ATMOSPHERE
Air Circulation
UNIT 7
Meteorology and Climate
Atmosphere-Ocean Coupling
 Why study atmospheric
circulation?
 Atmosphere and ocean
processes are intertwined
 Atmosphere-ocean
interaction moderates
surface temperatures,
weather and climate
 Weather: local atmospheric
conditions
 Climate: 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 Air
 Typical 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 Temperature
 Rising air expands and
cools
 Vapor condenses into
clouds and precipitation
 Sinking air is compressed
and warms
 Clear 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 Earth
 Solar energy absorbed unevenly over Earth’s surface –
why is air rising?
 Energy absorbed / unit surface area varies with:
 Angle of the Sun
 Reflectivity of the surface (i.e., ice versus ocean)
 Transparency of the atmosphere (i.e. clouds)
Solar Insolation Variations
with Latitude
Solar Heating of the Earth
 Sunlight heats the ground more intensely in the tropics
than near poles
July
January
Solar Heating and Seasons
 Seasons are caused by Earth’s 23.5° tilt
http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html
Solar Heat Energy
 Equator 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 Heat
 Convective heat transfer model’s Earth climate
 Heated air expands and rises, then cools and sinks
E
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Atmospheric Circulation
Cold, more
dense air sinks
near the Poles
Warm, less
dense air
rises near
the Equator
Wind from the north
Cold, more
dense air sinks
near the Poles
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?t
itle=Coriolis
http://science.nasa.gov/sciencenews/science-at-nasa/2004/23jul_spin/
http://ww2010.atmos.uiuc.edu/%28Gh%29/
guides/mtr/fw/gifs/coriolis.mov
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 Effect
 Strength of Deflection varies with latitude:
 Maximum at the poles
 Zero(!) 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 lowpressure at the surface!
 Converging air sets up counter-rotation (cyclonic)
Spinning clockwise
Bending to the right
Spinning counter clockwise
Bending to the left
Hurricanes – Low Pressure
 Hurricane 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 Pressure
 Air sinks and compresses and it
gets warmer and dry
 See clear air not clouds
 Pushes air away
 Rotate clockwise in northern
hemisphere
H
Atmospheric Circulation
 Sinking 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 Motions
 Rising Air: 0° and 60° Latitude
 Sinking Air: 30° and 90° Latitude
 Horizontal Motions
 Zonal winds flow nearly along latitude lines
 Zonal winds within each cell band
 DUE to DEFLECTIONS BY CORIOLIS!
Sea Breeze
 Land 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 Breeze
 Land 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 Layer
 Cold 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 Layer
 Cold 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 Layer
 Cold 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 Layer
 Cold 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 Layer
 Cold 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