Heat Transfer Due to Atmospheric Motion Page 224 to 228
Mar 31, 2015
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
• 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