Mary Jane King, age 13, of Fredericton, New Brunswick, Canada, for her question:
What are the atmosphere's different sections?
The atmosphere enfolds the solid globe in a series of airy shells. Its gaseous layers are sandwiched between the earth below and the sky above. Forces are exerted upon it by both the planet and the vast reaches of space. These various forces help to arrange the atmosphere in layered sections and cause other happenings in its mixture of gases.
The base of the atmosphere covers every square inch of the earth's surface and its outer limits extend up perhaps 1,000 miles. Altogether it is an enormous shell. A roomful of its gauzy gases at sea level weighs merely a couple of pounds. But its total weight is estimated to be five quadrillion tons, which is the figure five plus a tail of 15 zeros. The entire gaseous mass is hugged to the globe by the earth's gravity. This mighty force helps to section the airy shell in layers.
In the total atmosphere, most scientists distinguish four different layers, stacked one upon another, though the boundaries between them tend to be vague. The mixture of gases varies only slightly, but gravity affects its density. Its force decreases with distance from the planet's center. Hence, the densest layer is at the bottom and the air grows gradually thinner with altitude.
At the planet's surface, uneven patches of solar radiation cause uneven patches in the atmosphere. This, with help from the earth's rotation, creates the weathery troposphere. The height of this breezy bottom section varies from five to ten miles. Its temperature and its density decrease with altitude. Its roof is the floor of the stratosphere layer, which changes character as it ascends to about 50 miles above our heads.
The lower stratosphere is clear, bitterly cold and often calm. A few miles higher, the ozone content in its thin gases increases, which may attract heat from the sun. In any case, temperatures above the ozone layer rise to 30 degrees Fahrenheit and breezes often blow. These two lower layers are more strongly affected by the earth. The two upper layers are more exposed to the dynamic energies of outer space.
The top of the stratosphere merges with the amazing ionosphere. Its peppy ions are charged particles, created when cosmic rays and speeding particles from the sun shatter molecules in its thin gases. Its lower level reflects certain radio waves and angles them back to earth. This makes possible our long distance communication systems. Its upper levels shield life on earth from destructive solar radiations ¬and sometimes the energetic ionosphere entertains us with glimmering auroras.
About 300 miles above the surface, the upper ionosphere merges with the exosphere. Here, the gaseous molecules are very few and far between. But the thin air extends upwards for hundreds of miles, getting thinner with distance from the planet. A few stray molecules may escape the earth's gravity. Finally the thin air peters out and the upper limit of the atmosphere loses itself in space.