Clouds look white because the water droplets present in the clouds scatter visible light into all wavelengths equally. Due to identical scattering of all wavelengths, the sunlight remains white and clouds appear white.
Why do clouds look white?
Have you ever wondered why clouds appear white and sometimes turn gray before and during rains?
To understand why the cloud is white, we need to learn the scattering of light through the clouds.
We know that white light from the sun contains all the wavelengths of the visible spectrum at equal intensity. This visible light has different wavelengths in the range of 300-700 nm. Red light has the longest wavelength (700 nm) while blue light has the shortest wavelength (300 nm).
When this white or visible light enters the earth’s atmosphere, they get scattered by the tiny particles such as dust and ash present in the atmosphere. These tiny particles can easily scatter the shorter wavelength light that is blue light. This is the reason why the sky appears blue.
Why do clouds appear white?
Clouds are made up of millions of tiny water droplets and ice crystals. These water droplets are relatively larger than the tiny dust particles. When the sunlight passes through these water drops, the visible light gets scattered into all different wavelengths of color in equal proportion. As the cloud scatter all colors equally, the sunlight remains white even after getting scattered. Hence, clouds appear white.
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Why sometimes clouds turn grey?
On a normal cloudy day when the clouds are thin, most of the light can easily pass through it and the clouds look white. However, when the thickness of the cloud increases, the cloud looks darker and gray. Due to an increase in cloud thickness, less visible light is able to penetrate but it scatters all different wavelengths in equal proportion. Therefore, the base of the cloud looks gray while the top of the cloud appears white.
This is the reason why rain clouds appear dark and grey in color. The water droplets of rain clouds are relatively bigger in size. These water drops scatter all colors equally but they allow less sunlight to pass through them.
How Mie scattering causes grey and white color of the clouds?
Mie scattering occurs when the size of the particle is equal to the wavelength of the scattering light. The clouds suspended in the atmosphere are consists of water drops and ice crystals. The diameter of these water droplets is similar to the wavelengths of the scattered light. These water droplets scatter all wavelengths of visible light equally. Thus, the grey and white color of the clouds is caused by Mie scattering by water droplets.
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