Why is the sky blue?

Light particles come in all different colours and what that really means is that each different colour of particle has a slightly different energy. Red light is less energetic than green light which is less energetic than blue light and so on. If you go into space the sky is black and all the stars (including the sun) are white because the light particles they emit come in every different colour and when your eye averages them you see white. The light that they send out does not hit anything on its way to reach you because space is empty.

Down here on the earth we have tonnes of atmosphere between us and the sun. The white light comes in and most of it gets through. Except the blue light. That has a slightly higher energy than the rest and that makes it much more likely that when it hits an air molecule it'll get scattered. All this means that when you look at the sun it looks white MINUS the blue that has been scattered out - this means it looks yellowish. When you look elsewhere in the sky it is blue because the blue light gets scattered around everywhere and it hits your eyes from all directions at once.

Another interesting effect happens when you look near the horizon (when the sun isn't near it)- the sky looks paler here (whiter and not as bright). This is because blue light that goes near to the horizon and then back to your eye has to travel a long way. Chances are it'll get scattered again some other way. So from the horizon some of the blue light is taken out and the sky looks less of a deep blue and paler. When the sun is near the horizon it shines straight at you through more air than if it was above you and all the blue light scatters out so the sky is yellow during sunset. Dust and pollution in the air can take some of the green out too so that the sky becomes red. The same thing happens in the morning - shepherds warning! If you stand on Mars and look up the sky it should be blue but it is always red because there's so much dust in the air.

Clouds are different. They are made up of water and this scatters all colours of light the same. Any colour of light that goes into a cloud gets scattered in any direction so when you look at a cloud you see all the colours at once and it seems white to your eyes. Water weakly absorbs light so clouds don't appear brilliant white - they are a bit dimmer. Big wet stormclouds have more water in them and they absorb more light so they seem darker and greyer. Clouds are also very big and block out a lot of light from hitting their sides or their underbelly. When water freezes it doesn't absorb light very well. It transmits it or reflects it. Snow is white because it reflects all colours evenly and it doesn't absorb much light.

The colour of the moon is white. It reflects a bit of each colour of the light from the sun that hits it down to us. A bit of the blue can get scattered and make the moon yellow. Sometimes particles get into the air that scatter the red light very well and the light from the moon has all its red light taken out and it looks blue - a blue moon. It's a very rare occurence because to get these chemicals into the air you need a forest fire or a volcano. This effect never makes the sun seem blue because the sun is so bright - there is never enough of the chemical in the sky to remove enough of the red light.