Winds and the Global Circulation System
This chapter examines how unequal surface heating and the rotation
of the earth generate global circulation systems in the atmosphere
- The weight of air and the force of gravity pulling air towards
the earth create air pressure. Air pressure is greatest
at the earth's surface and decreases with altitude.
- Differences in pressure cause air to move horizontally. This
air in motion is called wind. Winds move from areas of
high pressure to areas of low pressure.
- Pressure differences between two places create pressure
gradients and the resulting pressure gradient force causes
air to move from high pressure areas to low pressure areas.
Land and sea breezes are examples of winds caused by
pressure differences that result from temperature differences
over land and water surfaces.
Wind direction is measured by a wind vane and wind
speed is measured by an anemometer.
- The Coriolis effect is due to the earth's rotation
and causes objects in motion to appear to be deflected off course.
This apparent deflection is to the right in the northern hemisphere
and to the left in the southern hemisphere. The effect is absent
at the equator and increases as you move toward the poles.
- The third force affecting the direction of wind is that of
- Air flow spirals into a low-pressure center and rises
while the air descends and flows out of a high-pressure center.
- The inspiral at a low-pressure center is counterclockwise
in the northern hemisphere and clockwise in the southern hemisphere.
The outspiral at a high-pressure center is clockwise in
the northern hemisphere and counterclockwise in the southern hemisphere.
Cyclones (low pressure centers) are associated with
cloudy or rainy weather. Anticyclones (high pressure centers)
are associated with clear, dry weather.
- At the equator, heating causes air to rise creating an area
of low pressure called the intertropical convergence zone
- At 30° latitude, air descends creating areas of high
pressure in the subtropical high-pressure belt. Air moves
out of these high pressure areas toward the equator creating the
trade winds. Winds also move toward the midlatitudes creating
- The monsoon is a seasonally reversing wind pattern
that brings heavy rains onto the Asian subcontinent in summer
and hot, dry conditions in the winter.
- Winds at an altitude of 5 to 7 km above the earth's surface
are influenced by pressure gradient force and Coriolis force but
not by the force of friction. These are the geostrophic winds
that flow parallel to isobars.
waves are large undulations in the flow
of the upper air Westerlies along the zone of contact between
cold and warm air. They allow warm air to penetrate northward
and cold air to penetrate southward.
Jet streams are narrow bands of high velocity air that
form along the polar front and above the Hadley cell in
- The uppermost layer of ocean water is the warmest.
Below this warm layer temperatures decline rapidly to around 0°
and remain cold in a layer extending to the ocean floor.
Ocean currents are persistent, mainly horizontal flows
of ocean water set in motion by the prevailing surface winds.
Coriolis force causes the flows to be deflected about 45 °
from the direction of the wind.
Gyres are circular movements of water that are driven
by the subtropical high pressure cells.
El Niño occurs when warm water replaces the
usual upwelling cold water that flows along the South American
coast. El Niño causes a decline in the numbers of
fish and birds and is thought to affect climate in other parts
of the world.
- Wind and wave energy are indirect forms of solar energy. Wind
power has been used for centuries and is on the rise again.
Technologies to exploit wave energy are in the planning and testing
intertropical anticyclones Rossby waves
convergence zone Hadley cells jet stream
pressure gradient doldrums thermocline
isobars Subtropical ocean currents
wind vane high-pressure gyres
anemometer belts El Niño
Coriolis effect monsoon La Niña
cyclones geostrophic wind
- What is the normal value of atmospheric pressure at sea level?
How and why does it change with altitude?
- Use the example of land and sea breezes to illustrate how
differential surface heating produces pressure gradients and winds.
- Define the Corliolis effect and describe how it influences
winds and ocean currents at the earth's surface.
- Describe the three-dimensional pattern of air flow in cyclones
and anticyclones in the northern and southern hemispheres. What
forces drive these air flow patterns?
- Sketch a diagram to show the following features of the global
atmospheric circulation system: the doldrums, equatorial trough,
Hadley cells, ITC, trade winds, subtropical high-pressure belts,
westerlies, polar front and polar easterlies.
- Describe the seasonal pressure and circulation patterns of
the Asian monsoon.
- What are local winds and what factors lead to their development?
- What are Rossby waves and what role do they play in the process
of poleward heat transfer?
- What are jet streams? How do they form and where are they
- Describe the thermal structure of the oceans.
- Sketch a diagram to show the general pattern of ocean currents
in the Pacific ocean. How are these current patterns related to
global wind patterns?
- What are El Niño and La Niña?
- What are some of the advantages and disadvantages of using
wind as a power source?
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