences?

Changing Pressure A rising barometer = increasing air pressure. This usually means: Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

Where’d the hula dancer go? Changing Pressure A falling barometer = decreasing air pressure. This usually means: Where’d the hula dancer go? Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.

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. Low Pressure System

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 Concepts The sun heats different areas of the earth by different amounts. This is called differential heating. Warm air expands and is less dense. Less dense air rises in the atmosphere. This creates areas of low pressure As air rises, the pressure on it decreases, and it cools.

Important Wind Concepts Cool air contracts and is more dense. More dense air sinks in the atmosphere. This makes areas of HIGH pressure. Differences in pressure between two areas causes air to move. (from high pressure to low pressure, always!!!!) Air moving across Earth’s surface is wind!

Local Winds Which 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.

Important Wind Concepts The sun heats different areas of the earth by different amounts. This is called differential heating. Warm air expands and is less dense. Less dense air rises in the atmosphere. This creates areas of low pressure As air rises, the pressure on it decreases, and it cools.

Important Wind Concepts Cool air contracts and is more dense. More dense air sinks in the atmosphere. This makes areas of HIGH pressure. Differences in pressure between two areas causes air to move. (from high pressure to low pressure, always!!!!) Air moving across Earth’s surface is wind!

Local Winds Which 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.

Local vs Global Winds Local Winds: Winds that blow over short distances. Caused by unequal heating of Earth’s surface within a small area. Global Winds: Winds that blow steadily from specific directions over long distances. Also caused by unequal heating, but occur over a large area.

SEA BREEZE C D B F WIND A A E High Pressure Low Pressure Land heats faster Water heats slower A E

Where is the best place to fly a kite?? Why? Local Winds Where is the best place to fly a kite?? Why?

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).

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. Low Pressure System

Can you figure out what happens here? Local Winds Animation

Sea Breeze Draw the land, water and Sun. Show circulation by convection. Label the low and high pressure areas. Draw an arrow showing wind direction. Valley Breeze Draw the mountain slope and Sun. Land Breeze Mountain Breeze

Sea Breeze Usually occur during the day. Land heats up faster than water. The air above the land expands, becoming less dense. Cool air over the water moves in to replace the warm air. This creates a sea breeze. Valley Breeze Occur during the day when the air heats over the mountains and rises. The air flows up along the mountains, creating a breeze that comes from the valley. Land Breeze Usually occur at night when air over cooler land flows toward warmer water, pushing the warm air upward. Mountain Breeze Occur at night when air cools high in the mountains and sinks into valleys.