Why Are Clouds White?

Guides · 3 min read · Jul 07, 2022

When it comes to how things look in the sky, it all usually comes down to the sunlight. Of course, there is no difference in why are clouds white since the answer is found in the effect of sunlight.

White clouds on a sky-blue background usually provide a nice view for every pilot, but rain clouds usually have an intimidating appearance and a view pilots would rather not have.

With this in mind, let’s try to better understand where the blue skies and white clouds come from.

Understanding blue light

The UK’s National Meteorological Service describes sunlight as a wave and a part of the electromagnetic spectrum, and when the spectrum is divided all the wavelengths are shown as the colors of the rainbow.

Now, blue light from the sun is the one with shorter wavelengths, living in the range of 300 to 400 nanometers. And these are the wavelengths that are usually more easily dispersed by atmospheric particles in the air.

Note that the wavelength of red light is in the range of 700 nanometers, and the combination of all the wavelengths results in white light.

White clouds against a blue sky.

Why the sky appears blue

When the sunlight reaches atmospheric particles blue light is dispersed, thus making us see a light blue sky.

Therefore, we could say that we have a blue sky because the blue light from the sun is scattered more effectively. So, to say it differently, we see the blue light reflected in the sky.

Why do clouds appear to be white or gray?

To answer why are clouds white, we need to keep in mind the effect of dividing the wavelengths of the light from the sun. We usually see white clouds on a sky-blue background because the sunlight is scattered equally when it reaches the clouds.

Clouds are formed by bigger particles or water droplets than those in the atmosphere. Therefore, when the light passes through the clouds no colors are dispersed more effectively, thus making the light stay white and we see the clouds white on the sky-blue background.

Now, it is essential to highlight that the sunlight is dispersed to the upside or laterally to the sides of the clouds, which makes those areas whiter than the base of the clouds.

As a result, gray clouds are those having bigger water droplets scattering more light and letting less light reach the base. In other words, a rain cloud appears gray because of the large water droplets.

How are white clouds and rain clouds formed?

We have already mentioned that white clouds look like that because they let more visible light pass to the base, but rain clouds look gray because they let less light reach the base. And all these because of the amount and size of water droplets.

Now, the amount and size of water droplets in a cloud are associated with the way clouds are formed.

When a cloud forms, it happens because the water vapor holds on to other particles like dust in the atmosphere. The vapor rises until the air temperature drops. Then, the vapor cools down and condenses to form a cloud structure.

We know that a cloud has formed because we see the color it takes. In other words, because we see that the light passes through it and is scattered by the ice and water droplets.

Of course, any formed cloud cannot be seen unless it acquires its own color. And some of them will look different because the water vapor may condense into bigger droplets and form grey clouds.

However, pilots usually avoid grey clouds by flying over them. Interestingly, when a pilot flies over a cloud that seems gray from below, a surprising bright white may be found while flying at a high altitude because the sunlight continues to reach that area of the cloud.

Clouds appear white

The truth is that saying that clouds are white is actually a simplification, because clouds only appear white. This is because when clouds are white they do not really have their own color, but we see the full spectrum of visible light that makes us think that clouds are white.

In addition, when it comes to the sunrise or the sunset, the position of the sun makes more light to be scattered, letting more red and yellow light reach us. This is why during these times of the day we see clouds and the sky with beautiful reddish and orangy colors.

A wing of an airplane on a white cloud and blue sky background.

Some final words

In summary, the sky is blue because the small particles in the atmosphere scatter shorter wavelengths more effectively, thus making us see only the blue light. Also, clouds are white as long as the sunlight continues to be white, and they remain white as long as less light is scattered by the cloud components. When you see a cloud that appears gray, what you are seeing is actually light at a different wavelength.

Certainly, it is easy to think that a cloud is a big and fluffy object in the sky, but now you know this is not a fact. What’s more, the next time you see intimidating gray clouds from the ground, you should remember they may be whiter than you think on the top.

As a passenger, you may just need to wait for your pilot to fly over them, and not worry. And if you are the pilot, wait until you reach your cruising altitude to see that clouds always remain white from the top.

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Carlos Collantes
Carlos Collantes
A mechanical engineer and aviation enthusiast dedicated to share some knowledge by creating top-notch content, especially in engineering and aviation topics.

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