Atmospheric Pressure and Winds

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.  30o N & S latitudes are high pressure belts.  Many of the world's deserts are situated at this latitude.
D.  60o N & S latitude are low pressure belts.

IV.  Global prevailing winds
A.  Between 0o and 30o, trade winds blow generally to the west
B.  Between 30o and 60o, the westerlies dominate, blowing from west to east
C.  Between 60o and 90o, 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.