CHAPTER 5
I. The Nature of Atmospheric Pressure
A. Pressure—the force a gas exerts on some specified area.
B. Atmospheric pressure—the force exerted by gas molecules in the atmosphere.
1. Omnidirectional force—exerted equally in all directions.
2. Force drops with increasing altitude because actual number of gas molecules also drops.
C. Pressure, Density, and Temperature
1. Variations in any one—pressure, density, and temperature of atmosphere—affect the other
two.
2. Density—amount of matter in a unit of volume. Density of gas changes easily because gas
expands as far as the environmental pressure will allow.
a) The denser the gas, the greater the pressure it exerts.
D. Mapping Pressure with Isobars
1. Barometer—instrument for measuring atmospheric pressure.
2. Millibar—an “absolute” measure of pressure--average sea-level pressure is 1013.25
millibars.
3. Isobar—a line joining points of equal atmospheric pressure.
4. “High” and “low” pressures are relative conditions, with the distinction depending on the
pressure of the adjoining areas.
5. Pressure gradient—the horizontal rate of pressure change, representing the “steepness” of
the pressure slope; has a direct effect on the speed of wind.
II. The Nature of Wind
A. Wind—horizontal movements of air; involve more area than do vertical motions.
B. Updrafts and downdrafts—small-scale vertical motions.
C. Ascents and subsidences—large-scale vertical motions.
D. Direction of Movement
1. Depends on the interaction of three factors:
a) pressure gradient
b) friction
c) Coriolis effect (Earth’s rotation)
(1) Coriolis effect—the apparent deflection of free moving objects to the right in the
Northern Hemisphere and to the left in the Southern Hemisphere, in response to the
rotation of Earth.
(a) Geostrophic wind—a wind that moves parallel to the isobars as a result of the
balance between the pressure gradient force and the Coriolis effect.
E. Cyclones and Anticyclones
1. Cyclone—low-pressure cell.
2. Anticyclone—high-pressure cell.
F. Wind Speed
1. Is determined by pressure gradient.
a) The steeper its slope, the faster the wind.
2. Most persistent winds are usually in coastal areas or high mountains.
III. Vertical Variations in Pressure and Wind
A. Atmospheric pressure usually decreases rapidly with height.
B. Wind speed usually increases with height; winds tend to move faster above friction layer.
IV. The General Circulation of the Atmosphere
A. Rotation of Earth and its variable surfaces is key in creating a complex circulation pattern for
atmosphere.
1. Only the tropical regions have a complete vertical cell.
a) Hadley cell—complete vertical circulation cells in which warm air rises to elevations of
about 50,000 feet (15 km), where it cools and moves poleward, then subsides. The cell’s
air rises at the equator and descends at about 30° of latitude (either north or south,
depending on cell).
(1) There are two.
2. At midlatitudes and high latitudes, vertical cells do not exist or are weakly and
sporadically developed.
B. The general circulation of the atmosphere has seven surface components:
1. Polar high
2. Polar easterlies
3. Subpolar low
4. Westerlies
5. Subtropical high
6. Trade winds
7. Intertropical convergence zone
C. Subtropical latitudes serve as the “source’ of the major surface winds of the planet.
D. Subtropical Highs
1. Subtropical highs— (STHs) large semipermanent high-pressure (anticyclone) cells
centered at about 30° latitude over the oceans; have average diameters of 3,200 kilometers
(2,000 miles) and are usually elongated east–west. Develop from the descending air of the
Hadley cells.
a) Horse latitudes—areas in the subtropical highs characterized by warm, tropical
sunshine and an absence of wind; created because weather within an STH is nearly
always clear, warm, and calm.
b) STHs also coincide with most of the world’s major deserts.
c) STHs serve as source for two of the world’s three major surface systems:
(1) Trade winds
(2) Westerlies
E. Trade Winds
1. Trade winds—the major wind system of the tropics, issuing from the equatorward sides of
the subtropical highs and diverging toward the west and toward the equator.
a) Most reliable of all winds, being extremely consistent in both direction and speed.
(1) Winds are named for the direction they blow from.
(a) Trade winds’ origin depends on which hemisphere they are in.
(i) In Northern Hemisphere, originate in northeast, so are sometimes called
northeast trades.
(ii) In Southern Hemisphere, originate in southeast, so are sometimes called
southeast trades.
b) Warming, drying winds capable of holding enormous amounts of moisture.
(1) Do not release moisture unless forced by a topographic barrier or pressure
disturbance.
(a) Pass over low-lying islands, which thus are desert islands.
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(b) Windward slopes in trade winds, as in Hawaii, are some of the wettest places on
Earth.
Intertropical Convergence Zone
1. Intertropical convergence zone— (ITCZ) a belt of calm air where northeast trades and
southeast trades converge, generally in the vicinity of the equator.
a) Also called equatorial front, intertropical front, and doldrums.
(1) Front—a zone of discontinuity between unlike air masses.
b) ITCZ zone’s thunderstorms provide the updrafts where all the rising air in of the
tropics ascends.
c) Often appears as a narrow band of clouds over oceans, but it is less distinct over
continents.
The Westerlies
1. Westerlies—the great wind system of the midlatitudes that flows basically from west to
east around the world in the latitudinal zone between about 30° and 60˚ both north and
south of the equator.
a) Two cores of high-speed winds at high altitudes in the westerlies:
(1) Polar front jet stream
(2) Subtropical front jet stream (see question 14)
b) Rossby waves—sweeping north–south undulations that westerlies frequently develop
in upper air.
(1) Rossby waves and the migratory pressure systems and storms associated with
westerly flow make midlatitudes have more short-run variability of weather than
any other place on Earth.
Polar Highs
1. Polar high—a high-pressure cell situated over either polar region.
a) Because it forms over an extensive, high-elevation, very cold continent, Antarctic high
differs greatly from Arctic high.
(1) Antarctic high is strong, persistent, and almost a permanent feature, while Arctic
high is much less pronounced and more transitory.
Polar Easterlies
1. Polar easterlies—a global wind system that occupies most of the area between the polar
highs and about 60° of latitude.
a) The winds move generally from east to west and are typically cold and dry.
Subpolar Lows
1. Subpolar low—a zone of low pressure that is situated at about 50° to 60° of latitude in both
Northern and Southern hemispheres and which often contains the polar front.
a) Characteristics vary in either hemisphere because the continents interrupt Northern
subpolar system, while Southern is virtually continuous over the oceans.
b) Polar front—the meeting ground of the polar easterlies’ cold winds and the westerlies’
warm winds.
Vertical Patterns of the General Circulation
1. Winds in upper elevations of troposphere are different from surface winds.
a) Most dramatic difference occurs between surface trade winds and the upper-elevation
antitrade winds.
(1) Antitrade winds—tropical upper air winds that blow toward the northeast in the
Northern Hemisphere and toward the southeast in the Southern Hemisphere.
V. Modifications of the General Circulation
A. The general circulation varies because of many factors, but the two principal modifications are
seasonal variations in location and monsoons.
B. Seasonal Variations in Location
1. The seven surface components of the general circulation shift latitudinally with the
changing seasons.
a) Affect weather only minimally in equator and polar regions, but significantly alter
weather in midlatitudes and their fringes.
C. Monsoons
1. Monsoon—a seasonal reversal of winds; a general onshore movement in summer and a
general offshore flow in winter, with a very distinctive seasonal precipitation regime.
a) Most significant disturbance to the pattern of general circulation.
(1) Offshore flow—wind movement from land to water.
(2) Onshore flow—wind movement from water to land.
b) Control the climates of regions with more than half of the world’s population.
c) Origin of monsoons is still not understood, though there is increasing evidence that it is
associated with upper-air phenomena, particularly jet stream behavior.
d) Monsoons have an essential impact—their failure or even late arrival of monsoonal
moisture inevitably causes widespread starvation and economic disaster.
VI. Localized Wind Systems
A. Lesser winds have a considerable effect on weather and climate on a localized scale.
B. Sea and Land Breezes
1. Cycle of sea breezes and land breezes is a common local wind system along tropical
coastlines and somewhat in summer in midlatitude coastal areas.
a) Essentially a convectional circulation caused by differential heating of land and water
surfaces.
(1) Land breeze—local wind blowing from land to water, usually at night (and
normally considerably weaker flow than that of sea breeze).
(2) Sea breeze—local wind blowing from sea toward the land, usually during the day.
C. Valley and Mountain Breezes
1. Daily cycle of airflow occurs with valley and mountain breezes.
a) Convectional circulation caused by differential heating of higher versus lower
elevations.
(1) Mountain air cools quickly at night, allowing cooler air to drain down the slope in
the evening. Conversely, valley air heats more rapidly during the day, allowing
warm air to move upslope during the day.
(a) Valley breeze—an upslope flow, during day.
(b) Mountain breeze—a downslope flow, during night.
(i) Air drainage—the sliding of cold air downslope to collect in the lowest spots,
usually at night; a modified form of mountain breeze common in winter.
D. Katabatic Winds
1. Katabatic wind—a wind that originates in cold upland areas and cascades toward lower
elevations under the influence of gravity.
a) Air is cold and dense, and usually colder than the air it displaces in its downslope flow.
(1) Mistral—a cold, high-velocity wind that sometimes surges down France’s Rhone
Valley, from the Alps to the Mediterranean Sea. Has considerable destructive power.
(a) Similar winds are called bora in Adriatic region and taku in Alaska.
E. Foehn/Chinook Winds
1. Chinook—a localized downslope wind of relatively dry and warm air, which is further
warmed adiabatically as it moves down the leeward slope of the Rocky Mountains.
a) Called foehn when it occurs in Europe.
2. Santa Anas—similar to chinook/foehn. Have high speed, high temperature, and extreme
dryness and prompt wildfires.