As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by the following:
* Air molecules
* Water vapor
* Clouds
* Dust
* Pollutants
* Forest fires
* Volcanoes.
This is called diffuse solar radiation. The solar radiation that reaches the Earth’s surface without being diffused is called direct beam solar radiation. The sum of the diffuse and direct solar radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.
Source: EERE, U.S. Department of Energy