Jeffrey Nordiand, age 14, of Columbus, Ohio, for his question:
WHAT MAKES UP OUR ATMOSPHERE?
Atmosphere is a mixture of gases that surround the earth and any celestial object that has a gravitational field strong enough to prevent the gases from escaping. The earths atmosphere is made up of 78 percent nitrogen and 21 percent oxygen. The remaining one percent is made up of small amounts of argon, carbon dioxide, water vapor and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton and xenon.
The water vapor content of the air varies considerably, depending on the temperature and relative humidity. With 100 percent relative humidity, the water vapor content of air. varies from 190 parts per million at minus 40 degrees Fahrenheit to 42,000 parts per million at 86 degrees Fahrenheit.
The atmosphere may be divided into several layers. Tn the lowest one, called troposphere, the temperature as a rule decreases upward at the rate of 3 degrees Fahrenheit per 1000 feet. This is the layer in which most clouds occur.
The troposphere extends up to about 10 miles in tropical regions and to about six miles in temperate latitudes.
Above the troposphere is the stratosphere. Tn the lower stratosphere the temperature is practically constant or increases slightly with altitude, especially over tropical regions.
Studies show that up to about 55 miles above sea level, the composition of the atmosphere is substantially the same as at ground level. The constant stirring produced by atmospheric currents counteracts the tendency of the heavier gases to settle below the lighter ones.
In the lower atmosphere, ozone a form of oxygen is normally present in extremely low concentrations. But at times atmospheric disturbances and downdrafts carry varying amounts of this ozone to the surface of the earth. Human activities add ozone in the lower atmosphere where it can become a pollutant that can cause extensive crop damage.
The ozone layer became a subject of concern in the early 1970s, when it was found that chemicals known as fluorocarbons, or chlorofluoromethane s, were rising into the atmosphere in large quantities because of their use as refrigerants and as propellants in aerosol dispensers.
The concern centered on the possibility that these compounds, through the action of sunlight, could chemically attack and destroy stratospheric ozone, which protects the earths surface from excessive ultraviolet radiation.
As a result, United States industries replaced chlorofluorocarbons in all but essential uses.
Subsequent atmospheric studies remain uncertain on the actual threat to the ozone layer by human activities.
Scientists have divided the atmosphere which surrounds the earth into four layers the troposphere, the stratosphere, the mesosphere and the thermosphere.