Why do some clouds appear dark gray? What prorties define cloud color?

49 views

Why do some clouds appear dark gray? What prorties define cloud color?

In: 16

If they’re darker, they’re holding more water. Clouds with less water in absorb less light coming from above it so they appear white, as more water accumulates they absorb more light and so they appear darker.

Darkness is just a matter of total amount of light getting through all the suspended water droplets. Dark clouds means that the light from above has hit a lot of water droplets and never made it down. Bright white clouds, fluffy cottonballs, are what you would see from the side where all the incoming light is actually going (reflected back out and away; water droplets scatter the light all over the place, mostly back where it came from-little random mirrors, in a way). Tops of clouds seen in an airplane tend to be pretty bright and white.

They do not tend to take on any particular color because the light is white as it comes from the sun. There is a small amount of red-end absorption of energy (the greenhouse problem) but not very efficient so the gray is often bluish but not blue.

Dark clouds and general darkness beneath the cloudy are means lots of cloud is above. Thick clouds. Expect lots of rain. Dark thunderheads go way up into the sky, filled with water droplets. Not much light gets through them.

Molecules of water cluster into separate small droplets in a cloud. Importantly: the size of the cloud’s individual water droplets will affect how light interacts. Clouds that scatter lots of light back to your eyes will appear white.

The scattering behavior of the incident sunlight depends very much on the actual size of each droplet. The math is surprisingly complicated, to calculate how much light will penetrate forward, and how much light will scatter at different angles: https://en.wikipedia.org/wiki/Mie_scattering

Many websites will say that clouds look gray when a thick layer absorbs light. That’s only one issue, not a full explanation. Mie scattering explains how the sun-facing side of storm clouds (having larger water droplets than a nearby white cloud) can also look gray — even when receiving direct sunlight.