Download Geography 310 Lecture 8 Pressure and Winds Needs: 3 GIF A

Geography 310
Lecture 8
Pressure and Winds
Needs: 3 GIF
A. Pressure-- force/area due to gravity (14.7#/in 2, 29.92"Hg, 1013.2 mb)
1. map representation = isobar (equal pressure isoline)
a. spacing = pressure gradient, 4 mb increment common, (reference level is mean
sea level)
b. low pressure center = "Low" , elongated area = "Trough" ^
c. high pressure center = "High" , elongated area = "Ridge" _
2. Causes of Pressure pat terns
a. temperature (cold dense, warm light)
I. thermal highs (Siberia in winter, poles)
ii. thermal lows (SW U.S. in summer)
b. simple global pattern with only temperature considerations
I. low in tropics, high near poles
ii. mid-latitude continent, low in summer, high in winter
iii. mid-latitude ocean, high over cold water, low over warm
c. dynamic (caused by physical movement of air)
I. dynamic highs (subtropics, air accumulates in upper atmos.
ii. dynamic lows (mid-lats, tropics, air removed in upper atmos.
d. mechanisms can complement or cancel out
I. low pressure storms-dynamic origin, strengthen over warm water (i.e.
hurricanes)
ii. high pressure-dynamic origin, strengthen over cold surface (continental
snow)
3. Surface Pressure patterns (generalized)
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* $ L$ H $ L$
*
* $ H$ L $ H$
*
*LLLLLLLLLLLLLL*
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general
summer
winter
a.
b.
c.
d.
e.
f.
g.
belts migrate seasonally following the sun's vertical rays
pressure gradient stronger in winter
oceans--high in summer, low in winter (see pattern above)
continents--high in winter, low in summer (see pattern)
equatorial low
subtropical high--summer intense over ocean (T+D reinforced)
subpolar low--winter intense over ocean, strong zone in southern hemisphere
(T+D reinforced)
h. polar high--related to cold temperatures + dynamics
Geography 310 Lecture 8
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I. summary for continents and oceans in mid-latitudes
I. cont. (summer) thermal low dominates in interior deserts
ii. cont. (winter) thermal high dominates interior
iii. ocean (summer) SPL weakens, STH intensifies and moves N
iv. ocean (winter) SPL intensifies, STH weakens and moves S
B. Winds--horizontal movement of air relative to sfc.
GIF1 1. Pressure gradient force--generates winds, acts at right angle to isobars from high to
low pressure H* Y* L weak* * stro ng5555
a. closer isobars, stronger PG force, stronger winds
2. Coriolis effect--if earth not rot ating winds blow directly from H to L,
however...rotation causes deflection
GIF2
a. earth rotates under atmosphere following a curved path, air follows straight line
path
b. earth imparts velocity to air which it conserves
c. effect increase in magnitude as wind speed increases, direction is to the right in
NH, and to the left in SH, with no effect (0) at the equator
d. PG + C = flow along isobars in NH
(show development)
HHHHH
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LLLLLL
3. Friction--winds cross isobars to low pressure in NH
(show development)
HHHHH
bbbbb
LLLLLL
4.circulation patterns (map view and cross section)
(NH shown, direction reversed in SH)
b
_
`La
_
a H`
b
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YY L ZZ
YYZZ
\
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5. Low = rising air -- clouds, precipitation, but need dynamic divergence aloft (define)
High = sinking air -- clear skies, but need dynamic convergence aloft (define)
6. Local Wind Systems
a. Land-Sea breeze
GIF3
I. effects from Lake Michigan
ii. effects along California Coast (enhanced by regional winds)
b. Mountain-Valley breeze
Geography 310 Lecture 8
7. Chaotic behavior in the atmosphere
a. laminar vrs. turbulent flow
b. the "butterfly" effect
c. model outcomes sensitive to initial conditions, which must be approximated
d. predictive accuracy decreases as time increases from initial conditions because
of b&c above (knowing system function may not lead to ability to predict)
e. assuming persistence can help improve some forecasts
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