Atmospheric pressure is the force exerted by the weight of the atmosphere above a unit area. The weight of the air exerts a force on the earth or on any object placed in the atmosphere. Pressure is a measure of this force per unit area: i.e., grams per square centimeter, pounds per square foot, etc. Pressure at any point on the earth's surface is the weight per unit area of the column of air above that point. In other words, if the pressure is 14 pounds per square inch, a column of air having one square inch cross section extending to the top of the atmosphere weights 14 pounds. (The average pressure at the surface of the earth is approximately 14.7 pounds per square inch.)

   We have mentioned that at any point in the atmosphere, pressure is the weight of the air above that point. Since pressure is the weight of the air above, and since less and less air lies above a point as it moves upward through the atmosphere, pressure must decrease with increasing altitude. The greater pressure at low altitude compresses the air more than does the lesser pressure at higher altitude. Therefore, the rate of decrease (lapse rate) in pressure with height becomes less with increasing altitude. For example, from sea level to 1,000 feet, pressure drops about one inch of mercury; but from 19,000 to 20,000 feet, pressure drops only about six-tenths of an inch.

   The rate of decrease of pressure with height, however, is not always constant. Like most substances, air contracts as it cools and expands as it becomes warmer. Therefore, when a sample of air cools, it occupies less space; when heated, it occupies more. As a result, the rate of pressure decrease with height in cold air is greater than in warm air.

   Since air is a gas, it may be compressed or permitted to expand. When air is compressed, a given volume contains more air, hence its density, or weight, is increased. Conversely, when air is permitted to expand, a given volume contains less air, thus its density, or weight, is decreased.

   Heat is a property of all matter. From early studies of science, we learned that heat is the motion of molecules. Heat is then defined as the total energy of motion of molecules. We also learned that dense air has more molecules than less dense air. The two might have the same average motion, and thus have the same temperature, but the total energy, and consequently the degree of heat is greater in the dense air with more molecules. We cannot measure heat directly, but we can measure temperature with the thermometer.

   A general gas law defines the relationship of pressure, temperature, and density when there is no change of state or heat transfer. Simply stated, this would be "density varies directly with pressure, inversely with temperature." Consequently, the higher we fly the less dense the air becomes due to less pressure and temperature.

   Atmospheric pressure is continually changing. It varies with both time and location. These pressure changes are caused primarily by changes in the air density (weight of air per unit volume) produced by variations in the distribution of temperature.