The ancient Greeks believed creation to be composed of four basic elements: earth, water, air and fire. These elements, according to Aristotle and his friends, were neatly arranged to form four concentric spheres - with earth in the middle, surrounded by water, and both in turn lying within a vast envelope of air. The fourth element, fire, occupied the outermost fringes of the world. This division between the elements, however, was not rigid.
Earth, for instance, obviously projected above the water here and there, and fire was often seen to descend to earth in the form of lightning. The elements were known to the Greeks, naturally enough, by their Greek names: "air" was meteoros, meaning "high" or "lofty", and hence what was happening in the air acquired the name by which we know it now - meteorology.
Although the concept of the elements has been long abandoned, meteorologists still, strangely enough, think of the atmosphere in terms of four concentric spheres. But now the regions are defined by their thermal characteristics, and are called, in ascending order from the ground: the troposphere, the stratosphere, the mesosphere and the thermosphere.
The troposphere, as we noted yesterday, is the lowest layer of the atmosphere, that immediately adjacent to the ground where the temperature decreases steadily with height. It is seven or eight miles deep at these latitudes, and contains nearly all phenomena we know as "weather" - the clouds, rain, snow, and so on.
The temperature decreases at an average rate of 10 to 12 Celsius per mile, up to the tropopause - the boundary between the troposphere and the stratosphere. Here an abrupt change in the temperature regime takes place: in the stratosphere, above the tropopause, temperature remains almost constant with height, at around 50 or 60 Celsius and sometimes even increases very slightly.
Next in order of height, from about 30 to 60 miles above the ground, comes the mesosphere, where the temperature falls with height at about the same rate as in the troposphere. But the mesosphere is deeper, so by the time we reach the mesopause - the top of the mesosphere - the temperature has fallen lower than anywhere else in our atmosphere; the average temperature is as low as 100 Celsius.
Above the mesopause, in the thermosphere, temperature again increases sharply with height, and 100 miles or so above the earth it is warmer than at sea level.
This relative warmth is caused by absorption of ultraviolet radiation, and as it happens, the air is so tenuous at these altitudes that very little energy is needed to produce a substantial increase in temperature.