<-previous | Geol 117 Home | Lectures | Review | next ->

Lecture 22: ATMOSPHERE CIRCULATION AND WINDS

Coriolis effect
Prevailing winds and vertical circulation
Zones of pressure, evap. & ppt.
Factors modifying global winds
-- Differential heating of land and sea

Powerpoint Lecture Slides


RADIATION IMBALANCE -- ultimate driving force of global winds
Warm, moist air rising at Equator
Cold, dry air sinking at poles

CORIOLIS EFFECT -- deflects winds (and ocean currents)
Earth's rotation changes the initial direction of winds & currents:
N. Hem.: deflection always to the right
S. Hem.: deflection always to the left
Cause -- Different latitudes rotate at different rates
Rotational velocity increases from poles to Equator
Winds and ocean currents have both....
an initial velocity and direction, and....
an initial rotational velocity that depends upon latitude.
As they cross latitudes, winds and currents are rotating
at different velocities than Earth's surface.
Result: Apparent deflection to Earth-bound observers.

Analogy -- launching a missile
1) Fire from North Pole (no rotation)
Earth rotates beneath it
Deflection -- to the west (right)
2) Fire from Equator (max. rotation)
Missile rotates faster than Earth beneath it
Deflection -- to the east (right)

Amount of wind/current deflection depends on ...
... time of travel (how long it moves)
... distance of travel (how far it moves)

Winds and currents are deflected a lot because they are in continuous motion over long distances.

GLOBAL WIND SYSTEM

Major zones

Trade Winds
Westerlies
Polar Easterlies

Other important features:

Convective vertical circulation

Convergent & divergent zones (surface and upper troposphere):

Convergent -- air masses coming together
Divergent -- air masses moving apart

Descending air creates zones of ....
high atm. pressure
sinking dry air- nice weather
Ascending air creates zones of:
low atm. pressure
high rainfall -- rising moist air


"Jet Streams" -- strong winds in upper atm. that lie
above boundaries between the major zones

MODIFICATION OF GLOBAL WIND PATTERNS
-- Differential heating of air and land

Seasonal &daily heating cycles
Oceans and lakes -- little T change
Land areas -- significant T change

1. Daily cycle of winds in coastal areas
Day: onshore winds (peak at mid-afternoon)
Night: offshore winds (peak in early morning)

2. Seasonal monsoons (India, southeast Asia)
Summer: hot continent, rising air, onshore winds (and rain)
Winter: cold continent (Tibet), offshore winds

3. Seasonal changes in wind patterns and pressure zones
over continents and oceans

Summer
Ocean (cool, high pressure) Land (warm, low pressure)

Winter
Ocean (warm, low pressure) Land (cool, high pressure)

Semi-permanent seasonal zones of different pressure

Winds (air flow) around pressure zones are deflected due to Coriolis effect:
Clockwise around (and out of) H-P cells (N. Hem.)
Counterclockwise around (and into) L-P cells (N. Hem.)

Wind speed -- spacing between lines of equal pressure
(= gradient of pressure change):
Strong winds: close spacing (high pressure gradient)
Light winds: wide spacing (low pressure gradient)


(Detailed Notes Start Here)

ATMOSPHERE CIRCULATION AND WINDS

Coriolis effect
Prevailing winds and vertical circulation
Zones of pressure, evap. & ppt.
Factors modifying global winds
-- Differential heating of land and sea

Effect of Earth's rotation -- the Coriolis effect. Winds (and ocean currents) that are moving long distances across the surface of the rotating Earth undergo continuous deflection (change in direction) as they move. The direction of deflection is always to the right in the Northern Hemisphere and always to the left in the Southern Hemisphere. Why does this happen? First of all, remember that the Earth rotates at a constant angular velocity -- 360 degrees per day. But because of Earth's spherical shape, a point on the Equator is moving (rotating) at a greater linear velocity than a point at any other latitude. Linear velocity of rotation decreases with increasing latitude to 0 (zero) at the poles. Thus, winds and ocean currents have both an initial velocity and direction, and an initial rotational velocity that depends upon the latitude where they started. As winds and currents cross latitudes, they are rotating at different velocities than Earth's surface (sometimes faster, sometimes slower). The result is a deflection with respect to the rotating Earth.

We can gain insight into the Coriolis effect on by considering how a missle launched in a particular direction is apparently deflected as it moves over the rotating Earth. Like winds and ocean currents, a missle is not "held" to Earth's surface by gravity but rather maintains a straight course. It is deflected because the Earth rotates beneath it. If we fire a missle from the North Pole, it will be deflected to the west (to the right of its original course) as the Earth rotates from west to east beneath it. If we fire a missle from the Equator to the North Pole, it will be deflected to the east (to the right of its original course). That happens because the missle started with a higher rotational velocity (maximum at the Equator) than the Earth over which it travels (higher and higher latitudes).

The extent of Coriolis deflection of winds and currents depends on (a) the time of travel (how long it is moving), and (b) the travel distance (how far it moves). Winds and currents are deflected a lot because they are in continuous motion over long distances.

The general circulation of the atmosphere -- driven by the latitude radiation imbalance and modified (deflected) by the Coriolis effect -- accounts for the pattern of global wind bands. The convective circulation of the atmosphere (troposphere) also generates regions in both the upper troposphere as well as at the surface where air masses converge or diverge.

Divergence at the surface (and convergence aloft) creates zones of descending air (e.g., at about 30 deg. N and S lat.). These are regions of semi-permanent high atmospheric pressure at the surface. They are also regions of relatively high evaporation, because air sinking from great heights is dry. The world's great deserts and maximum salinity in sea water occurs in these areas.

Convergence at the surface (and divergence aloft) creates zones of ascending air (e.g., at the Equator and about 60 deg. N and S lat.). These are regions of semi-permanent low atmospheric pressure at the surface. They are also regions of relatively high precipitation -- as moist air masses rise, they cool and water vapor condenses.

Jet streams are strong winds in the upper atmosphere (troposphere) that lie above boundaries of converging and diverging air masses.

Modification of global wind patterns. The global pattern of latitudinal belts of prevailing easterlies and westerlies are modified by continental land masses, because continents have different thermal properties than oceans -- land masses heat up and cool off more during seasonal and daily variations in solar radiation than water bodies. Differential heating of water and land alters wind patterns on different time scales.

For example, differential heating/cooling causes winds in coastal areas to blow onshore during the day and offshore during the night. Seasonal monsoons, such as those in the Indian subcontinent, are also the consequence of differential heating/cooling. During the summer, solar radiation causes air to rise over the subcontinent; winds blow onshore (from south to north) bringing welcome rain. During the winter, cooling of India and the Tibetan plateau generates offshore winds.

Differential heating/cooling also sets-up seasonal changes in pressure zones and wind patterns over continents and oceans in general. Just as in daily cycles and monsoons, air tends to sink over relatively cool continents during winter and rise over relatively warm oceans; the reverse holds during the summer. Winds (airflow) around the high- and low- pressure zones that are created are deflected due to the Coriolis effect. Winds don't simply blow away from (or out of) high-pressure zones, but actually blow around those zones -- clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Likewise, winds don't blow into low-pressure zones but around them -- counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Wind direction is parallel to lines of equal pressure around high- and low-pressure zones. Wind speed is high when the lines are close together and low when the lines are far apart. As we shall see, surface ocean currents behave in much the same way!


<-previous | Geol 117 Home | Lectures | Review | next ->