Why are some clouds black?
Today Mika and I wanted to play outside , but then we saw that the sky was completely dark, even though it was still the middle of the day. There were really dark grey and even black clouds in the sky. That made me wonder why clouds actually have different colors. When the sky is blue,there are fluffy white clouds,, when it’s a drizzly autumn day, the clouds are light grey, and during a thunderstorm they are almost black.
Are the dark clouds dirtier than others? Or do the clouds have their own colors?
I asked Linnu Bühler, who researches clouds in the Arctic. Linnu asks if I’ve ever flown above the clouds in an airplane and remembered what the clouds looked like. They were always white then! Linnu nods and says that’s because the clouds are always illuminated by the sun from above and are therefore bright. The sun always shines above the clouds during the day. When sunlight shines through the clouds, the tiny water droplets and ice crystals in the clouds bounce some of the sunlight back into space. This is called ‘scattering.’i.e. the sun’s rays bounce off the droplets and go back into space.
The bigger and thicker a cloud is, the more droplets there are for the sunrays to bounce off, which means that much less light reaches us on earth. And when there is little light, it looks dark to us, even almost black.
It’s like when you hold things of different thicknesses and transparency in front of a lamp at home, for example a sheet of paper (which still looks bright), or a whole book, i.e. many pages of paper (then you can’t see the light through at all).
This means that how bright a cloud is depends on how thick it is, how many droplets are in it and how big the droplets are. In dark clouds, the droplets are usually so big that they fall down and it rains.
Cool, so now we know how thick clouds are, even if we only look at them from below. And that we’d better put on rain gear if we want to go outside and play in dark clouds!
