Above this elevation there are so few molecules that an 'atmosphere' can be hardly said to exist. In this upper-most layer, the thermosphere, each molecule of gas, mainly oxygen and nitrogen, can travel 1km before colliding with another molecule. For comparison, at the surface of the Earth, each molecule in the atmosphere can travel only one millionth of a centimetre before colliding with another molecule. The top of the thermosphere occurs at 500km beyond which is 'space', or sometimes called the exosphere. The importance of the Atmosphere.
Many different disciplines study the atmosphere.
Astronomers and space scientists are concerned with the atmosphere as it is the layer through which they must peer before entering the realms of space. On a more practical level, meteorologists, climatologists and geographers are interested in the lower layers of the atmosphere from the points of view of predicting the weather, for investigating climatic regions and to examine the effects of climate and weather on human society. Weather and climate When we talk about the weather we are talking about the detailed conditions of the atmosphere at a particular point in time. Even in locations which we think of as having a constant weather pattern, for example the Mediterranean basin or the polar weather of Antarctica, the weather is constantly changing This because 'weather' comprises seven variables, each of which undergoes individual variation. Humidity Winds Cloud cover By taking many measurements at a specific location of these seven weather elements over thirty years we would be able to obtain a set of values that would describe the 'average weather' or climate for the site.