Why are clouds white? Because the water particles in the clouds scatter light equally.
When light comes out of the sun, it is white. That is to say, it emits all of the colors of the color spectrum. The sun emits light as radiation and it emits radiation across almost all of the electromagnetic spectrum, from radio waves to X-rays. It doesn’t emit gamma rays because they are produced in the core and have lost most of their energy by the time they get to the surface. All of this electromagnetic energy radiates out of the sun and some of it lands on Earth. When it does, some of the things on Earth absorb some of that energy and reflect other parts of that energy. If they happen to reflect energy that is in the visible spectrum of light, we can perceive that as a color. However, if they reflect all of the energy that has come from the sun in the visible spectrum, we see that as white because the light coming from the sun is white.
So, why does the sun look yellow to us? Our eyes take in all the light that they see and the light hits the cones at the back of the eye. These cones are sensitive to light in certain wavelengths: 420-440 nm (blue), 530-540 nm (green), and 560-580 nm (red). If more of the 420-440 nm cones are stimulated, our brain sees blue, and if more of the 560-580 nm are stimulated, we see red. If all of them are stimulated, we see white. When sunlight enters Earth’s atmosphere, all of the visible spectrum is still present and it is white. Once it enters the atmosphere, it starts to encounter all of the gas molecules that float there. These gas molecules are very small. Most of the longer wavelengths of light pass right through without hitting the tiny molecules. However, blue light has a smaller wavelength and it is far more likely to hit the gas molecules, getting scattered all over the place. The blue light is scattered, our eyes pick it up, and we see the sky as blue. Blue light is 16 times more likely to be scattered than other colors. This is called Rayleigh scattering. The rest of the light comes straight down from the sun into our eyes without being scattered, and we perceive that as yellow.
So, why are clouds white? Clouds are made up of water droplets. These water droplets are suspended in the clouds because they are very small. Gravity pulls them down, but air drafts push them back up again, and the air drafts are stronger than gravity. When they get very big, gravity overcomes air resistance and they fall as rain. While they are suspended in the cloud, light from the sun strikes them. Gas molecules in the air are small and only scatter the smallest wavelengths, but water molecules in clouds are much bigger and they scatter all of the light. The light bounces off the water molecules and comes out of the clouds, but all of the light comes out, and our eyes don’t pick up one color over all of the others. We pick up all of the colors equally, which is white. This type of scattering is called Mie scattering. When particles are the same size or bigger than the wavelength, they scatter it equally.
Rain clouds are much darker than regular clouds because the water molecules have clumped together and are larger. Larger water molecules scatter more light and most of it is scattered up or out of the side of the clouds. Most of the light has been scattered by the time it gets to the bottom of the clouds, leaving them much darker and gray or even black. However, if you go up in an airplane, all clouds, even rain clouds, are white.
Clouds change color at sunrise and sunset. This is because the sun is lower in the sky and the yellow and red wavelengths are able to get through. The blue light is stripped away, further over the horizon where the sun is still higher up, and the red and yellow light is left to carry on, where it gets scattered and makes the clouds look red or orange. And this is what I learned today.
Photo by Pixabay: https://www.pexels.com/photo/brown-field-and-blue-sky-46160/
Sources
https://www.nytimes.com/2013/03/12/science/why-do-clouds-appear-white.html
https://en.wikipedia.org/wiki/Rayleigh_scattering
https://www.space.com/what-color-is-the-sun
https://www.britannica.com/video/185569/yellow-Sun-sky-blue-light-colours-air
https://www.sciencefocus.com/science/what-colour-is-the-sun
https://en.wikipedia.org/wiki/Cone_cell
https://www.wtamu.edu/~cbaird/sq/2014/02/07/how-does-a-cloud-fill-up-with-water/