It is very rare to go a day without seeing a cloud, but have you ever wondered why are they white? Or why sometimes they grow darker and grey? Its mostly to do with sunlight and how it interacts with the contents of a cloud.
Why are clouds white?
Clouds are white because light from the sun is white. As light passes through a cloud, it interacts with the water droplets, which are much bigger than the atmospheric particles that exist in the sky.
When sunlight reaches an atmospheric particle in the sky, blue light is scattered away more strongly than other colours, giving the impression that the sky is blue.
But in a cloud, sunlight is scattered by much larger water droplets. These scatter all colours almost equally meaning that the sunlight continues to remain white and so making the clouds appear white against the background of the blue sky.
In more detail...
Sunlight or 'visible light' can be thought of as a wave and a part of the electromagnetic spectrum. We can see the full spectrum when it is split up and spread out as a rainbow. The spectrum is shared with other types of wave, from really short x-rays and gamma rays to really long radio waves.
Each visible colour has a different wavelength; blue light has the shortest wavelength at 400 nanometres and red light the longest at 700 nanometres. Smaller particles can scatter shorter wavelengths more efficiently, like those that are invisible to our eyes in the atmosphere, making the sky blue.
Bigger particles like water droplets within a cloud scatter all wavelengths with roughly the same effectiveness. If we consider that there are millions of water droplets in a cloud, the scattered light interacts and combines to generate a white colour.
So why are clouds sometimes grey?
Cloud bases are often grey as a result of the same scattering that makes them white. When light is scattered in a cloud it usually is sent back upwards, or out to the sides of the cloud, making the tops and sides of the cloud whiter than the base which receives less light.
This is more prominent in rain clouds because the cloud droplets are bigger, thus scattering more light, meaning even less light from the sun reaches the bottom of the cloud giving rain clouds their intimidating appearance.
Because the tops of clouds have a constant source of white light, they are always white! If you are ever on a plane look out the window when you are above the clouds, you will see that the tops of all the clouds will be a brilliant white.
At sunrise or sunset, clouds can take on a red or orange colour. This is because during sunrise and sunset, the sun is very low in the sky and so light has to travel through more of the atmosphere. As a result more of the blue light is scattered and deflected away allowing more red and yellows light to reach the earth. You can find out more here: Why is the sunset red?
When there are clouds in the sky, the light is also scattered by the water droplets inside the cloud. Unlike the gas molecules in the atmosphere, these water droplets are about the same size as the light's wavelength which means that they scatter light in a different way called Mie scattering. Instead of the different colours being scattered different amounts, they are all scattered equally strongly. The light being scattered from the cloud is therefore the same colour as the light that is shining on it from the sun - white.
When you are underneath a thick cloud it will seem a much darker grey colour because, as well as being scattered, most of the light is absorbed before it can get down to the bottom of the cloud. However, to someone standing further away who can see it in the sunlight, the cloud will still look white.