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Chapter 16

Air Pressure and


I. Atmospheric pressure

A. Force exerted by the weight of the air above

B. Weight of the air at sea level

1. 14.7 pounds per square inch

2. 1 kilogram per square centimeter

C. Decreases with increasing altitude.

D. Units of measurement

1. Millibar (mb) - standard sea level pressure is 1013.2 mb

2. Inches of mercury - standard sea level is 29.92 inches of mercury


E. Instruments for measuring

1. Barometer

a. Mercury barometer

1. Invented byTorricelli in 1643

2. Uses a glass tube filled with mercury

b. Aneroid barometer

1. " Without liquid "

2. Uses an expanding chamber

2. Barograph ( continuously records the air pressure )


II. Wind

A. Horizontal movement of air

1. Out of areas of high pressure

2. Into areas of low pressure

B. Controls of wind

1. Pressure gradient force

a. Isobars - lines of equal air pressure

b. Pressure gradient - pressure change over distance


Wind continued

2. Coriolis effect

a. Apparent deflection in the wind direction due to Earth’s rotation

b. Deflection is

1. To the right in the Northern Hemisphere

2. To left in the Southern Hemisphere

3. Friction with Earth’s surface

a. Only important near the surface

b. Acts to slow the air’s movement


Wind continued

C. Upper air winds

1. Generally blow parallel to isobars - called geostrophic winds

2. Jet stream

a. " River " of air

b. High altitude

c. High velocity ( 120 - 240 km / hr. )


III. Cyclones and anticyclones

A. Cyclone

1. A center of low pressure

2. Pressure decreases toward the center

3. Winds associated with

a. In the Northern Hemisphere

1. Inward ( convergence )

2. Counterclockwise

b. In the Southern Hemisphere

1. Inward ( convergence )

2. Clockwise

4. Associated with rising air

5. Often bring clouds and precipitation


B. Anticyclones

1. A center of high pressure

2. Pressure increases toward the center

3. Winds associated with

a. In the Northern Hemisphere

1. Outward ( divergence )

2. Clockwise

b. In the Southern Hemisphere

1. Outward ( divergence )

2. Counterclockwise

4. Associated with subsiding air

5. Usually bring " fair " weather


IV. General atmosphere circulation

A. Underlying cause is unequal surface heating

B. On the rotating Earth there are three pairs of atmospheric cells that redistribute the heat

C. Idealized global circulation

1. Equatorial low pressure zone

a. Rising air

b. Abundant precipitation


2. Subtropical high pressure zone

a. Subsiding, stable, dry air

b. Near 30 degrees latitude

c. Location of great deserts

d. Air traveling equatorward from subtropical high produces the trade winds

e. Air traveling poleward from the subtropical high produces the westerly winds


3. Subpolar low pressure zone

a. Warm and cool winds interact

b. Polar front - an area of storms

4. Polar high pressure zone

a. Cold subsiding air

b. Air spreads equatorward and produces polar easterly winds

c. Polar easterlies collide with the westerlies along the polar front


V. Circulation in the mid-latitudes

A. Complex

B. Occurs in the zone of the westerlies

C. Air flow is interruted by cyclones

1. Cells move west to east in the Northern Hemisphere

2. Create anticyclonic and cyclonic flow

3. Paths of the cyclones and anticyclones are associated with the upper-level airflow


VI. Local Winds

A. Produced from temperature differences

B. Small scale winds

C. Types

1. Sea and land breezes

2. Valley and mountains breezes

3. Chinook and Santa Ana winds


VII. Wind Measurement

A. Two basic measurements

1. Direction

2. Speed

B. Direction

1. Winds are labeled from where they originate ( e.g., North wind - blows from the north toward the south )

2. Instruments for measuring wind direction is the wind vane

3. Direction indicated by either

a. Compass points ( N, NE, etc.)

b. Scale of 0 0 to 360 0

4. Prevailing wind comes more often from one direction


Measurement continued

C. Speed - often measured with a cup anemometer

D. Changes in wind direction

1. Associated with locations of

a. Cyclones

b. Anticyclones

2. Often bring changes in

a. Temperature

b. Moisture conditions


VIII. Global distribution of precipitation

A. Relatively distribution of precipitation

B. Related complex wind and pressure patterns

1. High pressure regions

a. Subsiding air

b. Divergent winds

c. Dry conditions

d. e.g., Sahara and Kalahari deserts

2. Low pressure regions

a. Ascending air

b. Converging winds

c. Ample precipitation

d. E.g., Amazon and Congo basins

C. Related to distribution of land and water

1. Large landmasses in the middle latitude often have less precipitation toward their centers

2. Mountain barriers also alter precipitation patterns

a. Windward slopes receive abundant rainfall from orographic lifting

b. Leeward slopes are usually deficient in moisture

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