Heat Transfer Due to Atmospheric Motion

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What causes wind?

• Wind is the horizontal movement of air in the atmospheric system

• Winds result from differences in air pressure

• Differences in air pressure are caused by differences in temperature & gravity

• E.g. pressure falls rapidly with height

Low pressure area

High pressure area

wind

Average pressure at sea level is 1013mbLOW v HIGH

• Increased temperatures

• Air heats & expands

• Less dense & rises

• Creates low pressure below

• Drop in temperature

• Air cools & contracts

• More dense & sinks

• Creates high pressure below

• HIGH

• Out-blowing

• Descending

• Clockwise

• Gentle winds – gentle pressure gradient

Draw 2 diagrams to show facts below

• LOW

• Blows inwards

• Rising

• Anticlockwise

• Strong winds –steep pressure gradient

Air movement on a rotation free earth

• If earth did not rotate

• If earth was entirely either land or water

• There would be just one large cell

• Wind would move directly from high to low pressure

• HOW EVER EARTH IS NOT LIKE THIS

Tricellular Model• 1686 – Halley – one

cell

• 1735 – Hadley – expanded one cell theory

• 1856 – Ferrel – discovered three cells

• 1941 – Rossby – refined the 3 cell theory

Doldrums – gentle, variable winds

Horse latitudes – calm seas slowed sailors, no food for animals, dead horses thrown overboard

Mid-latitude depressions form

Towering cumulonimbus clouds typical of equatorial climate

Latent heat & moisture picked up from tropical oceans

Cumulonimbus clouds at ITCZ Mid-latitude depressions at the polar front

Clear skies due to high pressure

The Coriolis Force

• It is a deflecting motion or force

• Suggested by G.G. de Coriolis in 1835

• Developed by W. Ferrel in 1855

• The rotation of the earth causes a body moving across its surface to be deflected to the right in the N hemisphere and to the left in the S hemisphere

Coriolis Force

3 controls on wind:

-Pressure gradient

-Coriolis Force

-Friction with land surface

ROSSBY WAVES = a belt of upper air westerlies which often follow a meandering path.

• Evidence: pilots in WW 2 noticed:

- eastwards flights faster than westwards

- N-S flights often blown off course

Seasonal change in number of meanders – 4 to 6 in summer, 3 in winter

Jet streams = narrow bands of extremely fast moving air found within the Rossby Waves

• Their purpose is the rapid transfer of energy• Speeds can exceed 230km/hr• Can carry volcanic ash around earth in 1-2 weeks• 5 recognised jet streams:

2 significant - POLAR FRONT JET STREAM

- SUBTROPICAL JET STREAM

1 seasonal – EASTERLY EQUATORIAL J.S.

Normal path of J.S. over Britain is to NE so frequent wet & windy weather as warm air moves north

Polar Front Jet Stream(divides Ferral & Polar cells)

• Where J.S. moves south:

- Cold air- Descends- Clockwise- Dry- Stable- High pressure- Anticyclones

• Where J.S. moves north:

- Warm air- Rises- Anticlockwise- Strong winds- Heavy rain- Low pressure- Depressions

• SUBTROPICAL J.S.• 25-30 degrees• Divides Hadley &

Ferral Cells• Meanders less than

PFJS• Lower wind velocities• Similar west to east

path

• EASTERLY EQUATORIAL J.S.

• Seasonal• Associated with

summer monsoons• Indian subcontinent

• MONSOON = a seasonal reversal of wind direction in S.E. Asia

Balloon uses jet stream to orbit the earth