The Upper Air 103 
radio waves is very important for long distance radio com- 
munication because it acts as an invisible barrier preventing 
radio waves from escaping from the earth to outer space. It 
is presumably composed of charged particles of air, ions. The 
particles of air become charged by the strong solar radiation 
at great altitudes. A layer of electrically charged particles 
acts upon radio waves in much the same way as a mirror does 
upon light waves. The existence of an ionized layer at such 
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altitudes is sufficient proof that the atmosphere extends at 
least to altitudes of 50-60 miles even though it is very thin 
up there. 
The duration of twilight is another piece of evidence which 
indicates the height of the atmosphere. After the sun sets 
below the horizon, certain high objects like mountain peaks 
are still illuminated for a few moments by sunlight. Gradu- 
ally they disappear into darkness until only the highest 
