Lesson 5: Properties of the Atmosphere
The nature of the atmosphere is important to the study of aerodynamics in that it concerns the fluid, air. Air makes up the Earth’s atmosphere or the gaseous envelope surrounding the Earth, and represents a mixture of several gases. In this lesson, we will learn more the properties of the atmosphere, and discuss the effects of winds and turbulence.
Section 1 – A History and Summary
NATURE OF THE ATMOSPHERE
The atmosphere of the earth is thin and spherical. It is a shroud made up of a mixture of gases that is retained by gravitational attraction. Even though it extends to great height, conventional flight is only possible in its denser layers.
Approximately 99% of the total mass of air is found below 25 miles of 40 km. Commercial airliners fly below this height at only 30,000 to 50,000 ft above the ground. The supersonic Concorde reaches the highest altitude of 60,000 ft.
The atmosphere as a thin layer of air makes life on earth possible. Planets such as Mars with a smaller mass and a weaker gravitational attraction have a more tenuous atmosphere than ours on Earth.
Consider that smaller heavenly bodies such as the Moon cannot hold a gaseous shell at all.
However, Jupiter, the largest planet in our solar system can retain heavy gases at high surface pressures. This is possible as Jupiter has a mass of 315 times that of the earth and its acceleration of gravity is approximately 3 times greater.
On earth, gases with the lightest molecules, like hydrogen, have flown off into space. With escape speeds of over 7 mi/s or 11 km/s, they can escape from the gravitational grip of the earth to fly into space. This is just like a spaceship traveling to the moon.
The earth’s atmosphere shields us against excessive exposure to ultraviolet radiation from the Sun and against cosmic rays from outer space. Although there can be some exposure when there is damage to the ozone layer.
Meteors burn up at the extreme temperatures generated during their high-speed entry into the atmospheres. When most meteorites actually hit the ground, their mass eventually settles into a fine dust.
ATMOSPHERE COMPOSITION
The atmosphere is made up primarily of nitrogen, roughly 78% in volume, which was produced during the history of the earth by outgassing. Outgassing is the process during which rocks slowly give off stored gases.
Nitrogen does not easily enter into chemical reactions because of the nature of its molecule, N2, which is the diatomic molecule N-N.
The life-giving oxygen in the air of about 21% evolved in geological times since the formation of our planet about 5 billion years ago.
Argon, the heavy nonreactive noble gas, appears in small concentrations. Carbon dioxide which is produced in the biological cycle involving plant life, and water are found in smaller amounts.
Water exists as a vapor or as an invisible gas, or in liquid or solid form as rain, snow, or ice.
Since the water-vapor content of the air varies greatly with the local weather, and its average value changes with the seasons and the location of the earth, the standard atmosphere is assumed to be dry.
Carbon dioxide varies much less, and is affected by local conditions. The increased burning of fossil fuels by power plants and in auto engines, may accelerate the CO2 buildup. This is referred to as the greenhouse effect, since CO2 traps the heat radiated by the earth.
The entire atmosphere of the earth has only 1/240 the mass of oceans.
Air, in contrast to water, is a compressible medium. Therefore, its density change with altitude in the gravitational field of the earth. The lower layers of air carry more weight than the upper atmosphere which means they are also more compressed.
The atmosphere has greatly been studied since the 17th century. Daily and seasonal changes in various properties, climatic and geographical variations, and effects of altitude have been recorded.
Instruments carried by mountain climbers have yielded new data. Weather stations have been permanently established at different locations and altitudes around the world. Ships have provided movable platforms for observation at sea. Instruments have been carried aloft by manned and unmanned balloons, airplanes, rockets, satellites, and space vehicles.
From these observations, meteorologists have constructed a standard atmosphere possessing mean values of atmospheric composition, pressure and temperature in a motionless and stable atmosphere not affected by seasons.
Turbulent winds blowing near the surface, clouds and storms, jet streams at high altitudes, and global circulation patterns are all absent from these models of air at rest.
THE TROPOSPHERE
The lowest layer of the atmosphere where we dwell is called the troposphere. Troposphere comes from the Greek word that means turning. Here, turbulent air motion results in continuous mixing, and the troposphere is host to what we consider weather.
The temperature drops linearly in the troposphere. In other words, the decrease in temperature with altitude follows a straight line. The cooling of the air with increasing distance from sea level is approximately -3.6oF per 1000 ft. This rate of temperature drop is called the lapse rate. You can feel this cooling affect and drop in pressure if you ride a cable car to a mountain top.
Pressure and temperature at altitudes equivalent to the highest mountain ranges are too low for human life to exist comfortably.
The troposphere extends to approximately 7 miles. At its upper edge is the troposphere where there is a shift in the behavior of the temperature.
THE STRATOSPHERE
Beyond the troposphere is the cold stratosphere. This is the level at which jet planes cruise.
Traces of atmospheric components characteristic of lower altitudes, and other species produced by chemical reactions extend to great heights.
This means that orbiting satellites, space shuttles, and other spacecraft leaving the atmosphere and returning encounter aerodynamic forces at much higher altitudes than those in which ordinary airplanes fly. This is a subfield of aerodynamics referred to as low-density or rarefied gasdynamics.
Satellites in particular often have elliptical orbits in which the aerodynamic drag is uneven. Large retarding forces act on a satellite at the perigee of its orbit. This is the closest point to the earth.
When the craft encounters resistance in the denser air, it slows, resulting in a reduction of altitude at the apogee, the point of farthest distance from the earth.
In this case, the orbit decays. It is then necessary to predict this decay effectively in order to anticipate the moment when the satellite will enter the dense atmosphere to burn up in a fiery trajectory.
When the satellite reaches its final stage of flight, it becomes the equivalent of a man-made meteor, streaking luminously through the sky. It is sometimes possible for parts which do not vaporize to survive the aerodynamic heating and hit the ground as meteorites.
Note, however, that television and telephone stationary communication satellites are positioned at about 22,000 miles above us at the edge of space. Subsequently, aerodynamics have been considered to have little, if any, effect.
