atmosphere air circulation
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ATMOSPHERE Air Circulation
UNIT 7Meteorology and Climate
Atmosphere-Ocean Coupling
Why study atmospheric circulation? Atmosphere and ocean
processes are intertwined
Atmosphere-ocean interaction moderates surface temperatures, weather and climateWeather: local
atmospheric conditionsClimate: regional long-
term weather Atmosphere drives most
ocean surface waves and currents
Composition of the Atmosphere
Dry Air: 78% Nitrogen, 21% Oxygen
BUT it is never completely dry
Typically contains about 1% water vapor
Chemical residence time of water vapor in the air is about 10 days
Warm air holds much more water vapor than cold air
http://www.nature.com/scitable/knowledge/library/the-global-climate-system-74649049
Density of AirTypical air density ~ 1
mg/cm3
Temperature and pressure affect the density of air
Temperature: Hot air is less dense than cold air
Pressure: Air expands with elevation above sea level
http://www.physicalgeography.net/fundamentals/7d.html
Density and TemperatureRising air expands and
coolsVapor condenses into
clouds and precipitation
Sinking air is compressed and warmsClear air
Expanding Air Cools and Condenses
Like opening a pressurized bottle of soda
Air expands and cools
Water vapor condenses – cloud formation
Solar Heating of the EarthSolar energy absorbed unevenly over Earth’s
surface – why is air rising?
Energy absorbed / unit surface area varies with:Angle of the SunReflectivity of the surface (i.e., ice versus ocean)Transparency of the atmosphere (i.e. clouds)
Solar Insolation Variations with Latitude
Solar Heating of the EarthSunlight heats the ground more intensely in the
tropics than near poles
July
January
Solar Heating and SeasonsSeasons are caused by Earth’s 23.5° tilt
http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html
Solar Heat EnergyEquator absorbs more heat from the sun than it
radiates away
Polar regions radiates much more heat than they absorb from the sun
Energy in at equator and heat out at poles
Heat transfer from
E.g. Equator isn’t that hot – Poles aren’t that cold
Evidence that the atmosphere (2/3) and oceans (1/3) redistribute heat (wind and ocean currents)
Convective heat transfer moderates Earth climate
http://oceanmotion.org/html/resources/solar.htm#vishead
Convective HeatConvective heat transfer model’s Earth climate
Heated air expands and rises, then cools and sinks
Equator
Poles
Atmospheric CirculationCold, more dense air sinks near the Poles
Cold, more dense air sinks near the Poles
Warm, less dense air rises near the Equator
Wind from the north
Actual Atmospheric Circulation
Air rises and sinks
More than one convection cell
Earth spins once per day that amounts to a speed for us at the surface of Earth of 100’s of miles/ hour
Coriolis Effect
Coriolis Effect Movies
http://earthsciweb.org/GeoMod/index.php?title=Coriolis
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/gifs/coriolis.mov
http://science.nasa.gov/science-news/science-at-nasa/2004/23jul_spin/
The Coriolis Effect on Earth
Surface velocity increases from poles to equator
Points on the equator must move faster than points near the poles to go around once a day
Latitude velocity differences land to curving paths
Northern hemisphere deflected to right
The Coriolis EffectStrength of Deflection varies with
latitude:Maximum at the polesZero(!) at equator
Faster a planet rotates, the stronger the Coriolis effects
The larger the planet, the stronger the Coriolis effects (Jupiter spins once every 10 hours)
Hurricanes A storm with lots of clouds has rising air – thus
low-pressure at the surface!
Converging air sets up counter-rotation (cyclonic)
Spinning counter clockwiseBending to the left
Spinning clockwiseBending to the right
Hurricanes – Low PressureHurricane is rising and already
has moisture in it
Low pressure system at the surface – air rising so that means air is being sucked in at the base
Arrows get defected by Coriolis to the right
Set up a counter clockwise circulation in the northern hemisphere
L
High PressureAir sinks and compresses
and it gets warmer and dry
See clear air not clouds
Pushes air away
Rotate clockwise in northern hemisphere
H
Atmospheric CirculationSinking air at 30° –
deserts
Easterly winds – trade winds
Westerly winds at 30° and 60°
Atmospheric Circulation 3 convection cells in each hemisphere
Each cell: ~30° latitudinal width
Veritcal MotionsRising Air: 0° and 60° LatitudeSinking Air: 30° and 90° Latitude
Horizontal MotionsZonal winds flow nearly along latitude linesZonal winds within each cell band
DUE to DEFLECTIONS BY CORIOLIS!
Sea BreezeLand warms fastest during the day. Air during
the day expands and rises
Ocean surface temperature changes slowly. Air cools and becomes more dense, sinks then begins to rise over the land.
Result – wind from sea towards land
Land BreezeLand cools fastest at night. Low heat capacity. Air
contracts and sinks
Ocean surface temperature changes slowly. Air is pushed away and up by cooler denser land air.
Result – wind from land towards sea
Marine LayerCold waters, warm air: think cloud layer on
ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND OCEAN
Marine LayerCold waters, warm air: think cloud layer on
ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND OCEAN
Marine LayerCold waters, warm air: think cloud layer on
ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND OCEAN
Marine LayerCold waters, warm air: think cloud layer on
ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND OCEAN
Marine LayerCold waters, warm air: think cloud layer on
ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND OCEAN
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