Perhaps when we were little we have always wondered why the clouds are white, while the sky is completely blue. And not only that, but also why they change color when storms approach. Thus, we can say that the weather threatens a storm when we observe that they acquire a more grayish tone.
Colors and light
The color in which we observe a certain object depends on the multiple wavelengths that it reflects and that reach our retina. Thus, when we see an object of white color , what we are actually seeing are all the colors at the same time , that is, when all the wavelengths reach our eyes at the same time, our brain interprets them as the color white. And that is precisely the definition of the color white, the presence of all colors (however paradoxical it may be).
There are many ways to break down light. The best known is that of the light prism, but sometimes certain factors of nature can act like this prism such as water or our atmosphere and an example of this decomposition is the rainbow. The small gas particles that make up the atmosphere, such as oxygen or nitrogen, scatter those shorter wavelengths of the spectrum (which would correspond to the color blue and green). For this reason, the sky is blue and the sun is yellow. This phenomenon is called the Rayleigh effect.
So why are the clouds white? A simple way to explain this is that clouds are made up of tiny droplets of transparent water that are not yet large enough to fall as rain. Once sunlight hits these tiny droplets it is reflected. Unlike atmospheric oxygen and nitrogen, droplets reflect all lengths of light equally and in all directions, which is why we see clouds as white .
The reason for this difference is that atmospheric gas molecules are much smaller than the wavelength of light of the different colors, interacting with one and scattering it according to the Rayleigh effect. However, as the droplets are larger, they all scatter them equally, since when light interacts with molecules larger than its wavelength, a mechanism called Mie diffusion comes into play.
Why do the clouds darken
Although clouds usually appear white in color , at times when they are loaded with particles, they turn darker colors (gray to almost black). The main factors that influence this change are the thickness and density of the clouds .
A cloud known as a rain cloud is thicker, contains many more water droplets, and is larger. Comparing these clouds with the whitest clouds, the dark ones are more dense and do not have spaces between the particles, so that it is more difficult for light to pass through and we see them in a dark color.
To explain to the children why clouds are white and why they can turn dark, there is a very simple home experiment : we place an unfolded napkin over a lamp bulb and as we are placing more layers of napkins over the light outlet , less light will pass through. As a result of this, the light will dim to grayer and darker colors. For this reason, on cloudy days, the clouds appear more or less gray and when it is going to rain, almost black.
When a cloud discharges its drops, it can appear white again, because its density is less and less and allows the light to be scattered again. In addition, the particles are again farther apart and the probability of rain is much lower. In summary, we can say that clouds are black when their density does not allow light to pass through them.