Lecture 147 February 2005
Atmospheric and Oceanic Circulations(continued)
Chapter 6
Credit: www.physicalgeography.net
Wind simply put, wind is the horizontal flow of air
in response to differences in air pressurethese pressure differences are usually
due to uneven solar heating at the surface
wind flows because of
pressure gradient
‘heat rises’
Four forces that determine winds
1. Gravity - pulls gas molecules close to Earth density & pressure decrease with height
2. Pressure gradient force - the difference in air pressure between
areas3. Coriolis force - deflects wind from a
straight line to the right or left depending on hemisphere
4. Friction force - the drag on air flow from the Earth’s surface
Pressure Gradient Force
Pressure Gradient Force and Isobars
if there were no other forces acting on wind, it would flow in straight lines (perpendicular to isobars) from high to low pressure zones
Coriolis Force (just the facts)
• Rotation of Earth acts to deflect any motion
from a straight line
• Deflection is to right (NH) to the left (SH)
• Coriolis “force” act on a right angle to the
motion
• Coriolis Force is NOT a real “force” but is
caused by viewing motion on a rotating
planet
Figure Credit: “Earth’s Climate” by W.
Ruddiman
The Coriolis Force affects air flow in response to pressure gradients in the
atmosphere
geostrophic winds - PGF and Coriolis forces are opposite and balanced
Credit: www.physicalgeography.net
the CF deflects the wind to the right until upper troposphere wind flows parallel to
isobars
~7km
Friction Force
Surface friction reduces wind speed and the effects of the Coriolis force
Friction causes winds to move across
isobars at an angle
The friction force operates only in the bottom 0.5-1 km of the atmosphere,
and it acts opposite to the direction of motion
PGF + Coriolis + Friction Forces
Figure Credit: “Earth’s Climate” by W.
Ruddiman
Show shockwave 27_WindPatDev.swf
Useful things to remember
• Coriolis Force
Acts to the right of motion (left in SH)
Not a real force – matter of perspective
• Geostrophic wind – upper
troposphere/ocean
Horizontal pressure & Coriolis forces balance
Winds go ALONG isobars
• Surface friction – lower troposphere
Enables CROSS isobar flow
Useful things to remember
• Low pressure cells
Less dense –humid
Circulation is counter-clockwise (NH) – cyclonic
Convergent near ground – rising air masses
• High pressure cells
More dense – dry
Circulation is clockwise (NH) – anticyclonic
Divergent near ground – descending air masses
Heating & Humidity in Tropicssolar heating in the tropics expands air and decreases its density - leading to increased
buoyancy
How would this change the average molecular weight of air?
average molecular weight of air is ~29
g/mol
average density of air is 1.3 kg/m^3
what happens to air density if
you add water vapor?
It also gets more humid (adding water vapor)
Convection on your Stove
Convection on Earth
Warm, moist air rises and is replaced by cooler drier air from other sites
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as this air rises, it cools and water condenses out, leading to intense
precipitation
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A satellite (GOES) view of the ITCZ over the eastern Pacific
InterTropical Convergence Zone
the position of the ITCZ tracks the sun (it is in the summer hemisphere) - the location of the
ITCZ determines the rainy season in many tropical countries, especially those in Africa
the horizontal winds within the ITCZ are calm - the doldrums
The C in ITCZ
the intense uplift of air creates horizontal pressure gradients at the surface
Credit: NASA JPL
as a result, winds converge towards the equator from both hemispheres
what about the complete cycle - where does the uplifted air go?
Equator-to-pole cross section of circulation
Hadley cell circulation
this circulation refers to the complete circulation of rising air in the tropics,
descending air over 30 °N and °S, and trade winds converging at the equator
the descending branch of the Hadley circulation brings warm, dry air to the surface
leading to high pressure & reduced
precipitation
Subtropical high-pressure cellsthese cells occur where the tropical air
descends in either hemisphere
Equator-to-pole cross section of circulation
Figure 6.18
Jet Stream
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Show shockwave 04_GlobalWinds.swf
Figure Credit: http://www.geog.ucsb.edu/~joel/g110_w03/chapt10/vorticity/agburt2_10_07.jpg
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Figure Credit: physicalgeography.net
Monsoon Circulation
Asian monsoo
nintense, dry winds flow from the Asian interior in response
to the gradient between the
continental high pressure &
equatorial (ITCZ) low pressure
in summer, the ITCZ shifts north,
reversing the pressure gradient
–
winds flow from the Indian ocean & gain moisture
Asian monsoo
n
Daytime land-sea breeze
results from differential heating of land and sea - not from radiation differences - but
from the different specific heats of land and water
Nighttime land-sea breeze
at night, the land cools more rapidly than the sea and thus overlying air becomes more
dense and has a higher pressure
What time of day would you go jogging in Los Angeles?
WHY?