The Atmosphere Properties Temperature Density Pressure Composition 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] 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. 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. 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. 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. 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$^{2}$.  Figure 9-1.  Atmospheric mass by altitude. Altitude (feet) 53,000 18,000 Sea level 164,000 99.9% of mass below this level  90% of mass below this level  50% of mass below this level  9-2