Weatherwatch: Why the sky is blue

http://www.theguardian.com/news/2014/jun/09/weatherwatch-why-sky-blue

Version 0 of 1.

Look out of your window, and hopefully you can see some blue sky. But why is it blue and not green, say? It all depends on what is above your head that day.

When white sunlight (which is a mixture of all the different colours of light) hits Earth's atmosphere, some of the light is deflected by air molecules. In this case, oxygen and nitrogen molecules, which dominate Earth's atmosphere, preferentially scatter violet and blue light.

Longer wavelengths (such as red and yellow) usually make it all the way to the planet's surface without being scattered. So when we look up into the sky we see the scattered blue light.

On dismal days the sky is full of large water droplets, which scatter all the wavelengths of light, making the sky appear white or grey. Sometimes pollution gives the sky an exotic edge, creating a brown, yellow or even red hue, because the pollutants preferentially scatter those wavelengths of light.

By day the sun looks yellow, but at sunset it turns red and orange. That is because the light has to travel through more atmosphere at sunset, and only the longest (red and orange) wavelengths make it through without being scattered.

Other planets have different skies. To someone standing on Venus, the sky would look orange and red, thanks to the thick carbon dioxide atmosphere

Meanwhile, the thin dusty atmosphere around Mars makes the sky orangey-red by day, but gives blue sunsets. And on the moon the absence of any atmosphere makes the sky black.