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Why Are Clouds White, And Why Is The Sky Blue?

Clouds are a part of life for pilots. Have you ever wondered why they're white, or why they darken and turn gray? It's all about how sunlight interacts with the contents of a cloud.

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First, You Need To Understand How Sunlight Works.

According to the UK's National Meteorological Service, sunlight, or visible light, can be thought of as a wave and a part of the electromagnetic spectrum. When the spectrum is split up, you see all the colors as a rainbow.

Each visible color has a different wavelength along the spectrum. Blue light has the shortest wavelength at 300 nanometers. Red light has the longest at 700 nanometers.

As visible light passes through the Earth's atmosphere, small particles in the air can scatter shorter wavelengths more efficiently, like what you see on the left side of the diagram below.

Why Are Clouds White, And The Sky Blue?

When sunlight reaches atmospheric particles in the sky, blue light is scattered away more easily than other colors. Because it's scattered more effectively than longer wavelength forms of light, like red, yellow, green, and orange, the sky appears blue.

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So, what about the clouds? In a cloud, sunlight (which is white) is scattered by millions of relatively large water droplets. These droplets scatter all colors almost equally, meaning that the sunlight continues to remain white. This is why clouds appear white against the background of a blue sky.

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These same principles apply to vapor-filled "clouds" created by contrails.

What About Those Gray Clouds?

According to the UK's National Meteorological Service, clouds appear gray for the same reason they turn white. Light is normally scattered upwards, or to the sides of clouds. This makes the tops and sides of clouds whiter than the base, which receives less light.

When you see a rain cloud, the dark gray color is more noticeable because the droplets are even bigger and scatter more light. Less light reaches the base of the clouds, which gives them their intimidating appearance.

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Supersonic Flight And "Pressure Clouds"

Have you ever seen videos of jets breaking the sound barrier with a white cone forming around the wings? Aircraft wings generate areas of low pressure above them. As an aircraft reaches the speed of sound, the low pressure area grows and strengthens. The lowered pressure condenses water in the air, creating a vapor cloud.

According to the Scientific American, the regions of lowest pressure are usually behind the nose of the jet, on the wings, and around the fuselage. As the jet speeds up, the cloud being formed will appear farther toward the rear of the aircraft. Just as the jet reaches the sound barrier, the air is sharply disturbed by a shock wave. The cloud disappears, and the jet starts flying supersonic.

Check it out...

Now That You've Covered Blue Sky And White Clouds...

You may be wondering...what about sunsets? We'll dig into those in another article soon!

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and commercial pilot for Mokulele Airlines. In addition to multi-engine and instrument ratings, he holds a PIC Type Rating for Cessna Citation Jets (CE-525). He graduated as an aviation major from the University of North Dakota in 2018. He's the author of the articles, quizzes and lists you love to read every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures at http://www.swaynemartin.com.

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