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Atmosphere - PowerPoint PPT Presentation

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Atmosphere. 78% nitrogen, 21% oxygen. Water Vapor. up to 4% by volume leaves atmosphere as dew, rain or snow. Density of Air. Warm air is less dense than cold air Humid air is less dense than dry air. Air Movement. Air near sea level is packed by pressure As air rises - expands and cools

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  • 78% nitrogen, 21% oxygen

Water vapor
Water Vapor

  • up to 4% by volume

  • leaves atmosphere as dew, rain or snow

Density of air
Density of Air

  • Warm air is less dense than cold air

  • Humid air is less dense than dry air

Air movement
Air Movement

  • Air near sea level is packed by pressure

  • As air rises - expands and cools

  • As air descends- compresses and warms


  • Warm air can hold more water vapor than cool air

  • As air rises, it cools and water vapor may condense into clouds and eventually precipitation

Atmospheric circulation
Atmospheric Circulation

  • Wind = mass movement of air

  • wind patterns caused by variations in solar heating and earth’s rotation

Uneven solar heating atmospheric circulation
Uneven Solar Heating & Atmospheric Circulation

  • Air is warmed in the tropics and rises

  • Air is cooled near the poles and falls


Earths air

Circulation if




Fig. 7-5, p. 136


  • …putting all of this together…the hot, humid, air over the tropics is LESS dense (than all other air masses); this means:

    (a) Less dense air rises (like a hot air balloon);

    (b) As it rises (vertical) the pressure decreases;

    (c) As pressure decreases, air EXPANDS;

    (d) As it expands, it COOLS

    …then the reverse again…+ horizontal (wind) movement


…When you add the horizontal (wind) movement it pushes this (originating hot, dry, tropical air) BOTH up and out (North/South) for distribution throughout the entire atmosphere. How?



Tropical air rises, meets density around it (more dense than that above it but less dense than that below it) so it can’t “move” vertically …but it must go somewhere!

SIDEWAYS! (joins Atmospheric Circ. pattern, moving horizontally, toward POLES) & as it rises poleward it COOLS, gets more dense, and falls back (toward equator) again…and again…

Coriolis effect
Coriolis Effect

  • The eastward rotation of the earth deflects any moving object away from its initial course

  • the deflection is clockwise in the Northern hemisphere

  • the deflection is counterclockwise in the Southern hemisphere

Coriolis deflection
Coriolis Deflection

  • “The apparent deflection of objects moving across Earth’s surface to the right of direction of travel in the Northern Hemisphere and to the left in the Southern Hemisphere.”

  • Different because of difference of speed and width of Earth @ equator vs. poles

  • Increases poleward & as speed increases.

Wind patterns
Wind Patterns

  • At bands between cells air is moving vertically

  • winds are weak and erratic

  • doldrums or intertropical convergence zone (ITCZ) at equator

    • ascending air

  • Subtropical high pressure belt at 30o

    • descending air, very dry

Wind patterns1
Wind Patterns

  • Within cells air moves horizontally from high to low pressure areas

  • produces strong dependable winds

    • Trade winds or easterlies

    • westerlies

    • polar easterlies



  • Movement of air across a pressure gradient parallel to Earth’s surface is called a wind.

  • Winds are designated according to the direction from which they come.

  • In contrast, ocean currents are designated according to the direction towards which they travel.