Pressure and WindsGeneral Circulation of the Atmosphere

Geog 210 - Physical Geography

Geog 311 - Climatology

Global balance – local imbalances • Is the earth system in radiative equilibrium? The diagram below shows the annual mean, averaged around

latitude circles, of the balance between the solar radiation absorbed at the ground (in blue) and the outgoing infrared radiation from Earth into space (in red). The two curves generally balance over the entire globe, but not at every single latitude. In the tropics, there is an excess of radiation (solar radiation absorbed exceeds outgoing terrestrial radiation) in middle and high latitudes all the way to the poles, there is a deficit (Earth is radiating into space more than it receives from the sun). The atmosphere and ocean systems are forced to move about by this imbalance, and bring heat by convection and advection from equator to the poles. 

Source: Columbia University

The imbalance of radiative budget between the tropics and higher latitudes is the main cause that drives weather and climate! The

ultimate energy source is, of course, the sun.

Global weather phenomena as observed by NOAA’s geostationary satellite GOES-12 on Oct 7, 2003.

Whither the Wind?

Pressure Gradient

Coriolis Effect

Friction

•Pressure unevenness is the main cause of winds. This unevenness causes a pressure gradient from areas of high pressure areas of low pressure. The pressure gradient force causes winds to blow.

•In the absence of other forces, winds will blow from high pressure to low pressure. •The unit of pressure is millibars (mb). The ‘mean’ sea-level pressure is 1013.25 mb, or roughly 1000 mb.

Coriolis Force• But there are other forces that may

influence winds. One of the most important is the Coriolis force in honor of the French physicist Coriolis.

• Coriolis force is not a “real” force but an apparent force. It appears only because the observer (i.e., people like you and I) is on a rotating reference frame.

• Zero at equator; maximum at high latitudes

Coriolis Force• The same phenomenon occurs for any

motion on earth due to the earth rotation.

• When we observe the movement of an air parcel on earth, we find that the moment the air parcel starts to move, it will be subject to this force immediately. N

• ET in• The deflection of the movement is such that

the parcel will be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• Thus, the wind patterns are rotating in opposite sense near high and low pressure centers in Northern and Southern Hemispheres, as shown in the figure to the right.

Geostrophic Balance and Geostrophic WindsBalance between the pressure gradient force and Coriolis force

• Will the wind direction forever turn to right (left) in Northern (Southern) Hemisphere? The answer is no. It will keep turning until a balance between the pressure gradient force and Coriolis force is achieved.

• This balance happens when the wind direction becomes parallel to the isobars. This is called the geostrophic balance.

• The wind satisfying the geostrophic balance condition is called the geostrophic wind .

• Geostrophic winds are very close to (but not exactly the same as) the actually observed winds.

• Unisys Weather http://weather.unisys.com/

• Datastreme Project http://www.ametsoc.org/amsedu/dstreme/index.html

Northern Hemisphere

Friction

• Upper Atmosphere

Convergence and Divergence• In the real world, there are more forces (such as friction and curvature effect)

involved in shaping wind direction and magnitude aside from the pressure gradient and Coriolis forces. The final results are that winds tend to converge toward the low pressure center and diverge outward from the high pressure center.

Scale - “swirls within swirls”

Sea Breeze

Land Breeze

Single-Cell Circulation Model

The basis for average air flow around the earth can be examined using a non-rotating, non-tilted, ocean covered earth.

Heating is more intense at the equator, which triggers Hadley cells to redistribute rising heat from the tropical low to the polar highs.

3-Cell Model: simple!

• A simplified 3-cell model of the general meridional circulation of the atmosphere.

• In general, descending air has a drying effect while ascending air causes condensation.

The wind system on the earth surfaceThe earth is rotating, and rotation makes the wind system a bit more

complicated than implied by the 3-cell structure.

• In the northern hemisphere, the tropical ( 0 – 30ºN) prevailing winds are easterlies (NE winds)-called trade winds. In the middle latitudes (roughly 30 – 60ºN) the prevailing winds are westerlies (SW winds). The polar region’s prevailing winds are easterlies.

• The wind system in the southern hemisphere is like the mirror image of the northern hemisphere.

Hadley Cell

Seasonal shifts

ITCZ Limits

Winter Season

Summer Season

Winter

Coastal Summer WeatherCoastal Summer Weather

The semi-permanent Pacific high blocks moist maritime winds and rain The semi-permanent Pacific high blocks moist maritime winds and rain from the California coast, while the Bermuda high pushes moist tropical from the California coast, while the Bermuda high pushes moist tropical air and humidity over the eastern states.air and humidity over the eastern states.

Coastal Winter WeatherDuring winter months, the Pacific high migrates southward and allows for maritime winds with moisture and rains to reach California.

On the east coast, precipitation is rather even throughout the year, and moisture is always from the Gulf of Mexico and the tropical Atlantic.

January Winds AloftJanuary Winds Aloft

Land-sea temperature differences trigger ridges and troughs in the isobaric surface.

Whither the Westerlies?

Thermal wind is the most fundamental and significant dynamical balance controlling thelarge-scale circulation of the atmosphere and ocean. It is a consequence of hydrostatic and geostrophic balance, and relates horizontal buoyancy gradients to changes in the

horizontal wind with height.

“What goes up must come down, spinning wheel got to go ‘round.”

Blood, Sweat and Tearshttp://www.youtube.com/watch?v=8T97f2kBzOQ

Surface & 500 mb MapsSurface maps chart pressure contours, highs and lows, and wind direction.

Winds blow clockwise around highs, called anticyclones.

500 mb maps reveal patterns that on average are 5600 m above the surface, where westerly winds rise and fall across ridges and troughs.

Climagraphs of U.S. Citieshttp://drought.unl.edu/whatis/climographs.htm

World Climagraphshttp://cwx.prenhall.com/bookbind/pubbooks/lutgens3/medialib/abcontrol/pages/question.html

http://southhill.vsb.bc.ca/Departments/Humanities/Geogpraphy/Kyle/Notes/2_Atmosphere/Climographs/WorldClimates.html

World Climates (Ritter)

Please go to the website below and look at the climagraphs for

locations all over the world. Study the controlling factors.