I. Atmospheric pressure is the weight of the
overlying atmosphere.
A. Atmospheric pressure is measured by an
instrument called a barometer.
1. One
of the earliest traditional types of barometers is the mercury
barometer (see text).
2. A
barometer that works by enclosing a flexible chamber is called an
aneroid barometer.
B. Atmospheric pressure is measured in
different ways.
1.
Inches of mercury is a traditional way to express atmospheric pressure
in public weather reports.
a. The metric equivalent is
centimeters of mercury.
b. At sea level, average pressure is almost 30
inches of mercury (about 76 cm mercury).
2. A
bar is a unit of pressure roughly equivalent to the weight of the
atmosphere at sea level.
3. A
variation of the bar is the millibar. 1 bar = 1000 millibars
(remember how metric units work?)
4.
Pounds per square inch is not really a measure of atmospheric
pressure. Rather it is a measure of inflation pressure (how much
air to put in your car tires or a basketball).
II. Winds occur when air masses move from regions of higher
pressure to regions of lower pressure.
A. Isobars are imaginary lines that connect
points of equal atmospheric pressure on a given day.
B. Pressure gradient force is what determines
the strengthe of winds.
1.
Isobars that are widely spaced apart indicate gently blowing winds
(pressure gradient force is low)
2.
Isobars that are closely spaced together indicate strong winds (high
pressure gradient)
C. The Coriolis force determines that moving
air masses in the northern hemisphere veer to the right.
1. This
is caused by the spin of the Earth.
2. It
is the opposite in the southern hemisphere.
D. Pressure trends:
|
winds move
|
weather trends
|
vertical movement
|
rotational circulation,
N. hemisphere
|
low pressure -- cyclone
|
inward
|
cloudy skies, rains
|
air rises
|
counter clockwise
|
high pressure -- anticyclone
|
outward
|
clear skies, can be associated
w/ droughts & heat waves
|
air subsides
|
clockwise
|
III. Global pressure trends
A. Equatorial regions is hotter and the air
above expands, becomes less dense and rises. This produces a low
pressure belt at this latitude.
1.
Generally, the equatorial region has a warm, rainy climate.
B. Polar regions are colder, and air above
contracts, becomes denser, and subsides, producing a high pressure
region at this latitude.
C. 30
o N & S latitudes are high
pressure belts. Many of the world's deserts are situated at this
latitude.
D. 60
o N & S latitude are low
pressure belts.
IV. Global prevailing winds
A. Between 0
o and 30
o,
trade winds blow generally to the west
B. Between 30
o and 60
o,
the westerlies dominate, blowing from west to east
C. Between 60
o and 90
o,
the polar esterlies prevail.
V. Regional/seasonal wind patterns
A. High pressure systems can tend to form on
land during winter months
1. The colder
temperatures on land (compared to the ocean) cause the air above to
cool, contract, sink, and create high pressure systems.
B. Low pressure systems can tend to form on
land during summer months.
1. The
warmer temperatures on land (compared to the ocean) cause the
air above to warm, expand, rise, and create low pressure systems.
C. When this pair of phenomena dominates
in an area, a monsoon pattern is created.
1. Summer low pressure systems pull in warm, moist air from the
oceans and create rainy climate on land
2. Winter high pressure systems push away moist, ocean air w/ dry
air from land.
3. This pattern is best known in southern Asia (India,
Bangladesh), but may occur in the Southwest U.S.