1

1

Atmospheric Circulation: Key Ideas

Different amounts of solar energy areabsorbed at different latitudes.

Interaction of the ocean and atmospheremoderates surface temperature, shapesEarth’s weather and climates, and createswaves and currents.

Earth’s rotation causes the Coriolis effect, theapparent curvature of the path of moving air.

2

Composition and Properties of theAtmosphere

The lower atmosphere is a fairly homogeneousmixture of gases comprised mostly of N2 and O2.

Water vapor may comprise up to 4% of thevolume of the atmosphere.

The density of air is affected primarily bytemperature.

2

3

Atmospheric circulation is powered by sunlight. Since Earth is in thermalequilibrium, what assumption can be made about the input and output ofheat on Earth?

Heat Budget of Earth

4

3

5

Earth’s Energy Balance

Energy emitted = energy absorbed

Energy absorbed = incident radiation - reflectedsunlight

Earth’s thermalenergy (heat)

Energy absorbed

(Incoming solarradiation)

Energy emitted

(Outgoing infraredradiation)

If emitted and absorbed are not equal, then Earthwill become warmer or cooler until these becomeequal again

6

Unequaldistributionof solarradiation

4

7

8

Factors Governing Global AirCirculation

Uneven solar heating

The Coriolis effect

5

9

Uneven Solar Heating and Latitude

Earth as a whole is in thermal equilibrium, but different latitudes arenot. Moving masses of air and ocean currents transport energy fromlocations with a surplus to those with a deficit.

10

6

11

Summer sea surface temperatures

12

Surface Temperature Ranges at Different Latitudes

7

13

Density of Air

Decreases with: Increasing T Increasing humidity Increasing altitude

14

Air Density: Affected by Temperature,Pressure and Moisture Content

Warm air is less dense than cold air and moist air is less densethan dry air.

Air pressure is the weight of the air from Earth’s surface to the topof the atmosphere and equals 1.04kg/cm2 (standard air pressure,one atmosphere) at sea level.

Low pressure zone is where air density is lower than insurrounding areas because the air is warmer or has a highermoisture content.

High pressure zone is where air pressure is higher than insurrounding area because of cooling or lower moisture content.

8

15

Air Circulation

16

Monsoons are patterns of wind circulation that change with the season. Areas with monsoons generally have dry winters and wet summers.

Sea breeze is cool air from over the water moving toward land. Sea breezes occur after sunrise.

Land breezes occur after sunset when air warmed by the land blows toward the water.

Monsoons, Sea Breezes andLand Breezes

9

17

Sea Breezes And Land Breezes

Top: Sea breeze

Bottom: Land breeze

18

10

19

20

Temperature of Rising andFalling Air

Ascending air cools as itexpands. Cooler air canhold less water, so watervapor condenses intoclouds. Descending airwarms as it compressesand the cloudsevaporate.

11

21

Simple aircirculation ona non-rotating

Earth

22

A large circuit of air is called an atmosphericcirculation cell. Three cells exist in eachhemisphere.

Hadley cells are tropical cells found on each side of theequator.

Ferrel cells are found at the mid-latitudes.

Polar cells are found near the poles.

Atmospheric Circulation Cells

12

23

24

Three Major Convection Cells in EachHemisphere

The Hadley cell extends from the Equator to about30o latitude.

The Ferrel Cell extends from 30o to about 50o

latitude.The Polar Cell extends from 90o to about 50o

latitude.The Coriolis effect causes wind in these cells to

bend to the east or west to form the westerlies,easterlies, and the trade winds.

13

25

Fluids (air and water) flow from areas of highpressure to areas of low pressure.

Change in pressure across a horizontal distance is apressure gradient.

Movement of air across a pressure gradient parallelto Earth’s surface is called a wind

Winds are named for the direction from which theycome.

Fluid Motion

26

Rotation of the Earth InfluencesWinds

Global winds blow in response to variation inpressure related to uneven solar heating(insolation) of Earth’s surface.

Coriolis deflection is the apparent deflection ofobjects moving across Earth’s surface to theright of direction of travel in the northernhemisphere and to the left of direction of travelin the southern hemisphere.

14

27

28

The Coriolis Effect

The Coriolis effect is the observed deflection of a movingobject, caused by the moving frame of reference on thespinning Earth.

As air warms, expands, and rises at the equator, itmoves toward the pole, but instead of traveling in astraight path, the air is deflected eastward.

In the Northern Hemisphere air turns to the right.

In the Southern Hemisphere air turns to the left.

15

29

The Coriolis Effect

Air moves in cells, influenced by the Coriolis effect.

30

The doldrums are calm equatorial areas wheretwo Hadley cells converge

The horse latitudes are areas between Hadleyand Ferrel cells. There is little surface wind inthis area.

The trade winds are surface winds of Hadleycells.

The westerlies are surface winds of Ferrelcells.

Wind Patterns

16

31

32

Averageatmospheric

pressureduring July(top) andJanuary.

17

33

Summer (left) and winter prevailing windscontrolled by pressure cells.

34

18

35

Storms are regional atmospheric disturbances. Stormshave high winds and most have precipitation.

Tropical cyclones occur in tropical regions. These stormscan cause millions of dollars worth of damage andendanger life.

Extratropical cyclones occur in Ferrel cells, and are winterweather disturbances. These storms can also causeextensive damage.

Both types of storms are cyclones, or rotating masses oflow-pressure air.

Storms

36

The orange shaded areas and arrows designate the location andmovement of most hurricanes.

Storms