The establishment of a standard atmosphere was necessary to give scientists a yardstick to measure or compare actual pressure with a known standard. In the International Civil Aeronautical Organization (ICAO), the standard atmosphere assumes a mean sea level temperature of 59F or 15C and a standard sea level pressure of 1,013.25 millibars or 29.92 inches of mercury. It also has a temperature lapse rate (decrease) of 3.6F per 1000 feet or 0.65C per 100 meters up to 11 kilometers and a tropopause and stratosphere temperature of 56.5C or 69.7F.

Pressure at any point in a column of water, mercury, or any fluid, depends upon the weight of the column above that point.

Air pressure at any given altitude within the atmosphere is determined by the weight of the atmosphere pressing down from above. There-fore, the pressure decreases with altitude because the weight of the atmosphere decreases. It has been found that the pressure decreases by half for each 18,000-foot (5,400-meter) increase in altitude. Thus, at 5,400 meters one can expect an average pressure of about 500 millibars and at 36,000 feet (10,800 meters) a pressure of only 250 millibars, etc. Therefore, it maybe concluded that atmospheric pressures are greatest at lower elevations because the total weight of the atmos-phere is greatest at these points.

There is a change of pressure whenever either the mass of the atmosphere or the accelerations of the molecules within the atmosphere are changed. Although altitude exerts the dominant control, temperature and moisture alter pressure at any given altitudeespecially near Earths surface where heat and humidity, are most abundant. The pressure variations produced by heat and humidity with heat being the dominant force, are responsible for Earths winds through the flow of atmospheric mass from an area of higher pressure to an area of lower pressure.

Pascals Law is an important law in atmos-pheric physics. The law states that fluids (includ-ing gases such as Earths atmosphere) transmit pressure in all directions. Therefore, the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface that is exposed to the atmosphere.

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