why is clouds are black and sky is blue?

Visible light is part of a spectrum which is made up of many colours. At one end of the spectrum is red/orange gradually moving gradually through the other colours to the blue end. The red/orange end of the spectrum has the longest wavelength and the blue the shortest. The sky is blue because when light moves through the atmosphere it hits gas molecules, when it does most of the light waves pass straight through. The shorter wavelengths are absorbed by the molecules and this colour is then radiated, thus making the sky appear blue. Clouds are black in colour when they are filled with water. They are high in density therefore dont allow sunlight to pass through, making them appear dark.