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Heat Transfer Due to Atmospheric Motion Page 224 to 228 What causes wind? • Wind is the horizontal movement of air in the atmospheric system • Winds result from differences in air pressure Low pressure area wind High pressure area • Differences in air pressure are caused by differences in temperature & gravity • E.g. pressure falls rapidly with height Average pressure at sea level is 1013mb LOW v HIGH • Increased temperatures • Drop in temperature • Air cools & contracts • Air heats & expands • More dense & sinks • Less dense & rises • Creates low pressure below • Creates high pressure below Draw 2 diagrams to show facts below • LOW • HIGH • Blows inwards • Out-blowing • Rising • Descending • Anticlockwise • Clockwise • Strong winds –steep pressure gradient • Gentle winds – gentle 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 Mid-latitude depressions form Latent heat & moisture picked up from tropical oceans Horse latitudes – calm seas slowed sailors, no food for animals, dead horses thrown overboard Towering cumulonimbus clouds typical of equatorial climate Doldrums – gentle, variable winds 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 3 controls on wind: Coriolis Force -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
