Atmospheric Motion

Nonrotating Earth• Equator– Warming and rising of air– Rising air cools as it ascends– Surface winds blow towards

equator to replace ascending air

• Poles– Cool, dry air descends

Global Wind Bands• Hadley cells– Subtropical cells– Air rises at the equator due to solar heating, and

falls due to cooling at ~30º

Global Wind Bands• Ferrel cells– Temperate cells– Occurs between 30º and 50-60º latitude– Some air descending at 30º turns poleward

Global Wind Bands• Polar cells– Centered over each pole– At 50-60º, the air is warm and wet enough to

ascend, but too dense to mix with Ferrel cell

Wind Patterns• Between Cells:– Vertical air

movement– Weak surface

winds

• Within Cells:– Horizontal air

movement– Strong surface

winds

Wind Patterns• Between Cells:• Hadley cells– Doldrums• Near the equator• Decrease in

pressure gradient• Variable breezes

– Intertropical Convergence Zone (ITCZ)

Wind Patterns• Between Cells:• Hadley and Ferrel

cells– Horse latitudes• 30º latitude• Subtropical high

pressure• Sinking air; dry

– Many deserts are found at this latitude

Wind Patterns• Within Cells:– Hadley cell• Trade Winds

– Easterlies• Centered at 15º

– Ferrel cell• Westerlies• Centered at 45º

– Polar cell• Easterlies• 60-90º

Radiation and Latitude

• More heat gained at equator

• More heat lost at higher latitudes

Atmospheric Pressure

Atmospheric Pressure

High vs. Low Pressure High Pressure Cell

Anticyclonic rotation (clockwise) in N hemisphere

Downward vertical motion

Air is compressed and warmed

Dry weather

High vs. Low Pressure Low Pressure Cell

Cyclonic rotation (counter clockwise in N hemisphere)

Upward vertical motion

Air expands and cools as it rises

Wet weather

Fig. 8.16

Wind Band Modification• Seasonal Changes

Wind Band Modification• Seasonal Changes

Wind Band Modification• Seasonal Changes

Wind Band Modification• Seasonal Changes

Monsoons - Summer• Land heats more rapidly

than ocean• Air above land expands

and rises creating low pressure

• Cool air flows from ocean

• Cool air is heated, expands, rises and condenses

Monsoons - Winter• Land cools more rapidly than ocean• Air contracts and sinks creating high pressure• Dry surface winds move seaward

Onshore & Offshore Breezes(small, daily mini-monsoons)

• Onshore Breeze

Onshore & Offshore Breezes(small, daily mini-monsoons)

• Offshore Breeze

Topographic Effect• Windward = wet• Leeward = dry

• Rain shadow

Jet Streams• High speed winds in the

upper troposphere

• Polar• Subtropical• Weather of temperate

zone

Hurricanes• Easterly wave

• > 26.5 C (80 F)

• Typhoon or cyclone

Hurricanes

El Niño – Southern Oscillation

El Niño – Southern Oscillation

El Niño• Sea surface topography

and temperature

• Normal• El Niño• End of El Niño

La Niña• Colder phases

Storm Surge• Low atmospheric pressure• Strong onshore winds