Why Are Storm Clouds Gray?

 

Hey Steve,

Why are the clouds sometimes white, sometimes Gray or sometimes dark Gray? What gives them the color? Is it driven by the amount of water they are carrying or the height they are at?

Thanks,

Amit Shekhar

Thanks for the good question Amit.

Well cloud color is a little more in depth...but cloud brightness is an easy one.

Clouds usually look gray when we are directly underneath them. This is because sunlight does not shine directly on the bottom of clouds, but rather shines through the tops of clouds.

Sunlight gets blocked more when the cloud is thicker. So the bottom of the cloud looks dark. If the cloud is thin, it may appear more white because there is less cloud matter to block sunlight.

If you are under a tall dark cloud, wait until it passes, and look at it's side. The side of the cloud will be illuminated by sunlight, and thus all white. So, the brightness of the cloud (white versus gray or black) has a lot to do with where you are standing and the thickness of the cloud.

Sunlight has to travel thousands of feet through the cloud to our eyes. As the light travels through the thick cloud it gets scattered, absorbed, and reflected away from our eyes. In the end, this means that less and less light reaches our eyes. Notice the picture below.

However, if the cloud is thin, then we will see much more of the light, and the cloud will appear much more white...like the picture below.

And finally, if you aren't underneath the cloud...then the tall and thick cloud is illuminated by sunlight. This light does not travel through the cloud, but rather reflects off the side of the cloud and into your eyes. (Remember, all colors of light are reflected equally, that's why it's white). However,if you walked underneath this cloud, it would look much darker.

The cloud below is bright white from our perspective!

 

Notice that so far I am not really talking about color, but more about the brightness of clouds. That's because the reason for the color of clouds is much more complicated.

 

A Bit About Color & Clouds:

Color is light. The different wavelengths or parts of light are described as a color. When you see light white light, remember that it contains all colors of the rainbow.

So, Colors = Wavelengths = "Parts"

...at least for my purposes of explaining why clouds are white.

If all colors of the spectrum are present (Red, Orange, Yellow, Green, Blue, Indigo, Violet) then the color of light is white. If no light at all (not any of the colors above) are seen, then the color is called black.

White = all colors

Black = no colors

So, since clouds are white (typically) then clouds must reflect ALL colors of the rainbow, right?

You betcha. All wavelengths or "parts" of light are scattered equally when light hits clouds. This type of scattering is called Mie scattering, and is the reasoning behind clouds being white. If the light hitting clouds does not have all parts or wavelengths of light, then the clouds will scatter whatever colors. This is the case at sunset, when only the Red/Orange/Yellow wavelengths of light are hitting the clouds. Why does this happen? Scroll to the bottom of the page, I have a couple of special links for you.

 

Summary:

So, sunlight reflects equally off of the water and ice particles that make up a cloud. This is called Mie Scattering, and it means that all wavelengths of sunlight reach your eyes, thus clouds are white.

1) Cloud color is due to Mie Scattering
-This means that clouds scatter all wavelengths of light equally. All wavelengths of light = white color

2) Cloud brightness is due in part to where you are standing relative to the cloud.
-If underneath a tall thick cloud, it looks gray, but from a side view it looks white

3) Cloud brightness also depends on the type of cloud
-Thin clouds versus thick clouds

4) Cloud color also depends on the color of the light being reflected from the clouds
-If I shine a red light off of clouds, the clouds will look red

And this may sound confusing...

So, check out these savvy-licious web pages for more on sunrise, sunset, and even a green sky!

WeatherSavvy on The Green Sky Effect and Tornadoes

WeatherSavvy on Why Is The Sky Blue?

 

Are you learning? I hope so. Go get a cookie and let's learn some more...