The Sun is located at the center of our Solar System, at a distance of about 150 x 106 kilometers from Earth. The Sun produces solar heat energy. This energy travels through space in the form of electromagnetic waves. The process of heat transfer is known as radiation.Before reaching the sea-level the sun's rays have to pass through the whole thickness of the atmosphere.
So, the first surface that has the potential to come into contact with solar radiation are the clouds.