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Geol 117 Oceans
Lecture 16 Atmospheric circulation & Storms
I. Atmospheric Circulation
A. Wind -example non-rotating earth
1. Recall vertical circulation from heating near surface
a. Warm, moist air rises,
b. Expands & condenses water
c. Dry cool air Sinks
2. This sets-up circulation pattern
a. Near surface – air rushes in ‘to fill space’ left by rising air mass
b. Above surface – air rushes in the ‘to fill space’ left by sinking air mass
3. Consider simple non-rotating earth
a. Heating & rise at equator
b. Cool air sinking at poles
c. Large, Hemisphere – wide circulation cells –NOT OBSERVED – earth rotates!
B. Wind – example rotating earth
1. Earth’s surface rotates toward East
a. Speed is faster at equator vs. high latitude
b. b/c radius of latitude decrease away from equator
2. Coriolis Effect results form this difference in speed
a. As plane (air) leaves equator traveling north, but with an eastward velocity due to
rotation of the Earth
b. From the surface of the earth, the plane travels in an arc that veers to the right
(east in this case) because the planes eastward velocity is greater than that of the
ground that it is traveling over. Recall that the surface rotation speed decreases
with latitude.
c. in the southern hemisphere, deflection is too the left – think about it.
c. Animation (Coriolis) / Link
http://www.windpower.org/en/tour/wres/coriolis.htm
http://www.tam.uiuc.edu/courses/TAM195/2001.2/merry_go_round/results.html
3. Wind bands = Coriolis + vertical circulation
a. Consider Equator:
i. Air at surface heats, gain moisture rises
ii. Expands, cools during ascent and precipitates over tropics
iii. Flows N & S away from equator– deflected by Coriolis – sinking at ~ 30º
(subtropical high pressure)
iv. Easterlies: At surface flow from 30º to 0º deflected by Coriolis
b. Consider Polar regions:
i. Polar Easterlies: Cool dry air sinks at the poles and moves toward 60º deflected by Coriolis
ii. Begins heating and gaining moisture as it flows over the surface
iii. Rises, expands, cools and precipitates over temperate zones
c. Mid-latitudes:
i. Circulation follows neighboring cells
ii. Westerlies: at surface, flows from 30º to 60º - deflected by Coriolis
iii. Rises at 60º, expands cools & pcpts
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Geol 117 Oceans
Lecture 16 Atmospheric circulation & Storms
4. Predict Wet vs. Arid zones
a. Regions of rising air mass (tropics and 60º) = wet due to cooling of rising, moist
air
b. Regions of falling air masses (Subtropical high ~30º, and poles) are arid – no
moisture left in the air
5. Warm vs. Cold Fronts (image)
a. warm air rises causing precipitation
b. Cold fronts are steep – storms are brief
C. Land effects: Monsoons and Sea breezes
1. Sea breezes
a. Onshore = day time
i. Hot land (low Heat Capacity) heats air & it rises
ii. Cool ocean breeze blows on-land to replace rising air
b. Offshore -= night
i. Land cools (low heat capacity) and cools air
ii. Warm air over ocean rises
iii. Cooler air from land flows out to replace it
2. Monsoons = large scale onshore sea breeze
a. Summer heating of equatorial continent – air rises
b. Flow of moist ocean air over hot land
i. Heats, rises expands, condenses – precipitates
ii. Continued circulation = continued precipitates
II. Tropical Cyclones (Hurricanes or typhoons in W Pacific)
A. Cause: low pressure cell breaks away from equatorial region and flows into N or S
hemisphere
1. Heat sea surface to >27ºC
2. Transfer E (and vapor) to Atmosphere
a. Warm moist air rises, expands & precipitates
b. Pcpt releases E (4% to generate wind)
3. Circulation/spiral
a. Caused by Coriolis effect as air convects
b. Cannot form on equator – no Coriolis
B. Result
1. Damaging high winds & waves
2. Storm surge
3. Flooding
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