Earth
Science, 10th edition
Chapter
17: Air Pressure and Wind
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 pressure is 29.92 inches of mercury
E. Instruments for measuring
1. Barometer
a. Mercury barometer
1. Invented by Torricelli
in 1643
2. Uses a glass tube
filled with mercury
b. Aneroid barometer
1. "Without
liquid"
2. Uses an expanding
chamber
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
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
a. Only important near the
surface
b. Acts to slow the air's
movement
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
kilometers per hour
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. Anticyclone
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 atmospheric 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 the 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
D. Influence of continents
1. Seasonal temperature differences
disrupt the
a. Global pressure patterns
b. Global wind patterns
2. Influence is most obvious in the
Northern Hemisphere
3. Monsoon
a. Seasonal change in wind
direction
b. Occur over continents
1. During
warm months
a. Air flows onto
land
b. Warm, moist air
from the ocean
2. Winter months
a. Air flows off the
land
b. Dry, continental
air
V.
Circulation in the mid-latitudes
A. The zone of the westerlies
B. Complex
C. Air flow is interrupted 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. Land and sea breezes
2. Mountain and valley breezes
3. Chinook and
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. Instrument for measuring wind
direction is the wind vane
3. Direction indicated by either
a. Compass points (N, NE,
etc.)
b. Scale of 0° to 360°
4. Prevailing wind comes more often
from one direction
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
IIX. Global
distribution of precipitation
A. Relatively complex pattern
B. Related to global wind and pressure
patterns
1. High pressure regions
a. Subsiding air
b. Divergent winds
c. Dry conditions
d. e.g.,
2. Low pressure regions
a. Ascending air
b. Converging winds
c. Ample precipitation
d. e.g., Amazon and
C. Related to distribution of land and
water
1. Large landmasses in the middle
latitudes 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