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Why Are Clouds White?

October 14, 2013

Why Are Clouds White?

Cumulus Clouds on a Sunny Day

Cumulus Clouds on a Sunny Day

White and cottonball like, cumulus humilis clouds indicate fair weather.

We know what clouds are and how they form, but what makes them white?

In previous articles, we’ve answered similar questions like “Why is the Sky Blue?” and “Why is the Ocean Blue?”  The common component in each of these topics is the sun. 

The Sun's Electromagnetic Spectrum

The Sun's Electromagnetic Spectrum

This image from NASA, shows the large range of electromagnetic radiation emitted by the Sun.

Clouds Scatter All Visible Light

Clouds Scatter All Visible Light

Water droplets in the clouds scatter the entire visible spectrum, making clouds appear white.

The sun is also the main contributing factor as to why clouds often, but not always, appear to be white.

(MORE: Why Are There Seasons?)

First, let’s talk about the composition of a cloud. Clouds are composed of tiny round water droplets, which form when water vapor condenses from a gas to a liquid. 

Next is the electromagnetic spectrum. The sun emits energy over a wide rangem and humans aren't able to see most of that energy with the naked eye. We're only able to see the visible spectrum: red, orange, yellow, green, blue and violet light. When those colors are combined, we see white light.  

As the sun’s light passes through these small cloud droplets, it scatters. In other words, it’s pushed out of the droplet in all different directions.  

Unlike scattering in the sky or ocean, in which molecules are so small they only scatter blue and violet light, water droplets are large enough the scatter the entire spectrum.  

That's why clouds appear to be mostly white.  

There are times when clouds may not appear to be totally white, even on a sunny day.   This could be due to shading from other clouds or the background color filtering through a cloud. 

MORE: Cumulus Clouds

Cumulus Clouds

Cumulus Clouds

iWitness Weather

iWitness contributor PrairieMan sent this picture of a "cumulus convention" from Spokane, Wash. (iWitness/PrairieMan)

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