Planet earth is evenly surrounded by a layer of air. This layer of air is a mixture of different gases, the composition of which is fairly constant all over the surface of the earth. At the boundary of the atmosphere at about 900 km and beyond, only solitary gas molecules will be found so that the air pressure can practically be equated with the absolute vacuum and the temperature with absolute zero.
The ionosphere starts at about 80 km height and is a zone consisting of electromagnetically charged layers of molecules which reflect emitted radio waves to the receiver. Even in the stratosphere, which is below the ionosphere, there are no weather phenomena and the median temperature in this region amounts to only about -60°C.
Figure 1.1.1
The lowest layer of the air envelope, the troposphere, reaches a height of 10 km with its upper limit. The temperature at the extremes of the troposphere amounts to about -50°C. The temperature gradient from there to the earth's surface is 6.5°C per km on average.
Without atmospheric air, the daily temperature differences would be so large as to make life impossible. Atmospheric air permits the evening out between the heat of the day and the cold of night.
