Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
PRESSURE, AND WINDS STUDY GUIDE
Assigned reading: Elemental Geosystems, Christopherson, Chapter 4, Pages 111-143
Web Learning Module: Pressure and Winds
- Winds perform important work throughout the global
ecosystem. Their most important function in the atmosphere is to move excess
heat from the tropics toward the poles and to move water vapor from above
oceans to the land. Winds also drive waves and ocean currents and shape
A wind is a horizontal movement across a surface. Winds
are created by air pressure differences due to unequal heating.
Just as unequal heating in a British living room in winter sets up pressure
differences that create drafts, so the sun’s unequal heating of the earth
In general, air that is warmer than surrounding air
is lighter and rises creating a low pressure at the earth's surface.
Because cold air is denser than warm air, cold air sinks and builds up
high pressure. (An important exception to this generalization
is the dynamically-induced subtropical high pressure areas.)
Low pressure areas (cyclones) are zones of rising
air and converging winds. Low pressure areas are often wet and stormy.
High pressure areas (anticyclones) are zones of descending air currents
and diverging winds with clear, fair weather.
Like winds, air pressure is invisible. Meteorologists
show differences in air pressure by drawing isobars on maps. Isobars
are lines joining places with the same air pressure. Isobars form roughly
concentric circles around centers of relatively low and high pressure.
Winds always blow from high to low pressure. The greater
the difference between the high and low pressure (pressure gradient)
the greater the speed of the wind.
Due to the earth’s rotation, however, winds appear to
be deflected to the right of the pressure gradient in the northern hemisphere
and to the left in the southern hemisphere. This deflection is called the
Coriolis effect. At the earth’s surface, winds cross isobars at
about a 45 degree angle but in the upper atmosphere, the winds circulate parallel to the
Unequal heating of land and water in coastal areas leads
to unbalanced pressures that drive air from high to low pressure. The difference
in temperature between the land and sea produces land breezes
at night and sea breezes during the afternoon. This entire mechanism
is called a heat engine.
On a global scale, the atmospheric circulation system
is driven by the solar heat engine with its unequal heating between polar
and equatorial regions.
If the earth did not rotate, warm, rising air from the
equatorial low would rise and move toward the poles at high altitudes,
sink and then cold, dense air would return to the equator near the earth's
The earth's rotation, however, causes rising air currents
at the equator to be turned back toward the earth’s surface near latitude
30 degrees. This creates subtropical high-pressure cells (horse latitudes). Part
of the subsiding air turns toward the equator creating trade winds
(10 - 25 degrees latitude), and part turns poleward creating westerly winds (35
- 60 degrees latitude). Where the northeast and southeast trade winds meet they form the intertropical convergence zone (ITCZ). The ITCZ is a belt of low pressure, instability, and rains.
- The westerly winds (westerlies) bring warm air
from the tropics into contact with cold air from the polar easterlies.
The polar easterlies blow out from the polar high. This low pressure zone
of contact between the westerlies and polar easterlies is known as the
polar front and is the birthplace of most mid-latitude cyclonic
All of the earth’s planetary wind and pressure belts
shift 5 degrees latitude north and south of the equator with the seasonal migration of the sun's vertical rays.
At upper levels along the polar front, wave-like undulations
known as Rossby waves often develop where warm and cold air are
in contact. Jet Streams develop in these upper air waves forming
narrow zones of high velocity, pulse-like winds (200 - 250 mph).
Over large continental areas such as Eurasia, unequal
heating of land and sea produces a seasonal reversal of winds called monsoons.
In hot South Asia (India) in summer, hot air rises creating low pressure.
Moisture-bearing winds from the ocean are then sucked in by the low pressure
bringing torrential rains of the summer monsoon to the land. In
winter, cold air sinks over Siberia producing the Asiatic high pressure
with its dry, out-blowing winter monsoon winds.
Locally, special winds may develop due to pressure differences.
In mountains, differences in heating and cooling give rise to mountain
and valley winds. When air descends and warms by compression, hot, dry
winds occur (e.g., Chinook, Santa Ana).
Atmospheric circulation and related oceanic circulation
play an important role in transferring toxic substances, dust, and biological
organisms far from their source of origin (12 - 25 days in mid-latitudes).
Describe five functions of the wind in the global ecosystem.
Define a wind. What is the fundamental cause of wind?
Explain the relationship between wind and temperature.
Describe the wind and weather characteristics of high
and low pressure areas (cyclones and anticyclones).
Define an isobar.
Describe the relationship between pressure and winds.
Winds blow from ___ to___. What is the pressure gradient?
Give the cause of the Coriolis effect and explain how
it influences the movement of winds in the Northern and Southern Hemispheres.
Describe the effect of unequal heating of land and sea
upon pressure and wind, on a daily basis with diagrams. What is a heat
- What drives the atmospheric circulation system?
- Describe the global circulation system if the earth did not rotate.
Describe the atmospheric circulation between the equator
and the mid-latitudes and between the mid-latitudes and the poles.
Explain the movement of the earth's pressure and wind
belts with the seasons.
What happens along the polar front? Define the jet stream.
Describe with the help of diagrams the pressure and
wind patterns associated with winter and summer monsoons over continents.
List some local winds and how they develop.
Explain how atmospheric and oceanic circulation systems
connect the entire world by transferring pollutants far from their sources.
Draw a diagram to show the pressure and winds on
a hypothetical earth with a uniform surface during an equinox. Assume that
the earth is rotating. Make sure that you include the following global
pressure and wind belts:
Subpolar low and Polar Front in both
Subtropical High Pressure in both hemispheres
Intertropical Convergence Zone (ITCZ)
in both hemispheres
Westerlies in both hemispheres
Northeast Trade winds
Southeast Trade winds
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Created by Rita D. Haberlin, Geography Professor
Peralta Community College District
333 East Eighth Street
Oakland, CA 94606
This Geography Site Is Maintained By Patricia A. Kulda
Last Update July 14, 2010