Air Pressure and WindAir Pressure and Wind

D 34-45D 34-45

How can air pressure change?How can air pressure change?

1.1. VolumeVolume large volume of air means large volume of air means there will be less pressurethere will be less pressure

2.2. Height above Earth’s surfaceHeight above Earth’s surface depends on the weight of the depends on the weight of the molecules pressing down on a given molecules pressing down on a given area; molecules are closer together area; molecules are closer together at sea leavel at sea leavel

• Denser air weighs more than an equal Denser air weighs more than an equal volume of less dense air and pushes volume of less dense air and pushes down harderdown harder

3.3. TemperatureTemperature When air is heated, the When air is heated, the molecules speed up molecules speed up

4.4. Amount of water vaporAmount of water vapor Air is a mix of Air is a mix of nitrogen, oxygen, and other gases; water nitrogen, oxygen, and other gases; water molecules weigh less than oxygen and molecules weigh less than oxygen and nitrogen moleculesnitrogen molecules

• Lighter molecules exert less pressure Lighter molecules exert less pressure because they weigh less because they weigh less

• In moist air lighter water molecules take the In moist air lighter water molecules take the place of heavier oxygen and nitrogen place of heavier oxygen and nitrogen molecules so there is less pressure than dry molecules so there is less pressure than dry airair

Why do winds blow?Why do winds blow?

WindWind air that moves horizontally air that moves horizontally Convection cellsConvection cells the unequal the unequal

heating and cooling of the air often heating and cooling of the air often makes a pattern of rising air, sinking makes a pattern of rising air, sinking air, and windair, and wind– A part of the atmosphere where air A part of the atmosphere where air

moves in a circular pattern because of moves in a circular pattern because of the unequal heating and coolingthe unequal heating and cooling

What are sea and land breezes?What are sea and land breezes?

Sea breezeSea breeze a wind that blows a wind that blows from the sea toward the landfrom the sea toward the land

Land breezeLand breeze a wind that blows from a wind that blows from the land toward the waterthe land toward the water

What is the Coriolis effect?What is the Coriolis effect?

Earth’s rotation affects winds blowing Earth’s rotation affects winds blowing across its surfaceacross its surface

The Earth rotates clockwise from the The Earth rotates clockwise from the North Pole; the same thing happens North Pole; the same thing happens with the winds blowing from the with the winds blowing from the North Pole; because the Earth rotates North Pole; because the Earth rotates the winds seem to curve to the right the winds seem to curve to the right as they head southwardas they head southward

No matter which way the wind blows No matter which way the wind blows it will curve to the right in the it will curve to the right in the Northern hemisphere (This is the Northern hemisphere (This is the Coriolis effect)Coriolis effect)

In the Southern hemisphere it causes In the Southern hemisphere it causes the winds to curve to the leftthe winds to curve to the left

How are global wind patterns How are global wind patterns produced?produced?

Year round the equator is being heated Year round the equator is being heated strongly by the sunlight; this causes the strongly by the sunlight; this causes the air to be very warm; it also caused air to be very warm; it also caused evaporation, which leads to moist airevaporation, which leads to moist air warm moist air over the equator creates a warm moist air over the equator creates a zone of low pressure around the globezone of low pressure around the globe

As this air warms it becomes less dense As this air warms it becomes less dense and starts to rise; as it rises it begins to and starts to rise; as it rises it begins to spread out north and southspread out north and south

At about 30 degrees north and sound At about 30 degrees north and sound latitudes it begins to cool and sink latitudes it begins to cool and sink towards the surface; this causes a towards the surface; this causes a high pressure zone on both sides of high pressure zone on both sides of the equatorthe equator– This causes a belt of winds to be set in This causes a belt of winds to be set in

motion around the Earth by air moving motion around the Earth by air moving from the high pressure zones towards from the high pressure zones towards low pressure at the equator. These are low pressure at the equator. These are know as trade windsknow as trade winds

The poles get very low angle sunlight and The poles get very low angle sunlight and the air is very coldthe air is very cold– This air is cold and dry so it has high pressureThis air is cold and dry so it has high pressure

Air at the poles moves towards 60 degrees Air at the poles moves towards 60 degrees latitude forming winds called easterly latitude forming winds called easterly winds (winds that blow from the east)winds (winds that blow from the east)

Other winds occur between 60 degrees Other winds occur between 60 degrees and 30 degrees latitudes called westerly and 30 degrees latitudes called westerly winds (winds that blow from the west) winds (winds that blow from the west) (The US is in this zone)(The US is in this zone)

What are isobars?What are isobars?

Knowing where the air pressure is Knowing where the air pressure is high or low allows you to predict high or low allows you to predict which way the air will movewhich way the air will move– Meteorologists start by plotting the air Meteorologists start by plotting the air

pressure at many different locations pressure at many different locations then connect all the places with the then connect all the places with the same air pressure with a linesame air pressure with a line

– A line on a map connecting places with A line on a map connecting places with equal air pressure is called equal air pressure is called isobarisobar

High-pressure systems (H) have a High-pressure systems (H) have a center of higher pressure than the center of higher pressure than the surrounding area and the wind in the surrounding area and the wind in the center blows clockwisecenter blows clockwise

Low-pressure systems (L) the center Low-pressure systems (L) the center is surrounded by higher pressure and is surrounded by higher pressure and the winds in the center blow the winds in the center blow counterclockwisecounterclockwise

How do we study the wind?How do we study the wind?

1.1. Weather vaneWeather vane has a pointer that blows has a pointer that blows around in the wind; the pointer is around in the wind; the pointer is mounted to it can point to the different mounted to it can point to the different compass markingscompass markings

2. Anemometer2. Anemometer a a series of cups series of cups mounted on a mounted on a shaft that can shaft that can spin freely; when spin freely; when the wind blows the wind blows against the cups against the cups they spin like a they spin like a pinwheel (used to pinwheel (used to measure wind measure wind speed)speed)

3. Beaufort Scale3. Beaufort Scale created in 1805 by Sir created in 1805 by Sir Francis Beaufort of the British Navy; a way Francis Beaufort of the British Navy; a way to measure wind speed by observing it’s to measure wind speed by observing it’s affect on the seaaffect on the sea