ATMOSPHERE

The entire surface of the Earth is covered by an immense 'sea' of air, stretching upwards to a height of several hundred miles. This covering of air is known as the atmosphere. It is usually thought of consisting of three layers; the troposphere, which stretches from sea level to between 5 and 10 miles up; the stratosphere, which extends from between 5 and 10 miles up; and the ionosphere, which lies beyond.

The air becomes 'thinned-out' as we move away from the Earth's surface. The oxygen that air contains is essential to all living creatures, so that man cannot survive at any great height without some kind of mechanical aid. Even at heights of three miles or less, the thinning-out of the air is very noticeable, causing an uncomfortable shortness of breath. For this reason, mountaineers, especially when climbing the highest peaks, such as Mount Everest, are often forced to use oxygen masks to breathe.

Life as we know it is possible because our Earth has an atmosphere. To appreciate the importance of the atmosphere, consider the Moon, which has no atmosphere. Temperatures in the daytime are so high that if the Moon had any water it would boil. And at night it is very much colder even than the coldest places on Earth. But on Earth the atmosphere acts as a shield during the day, protecting us from the sun's ray, and as a blanket at night, holding the heat in.

We do not usually think of air as having weight, but in fact it has. The total weight of the atmosphere is nearly 6,000 million tons. The column of air rising above each of us weighs about a ton and exerts a pressure of about 15 pounds on every square inch at sea level. We do not notice this tremendous pressure because the pressure inside our bodies is the same as that outside, just as the pressure inside the deep-sea creatures is the same as the tremendous pressure of the surrounding water.

The higher we go in the atmosphere, the lower the pressure, because there is less air above us and, as a result, less weight of air. Thirty miles up, the air pressure is only one-thousandth of the air pressure at sea level.

Conditions in the troposphere, the lowest layer of the atmosphere, create the world's weather. In the troposphere temperatures usually decrease with height at the rate of 3.5 degrees Fahrenheit for every thousand feet (6 degrees Celsius for every thousand meters). But at the boundary between the troposphere and the stratosphere above, the temperature stops falling for a while. This is called tropopause. In certain places among the tropopause there are winds of over 100 miles per hour. These are called jet streams.

The stratosphere is generally calmer than the troposphere and is used by high-flying aircraft to avoid bad weather conditions.

The outermost layer of the atmosphere, the ionosphere, is important in long range radio communications. It reflects medium and long radio waves back to the ground, making it possible for them to travel around the Earth's curved surface.