# Air Pressure and Wind Chapter 10 Lesson 6.

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Air Pressure and Wind Chapter 10 Lesson 6

Height and Air Pressure
Air Pressure depends on the weight of the molecules pressing down on a given area. Molecules are denser (or compacted) at sea level, rather than higher in the atmosphere. You could take the same amount of air from sea level and from up in the atmosphere, and the air that is at sea level would be more dense than the air taken from the atmosphere.

Air Pressure and Temperature
When air is heated its molecules begin to speed up and spread out. The same amount of air will weigh less and the pressure will be decreased.

Water Vapor and Air Pressure
Air is composed of nitrogen, oxygen, and other gases. Water Vapor (which is not a gas, but tiny particles of water), weigh less than the gasses in the air. Moist air exerts less pressure than dry air.

What makes wind and How does it Blow!
Think of play dough. When you squish it under your flat palm, the play dough moves to cracks between your fingers, or even tries to squish out from under your hand. Wind is the same concept. Dense Air always tries to find Areas of Less Dense Air. This horizontal movement is called Wind. Air that moves upward is called an updraft. Air that moves downward is called a downdraft.

What is a Convection Cell
A convection cell is a part of the atmosphere where air moves in a circular pattern because of unequal heating and cooling. Isolation (or the suns rays play an important role when it comes to making wind. WHY?)

What Makes a Convection Cell?
The Sun’s rays hit an area, and they transfer energy to the air, heating it up. The warmer the air the less dense it becomes. The warm air rises, creating an updraft. While the warm air rises, denser air (cooler air takes its place) The warm air after it is in the atmosphere starts to cool off and sink. This circular pattern is called a convection cell and also describes how winds blow.

What is a Sea Breeze Happens when…Its DAY TIME.
Air over Land heats up faster than Air over water. The warm air rises and the denser air over the water moves horizontally to the land. A wind that blows from Ocean to Land is called a Sea Breeze. Sea and Land Breezes

What is a Land Breeze Happens when… It is NIGHT TIME.
The Air over the land cools more rapidly than the air over the water. Hot Air over the water rises and cooler air from the land moves horizontally to the sea. A Land Breeze is when Air moves from Land to Sea.

Valley Breezes During the Day, the sun shines on an area of higher elevation. (such as a Mountain) The air on the slope of the mountain will heat up faster than the air at the bottom of the valley floor. The cooler air of the valley flows up the side of the mountain to replace the warming air. This is called a Valley Breeze.

Mountain Breeze A Mountain Breeze happens when it is Night Time.
The valley air is warmer than the side of the Mountain. (this happens because of a rapid cool down) The valley air will rise, and be replaced with a breeze coming from the top of the mountain. Air moving down the slope of a Mountain is called a Mountain Breeze.

What is the Coriolis Effect?
The Coriolis Effect is the curving of the wind due to the rotation of the Earth. The Earth Spins to the right, and so the air in the Northern Hemisphere curves to the right, while the wind in the Southern Hemisphere curves to the Left. The Coriolis Effect also affects the Oceans Currents. Coriolis Effect

Side Note (this will help in upcoming lessons)
In the Northern Hemisphere wind always blows counterclockwise around the center of a low pressure system. The wind will blow clockwise around the center of a high pressure system.

How are Global Wind Patterns Produced?
Year round the Equator is heated by strong sunlight. This causes the air in this region to be warm and moist. It creates a zone of Low Air Pressure around the word, that we know as the Equator. As the air warms it becomes less dense and rises. As it rises it hits the top of the Troposphere and begins to spread out to the North and South and slowly starts to cool down and sink to the surface of the Earth.

Global Wind Patterns At about 30 degrees North and South of the Equator, the warm air that has been in the atmosphere, is now starting to sink and fall down wards. This will create an area of High Air Pressure from 30 degrees North through the Equator to 30 degrees South. Because, the High Air Pressure is seeking out the Low Air Pressure, a belt of wind is set into motion. We call these winds the Trade Winds.

Global Wind Patterns The Poles have very little sunlight and very cold air. Cold air can not hold a lot of water vapor, and because cold air can hold little water vapor, the air at the poles will be very dry air. This dry cold air, gives the poles a high air pressure. The winds that can be found between the 90 and 60 degree mark are called the Polar Easterlies.

Global Winds Between the 60 and 30 degrees lines on a globe, an area of Low air pressure exists. This area also has winds known as the Prevailing Westerly Winds. The United States can be found in this zone.

Isobars Isobars are lines that connect areas of equal air pressure, on a map. Isobars help us to see patterns of air pressure.

Air Pressure Systems High Air Pressure Systems have winds that blow from the center of the system to the outside of the system, in a counter clockwise rotation. Low Air Pressure Systems have winds that blow from the outside to the inside of the system. Be Careful because in the center of a low air pressure system you can find a high air pressure system.

Isobars. When you look at a map that has Isobars on it, you will notice that some lines are closer together than others. Lines that are close together mean that the winds are really strong. However; the further away the lines are from each other the weaker the winds.