The Atmosphere

Geometric altitude vs. temperature, pressure, density, and the speed of sound derived from the 1962 U.S. Standard Atmosphere.

In this figure, the pilot is attempting to fly from point A to point B, but winds blowing crosswise to his intended flight path push him to point C. The pilot should have pointed the aircraft slightly into the wind, which would have canceled out any drifting off course.

A typical statistical maximum wind speed curve.

Flight path of an aircraft through various forms of turbulence. Relatively stable air exists about thunderstorms.

The Atmosphere

Aircraft fly within the Earth's atmosphere, the gaseous envelope that surrounds the planet. The atmosphere's weather, temperature, and properties all affect aircraft flight, as does the way in which air, the fluid of interest to aerodynamicists, moves around and over the parts of an aircraft.

A fluid is a continuous and shapeless substance that assumes the shape of its container and whose molecules move freely past one another. All fluids are compressible to some extent, that is, their molecules can be compacted and their density increased under increasing pressure, but liquids are, as a rule, highly incompressible compared with gases. Even gases, like air, though may be treated as incompressible if their speeds are not great. For subsonic airflow over an airplane traveling below about 150 meters per second (about 492 feet per second), air may be treated as incompressible (that is, there is no change in density throughout the flow). At higher speeds, the behavior of the airflow changes and the effects of compressibility, or change in density, must be taken into account.

The primary ingredients in the Earth's atmosphere are nitrogen (78 percent) and oxygen (21 percent). The remaining one percent consists of argon, carbon dioxide, several trace gases (extremely small amounts), and water vapor. Above about 56 miles (90 kilometers) from the Earth's surface, the different gases begin to settle or separate out according to their respective densities. In ascending order one would find high concentrations of oxygen, helium, and then hydrogen, which is the lightest of all the gases.

Researchers have studied the Earth's atmosphere since the 17^th century, documenting variations across seasons, in different geographic areas and climates, and at varying altitudes. Beginning in the 1920s, and updated many times since, meteorologists in the United States and around the world have used a large amount of data to construct what is called the standard atmosphere. The standard atmosphere is a hypothetical model that lists average (mean) conditions for atmospheric composition, pressure, temperature, and several other parameters in a motionless, stable atmosphere from sea level to an altitude of 1,000 kilometers (about 621 miles) and without considering the variations that occur with the seasons. The standard atmosphere is useful when the vertical distribution of pressure, temperature, density, and speed of sound is required such as when calibrating aircraft altimeters and determining aircraft and rocket performance and design.

The Earth's atmosphere is divided into different levels or regions primarily by temperature. The lowest region of the atmosphere is the troposphere, which begins at the Earth's surface and extends to an altitude of approximately 10 miles (16 kilometers), or 55,000 feet—about 10.4 miles (16.8 kilometers), above sea level at the equator. Around the North and South Poles, the troposphere is only a little more than 5 miles (8 kilometers), or 28,000 feet (8,805 meters), deep. The temperature of the troposphere decreases about 2 degrees Celsius, or 3.5 degrees Fahrenheit per 1000 feet. Humans live in the troposphere and most weather occurs here.

The tropopause is the dividing line between the troposphere and the next region, the stratosphere, which is found between approximately 10 miles and 30 miles (16 and 48 kilometers) above sea level. The stratosphere contains a type of oxygen called ozone (O₃) that absorbs sunlight, resulting in temperatures similar to those found near the Earth's surface. The ozone layer absorbs harmful solar ultraviolet radiation and protects the Earth. The temperature in the stratosphere rises with altitude, reaching about -40 degrees F at 30 miles (48 kilometers) up. Almost all aircraft flight occurs in the troposphere and stratosphere.

Between altitudes of approximately 30 and 55 miles (48 and 89 kilometers) is the mesosphere. In this region, the temperature first increases to about 50 degrees F (10 degrees C), then decreases until about 50 miles altitude (80 kilometers) the mesopause it drops to as low as –130 degrees F (-90 degrees C). The thermosphere begins at approximately 50 miles (80 kilometers) above sea level and extends to 6,000 miles (9,656 kilometers), the outer limit of the Earth's atmosphere. Temperature in the thermosphere increases to about 2,200 degrees F (1,204 degrees C). The exact temperature depends on solar activity. The region beyond the Earth's atmosphere is referred to as space, or outer space.

The regions of the atmosphere can also be characterized by the distribution of various chemical processes that happen within them, by their molecular composition, and also by the dynamic and kinetic processes that occur within each region.

Weather conditions that occur within the troposphere affect flight and are carefully studied partly for that reason. Conditions such as wind, temperature, water in the atmosphere, atmospheric pressure, and turbulence all affect the way an aircraft flies.

Winds are a natural motion of the air parallel to the Earth's surface caused by the uneven heating and cooling of the Earth and atmosphere. Air that is heated rises because the heat applied to air decreases the air's density to the point where it is lighter in weight than the surrounding cooler air. Air at higher altitudes also exerts less atmospheric pressure because fewer air molecules are present and because of the lesser effect of gravity. (Atmospheric pressure is the force exerted by the air over a specified area.) When less dense air rises, it displaces the cooler, denser air, which moves horizontally to fill the lower pressure area created. This horizontal motion is wind. Motion in a vertical or nearly vertical direction is called a current.

Pilots have to compensate for the direction and velocity of winds to stay on course. Although statistical averages of wind speed as a function of altitude have been calculated, real wind velocity at any particular time and place varies considerably from the statistical average. To avoid drift as a result of wind, pilots should consult local airports for wind conditions and forecasts along their intended flight path.

Differences in temperature and pressure within an airflow result in turbulence, or small-scale motion of the atmosphere. An aircraft experiences turbulence because small currents of wind are moving in a different direction from the main flow of wind. Turbulence also occurs because of winds blowing over irregular terrain. In passenger aircraft, turbulence may cause minor problems such as spilled coffee and in extreme cases, injuries if seat belts are not fastened. Excessive shaking or vibration may render the pilot unable to read instruments. In cases of precision flying such as for air-to-air refueling, bombing and gunnery, or aerial photography, turbulence-induced motions of the aircraft are a nuisance. Turbulence-induced stresses and strains over a long period may cause fatigue in the airframe and particularly heavy turbulence may cause the loss of control of an aircraft or even immediate structural failure.

A thunderstorm is the most violent of all turbulences. In a thunderstorm strong updrafts and downdrafts exist side by side. The severity of the aircraft motion caused by the turbulence will depend upon the magnitude of the updrafts and downdrafts and their directions. Many private aircraft have been lost to thunderstorm turbulence because of structural failure or loss of control. Commercial airliners generally fly around such storms for the comfort and safety of their passengers.

The atmosphere contains moisture in the form of water vapor. Water vapor is less dense than dry air and consequently, humid air (air containing more water vapor) is less dense than dry air. Because of this, a plane's takeoff roll will be longer, its rate of climb slower, and its landing speed higher in humid air than in denser dry air. Further, forms of precipitation such as icing on aircraft wings, zero visibility in fog or snow, and physical damage caused by hail all affect aircraft performance.

Air density is a very important factor in the lift, drag, and engine power output of an aircraft and depends upon the local temperature and pressure. Since the standard atmosphere does not indicate true conditions at a particular time and place, it is important for a pilot to contact a local airport for local atmospheric conditions. From these local temperature and pressure readings, density may be obtained and, hence, takeoff distance and engine power output may be determined.

All these weather conditions affect flight. It is the responsibility of the pilot to obtain as much information as possible to ensure a safe, efficient flight.

—Dan Johnston

References:

Allen, Oliver E. Planet Earth: Atmosphere. Alexandria, Va.: Time-Life Books Inc., 1983

Civil Air Patrol. Aerospace: The Journey of Flight. Maxwell Air Force Base, Alabama, 2000.

Looking at Earth From Space: Glossary of Terms. National Aeronautics and Space Administration. Office of Mission to Planet Earth. August 1994.

Talay, Theodore A. Introduction to the Aerodynamics of Flight. SP-367, NASA, Washington, D.C. 1975. Available at http://history.nasa.gov/SP-367/cover367.htm.

U.S. Department of Transportation, Federal Aviation Administration. Pilot's Handbook of Aeronautical Knowledge. Washington, D.C.: Government Printing Office, 1997.

Williams, Jack. The Weather Book. USA Today. New York: Vintage Books, 1992.

On-Line References

Educational Organization	Standard Designation (where applicable)	Content of Standard
International Technology Education Association	Standard 3	Students will develop an understanding of the relationships among technologies and the connections between technology and other fields of study.
National Science Education Standards	Content Standard B	Students should develop an understanding of motions and forces.
National Science Education Standards	Content Standard E	Students should develop understandings about science and technology.