PHYSICAL PROPERTIES OF THE ATMOSPHERE
The total mass of the atmosphere is about onemillionth of the total mass of the earth. It has several layers, each with distinctive thermal, physical, chemical, and electrical properties. Approximately half the atmosphere is contained in the first 5.6 kilometers, and 99 percent of it lies below 35 kilometers.
Our weather takes place in the bottom layer, called the troposphere. Eleven kilometers up, the temperature drops to -55°C. Above this region lies a 40kilometer-thick layer, the stratosphere, where the temperature slowly rises, reaching a maximum of about O°C at 50 kilometers, and somewhat below this altitude an absorbing layer of ozone screens out most of the incoming ultraviolet radiation. Within the next layer up, the mesosphere, the temperature rapidly drops to a minimum of -85°C at its upper limit, 90 kilometers.
Above the mesosphere is the thermosphere. Here the still more dangerous X rays and gamma rays are effectively filtered out by molecular oxygen and nitrogen and by their dissociated atoms at even higher
altitudes. The temperature climbs steadily th roughout the thermosphere and into the exosphere, the atmospheric fringe several hundred kilometers above sea level.
