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 (N2) and oxygen (O2), 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 (H2O), 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/in2). From the outer atmosphere to sea level, a typical value of atmospheric pressure is 14.7 lb/in2.
In aviation weather reports, the units of pressure are inches of mercury (“Hg) and millibars (mb) and 29.92 “Hg equals 1013.2 mb. This force or pressure is created by the moving molecules act equally in all directions when measured at a given point.
In the METAR report, see Figure 9-3, the local altimeter setting “A2955”, read as 29.55, is the pressure “Hg. In the remarks (RMK) section of this report, sea level pressure expressed as “SLP010”, the value expressed in millibars (hPa), is used in weather forecasting.
Dry air behaves almost like an ideal gas, meaning it obeys the gas law given by P/DT = R, where P is pressure, D is density, T is temperature, and R is a constant. This law states that the ratio of pressure to the product of density and temperature must always be the same. For instance, at a given pressure if the temperature is much higher than standard, then the density must be much lower. Air pressure and temperature are usually measured, and using the gas law, density of the air can be calculated and used to determine aircraft performance under those conditions.
Using a representative vertical distribution of these variables, the standard atmosphere has been defined and is used for pressure altimeter calibrations. Since changes in the static pressure can affect pitot-static instrument operation, it is necessary to understand basic principles of the atmosphere. To provide a common reference for temperature and pressure, a definition for standard atmosphere, also called International Standard Atmosphere (ISA), has been established. In addition to affecting certain flight instruments, these standard conditions are the basis for most aircraft performance data.
At sea level, the standard atmosphere consists of a barometric pressure of 29.92 “Hg, or 1,013.2 mb, and a temperature of 15 °C or 59 °F. Under standard conditions (ISA), a column of air at sea level weighs 14.7 lb/in2.
Since temperature normally decreases with altitude, a standard lapse rate can be used to calculate temperature at various altitudes. Below 36,000 feet, the standard temperature lapse rate is 2 °C (3.5 °F) per 1,000 feet of altitude change. Pressure does not decrease linearly with altitude, but for the first 10,000 feet, 1 “Hg for each 1,000 feet approximates the rate of pressure change. It is important to note that the standard lapse rates should be used only for flight planning purposes with the understanding that large variations from standard conditions can exist in the atmosphere. [Figure 9-4]
Layers of the Atmosphere
The earth’s atmosphere is divided into five strata, or layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. [Figure 9-5] These layers are defined by the temperature change with increasing altitude. The lowest layer, called the troposphere, exhibits an average decrease in temperature from the earth’s surface to about 36,000 feet above mean sea level (MSL). The troposphere is deeper in the tropics and shallower in the polar regions. It also varies seasonally, being higher in the summer and lower in the winter months.
Almost all of the earth’s weather occurs in the troposphere as most of the water vapor and clouds are found in this layer. The lower part of the troposphere interacts with the land and sea surface, providing thermals, mountain waves, and seabreeze fronts. Although temperatures decrease as altitude increases in the troposphere, local areas of temperature increase (inversions) are common.
The top of the troposphere is called the tropopause. The pressure at this level is only about ten percent of MSL (0.1 atmosphere) and density is decreased to about 25 percent of its sea level value. Temperature reaches its minimum value at the tropopause, approximately –55 °C (–67 °F). For pilots, this is an important part of the atmosphere because it is associated with a variety of weather phenomena, such as thunderstorm tops, clear air turbulence, and jet streams. The vertical limit altitude of the tropopause varies with season and with latitude. The tropopause is lower in the winter and at the poles; it is higher in the summer and at the equator.
The tropopause separates the troposphere from the stratosphere. In the stratosphere, the temperature tends to first change very slowly with increasing height. However, as altitude increases the temperature increases to approximately 0 °C (32 °F) reaching its maximum value at about 160,000 feet MSL. Unlike the troposphere in which the air moves freely both vertically and horizontally, the air within the stratosphere moves mostly horizontally.
Gliders have reached into the lower stratosphere using mountain waves. At these altitudes, pressurization becomes an issue, as well as the more obvious breathing oxygen requirements. Layers above the stratosphere have some interesting features that are normally not of importance to glider pilots. However, interested pilots might refer to any general text on weather or meteorology.