Why are some clouds bright and others dark?
Clouds come in all different types, shapes, sizes and, sometimes, colours.
Aside from the sometimes colourful clouds we see at dawn and dusk, most would classify clouds as being white or grey. Sometimes, however, clouds are much darker.
So, what determines how bright or dark clouds are? First, let’s recall how they form.
When rising air condenses gaseous water vapour into either liquid water droplets or ice crystals, clouds form. This happens when parcels of air reach the point of saturation and have no choice but to condense onto particles such as dust.
Once the cloud has formed, we turn to the visible light emitted from the sun. Visible light includes all colours of the rainbow – each with a different wavelength that influences how it's scattered and reflected.
However, particles in clouds can scatter and reflect all colours equally, which makes up white light and is why clouds appear white.
The base of a cloud is often darker than the top of clouds because light scattered from the cloud is sent up or outwards
from the cloud, causing the cloud base to receive
less light. However, if clouds are more abundant or larger – containing more water droplets or ice crystals, even more light can be absorbed and scattered from the tops of the clouds versus the bottom, allowing the cloud to have a darker, sometimes stormy appearance.
Cool fact – no matter the cloud or weather, the tops of clouds are always white.
At the end of the day, it comes down to the size of the cloud, the amount of cloud coverage in our sky, and just how much scattering and reflecting of visible light is happening in the cloud to make it appear bright or dark.