Global Wind Patterns

The circulations of the atmosphere The circulations of the atmosphere (winds) and oceans (currents) are (winds) and oceans (currents) are driven by sunlight hitting the earth’s driven by sunlight hitting the earth’s surface more directly near the surface more directly near the equatorequator

a LOT of this sunlight energy hits a LOT of this sunlight energy hits the tropics, between 23.5the tropics, between 23.50 0 North North and South of the equator.and South of the equator.

Caused by uneven heating

IInternterTTropical ropical CConvergence onvergence ZZoneoneITCZITCZ

the “doldrums”

If Earth did not rotate, wind patterns would If Earth did not rotate, wind patterns would be simply N to South along the surfacebe simply N to South along the surface

• Thermal convection leads to formation of convection cell in each hemisphere

• Heat Energy is transported from the tropics toward poles in upper atmosphere

• Cold air sinks and shuttles back toward equator along surface

Coriolis effectCoriolis effect

But the earth rotates, so there is a tendency for objects moving over long distance (like air masses) to curve due to “coriolis effect”

• Curve to right in N. Hemisphere

• Curve to left in S. Hemisphere

• This leads to winds generally blowing in east/west directions and having spin to them

CORIOLIS EFFECTCORIOLIS EFFECT• THE ROTATION OF THE EARTH CAUSES

THE AIR TO MOVE AT ANGLES FROM THE EQUATOR TO THE POLES AND BACK.

• http://uwf.edu/atc/projects/coriolis/main.swf

• http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization

See coriolis effect as high pressure wind blows See coriolis effect as high pressure wind blows into a low pressure air massinto a low pressure air mass

• Wind blowing in curve to the right in N. hemisphere

• Thus, low pressure system has air spinning counterclockwise

• This pattern creates tornadoes and hurricanes as well

Heat Exchange in air massesHeat Exchange in air massesWarm, humid air rises, dry clear air sinksWarm, humid air rises, dry clear air sinks

Creates 3 huge convection/circulation cells in each hemisphereCreates 3 huge convection/circulation cells in each hemisphere

Prevailing winds are created by coriolis effect Prevailing winds are created by coriolis effect curving their pathscurving their paths

Add these prevailing winds to your globe from north to south:

Polar Northeasterlies

Westerlies

Northeasterly Tradewinds

Doldrums

Southeasterly Tradewinds

Westerlies

Polar Southeasterlies

Wind Wind • Wind blows from high pressure air masses to low pressure

• Winds named westerly wind IF blowing from the West

• Wind spead increases as pressure gradient increases (steeper isobars)

Low pressure areasLow pressure areas•Warm, humid air rises, expands and condenses = causing a low pres air mass at surface and condensation above (lots of rain or snow)

•Equator and polar front, 00 and 600 N and S.

High Pressure AreasHigh Pressure Areas

• Dense, dry air (can by cold dense or hot arid) sinks due to gravity = creating high pressure at surface and outward blowing wind, very little precipiation

• 30° N and S; 90° N and S at poles

wet

wet

wet

dry

dry

dry

dry

Notice location of major deserts Notice location of major deserts near 30 Latitudenear 30 Latitude

Hadley Cells

Ferrel

cell

Key features of three cell modelKey features of three cell model• Hadley cell takes heated air away from the tropics - TROPICS: low pressure air rises near equator -

MUCH RAIN-DESERT ZONE: condensed, high pressure air descends

near 30 degrees - clear skies! • Ferrel Cell - driven by heat transports of eddies - Polar air rises near 60 degrees and descends near 30

degrees - explains surface westerlies from 30-60• Weak (WUSSIE-BABIES) winds found near

– Equator (doldrums)– 30 degrees (horse latitudes)

• Boundary between cold polar air and mid-latitude warmer air is the polar front

Understanding Understanding the General Circulationthe General Circulation

1. Unequal solar heating between the equator and poles. Atmospheric general circulation moves heat toward the north and south poles..

2. In a Hadley cell, warmer air rises and moves poleward. Equator-to-pole Hadley cell is impossible. The Earth rotates!

3. In the Northern Hemisphere, air is pushed to the right as it moves; in the Southern Hemisphere, air is deflected toward the left. The Coriolis effect.

- rotation produces trade winds; surface westerlies in NH; upper tropospheric jet streams.

4. Ferrel cell runs backwards! Transports heat the wrong way, towards the equator!

5. The Ferrel cell is weak, so does not move as much energy as the other cells.

Global Wind and Pressure Patterns

Hadley Cell: low-latitude atmospheric circulation cell with rising air over the equatorial trough and sinking air over the subtropical high-pressure belts

Intertropical Convergence Zone (ITCZ): Where convergence of air masses along the equatorial trough

Deserts: belts of persistent high atmospheric pressure centered at about lat. 30º N and S

Polar Front: front lying between cold polar air masses and warm tropical air masses

Thermal Wind Balance Thermal Wind Balance Produces Jet StreamsProduces Jet Streams

• Hot tropics and cold poles produce westerly high altitude jet streams at middle latitudes like ours.