Why is water in the ocean blue when you look at it, but water in lakes and rivers looks clear?
This is all to do with how water interacts with light. When water is a vapour in the atmosphere, the molecules are not glued together in the same way they are in a liquid. Water molecules tend to stick together with hydrogen bonds, but in a vapour they can spread out. In this state they are very good at soaking up infra-red light, but not visible light.