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## The Atmosphere 

The atmosphere is a mixture of gases surrounding the earth. Without it, there would be no weather (wind, clouds, precipitation) or protection from the sun’s rays. Though this protective envelope is essential to life, it is extraordinarily thin. When compared to the radius of the earth, 3,438 nautical miles (NM), the vertical limit of the atmosphere represents a very small distance. Although there is no specific upper limit to the atmosphere—it simply thins to a point where it fades away into space—the layers up to approximately 164,000 feet (about 27 NM) contain 99.9 percent of atmospheric mass. At that altitude, the atmospheric density is approximately one-thousandth the density of that at sea level. [Figure 9-1] 

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## Composition 

The earth’s atmosphere is composed of a mixture of gases, with small amounts of water, ice, and other particles. Two gases, nitrogen (N$_{2}$) and oxygen (O$_{2}$), comprise approximately 99 percent of the gaseous content of the atmosphere; the other one percent is composed of various trace gases. Nitrogen and oxygen are both considered permanent gases, meaning 

their proportions remain the same to approximately 260,000 feet. Water vapor (H$_{2}$O), on the other hand, is considered a variable gas. Therefore, the amount of water in the atmosphere depends on the location and the source of the air. For example, the water vapor content over tropical areas and oceans accounts for as much as 4 percent of the gases displacing nitrogen and oxygen. Conversely, the atmosphere over deserts and at high altitudes exhibits less than 1 percent of the water vapor content. [Figure 9-2] 

Although water vapor exists in the atmosphere in small amounts as compared to nitrogen and oxygen, it has a significant impact on the production of weather. This is because it exists in two other physical states: liquid (water) and solid (ice). These two states of water contribute to the formation of clouds, precipitation, fog, and icing, all of which are important to aviation weather. 

## Properties 

The state of the atmosphere is defined by fundamental variables, namely temperature, density, and pressure. These variables change over time and, combined with vertical and horizontal differences, lead to daily weather conditions. 

## Temperature 

The temperature of a gas is the measure of the average kinetic energy of the molecules of that gas. Fast-moving molecules are indicative of high kinetic energy and warmer temperatures. Conversely, slow-moving molecules reflect lower kinetic energy and lower temperatures. Air temperature is commonly thought of in terms of whether it feels hot or cold. For quantitative measurements, the Celsius (°C) scale is used in aviation, although the Fahrenheit (°F) scale is still used in some applications. 

## Density 

The density of any given gas is the total mass of molecules in a specified volume, expressed in units of mass per volume. Low air density means a smaller number of air molecules in a specified volume while high air density means a greater number of air molecules in the same volume. Air density affects aircraft performance, as noted in Chapter 5, Glider Performance. 

## Pressure 

Molecules in a given volume of air not only possess a certain kinetic energy and density, but they also exert force. The force per unit area defines pressure. At the earth’s surface, the pressure exerted by the atmosphere is due to its weight. Therefore, pressure is measured in terms of weight per area. For example, atmospheric pressure is measured in pounds per square inch (lb/in$^{2}$). From the outer atmosphere to sea level, a typical value of atmospheric pressure is 14.7 lb/in$^{2}$. 

