This is because the air density is lower in the upper part of the earth and the amount of dust in the air is less. As a result, the sun's rays are less scattered in the dust above. As a result , not only the temperature but also the scattering of light is reduced so that at one time the sky can be seen as blue or black. Thus the temperature decreases up to 75 km - 62 km above the surface of the earth - the temperature is 190K . The Van Allen Belt then began to raise the temperature for sunlight and other cosmic radiation.