With a longer path length, there is an increased opportunity for the incoming radiation to be absorbed or reflected back into space by atmospheric constituents. During winter, as you approach the pole, the solar zenith angle increases, and the length of the day decreases. As a result, the total radiation received from the sun at the top of the atmosphere drops off sharply. In summer, a larger sun angle is compensated for by longer day lengths as you travel toward the pole. Therefore, the amount of radiation received at the top of the atmosphere varies little between the equator and about 60 latitude.
Thick clouds absorb uv radiation effectively.
However, cumulus clouds can sometimes have the opposite effect as uv radiation is scattered at the edges of these clouds and subsequently reaches Earth's surface. Uv radiation that makes it all the way to Earth's surface can be either reflected or absorbed. All rights reserved.