Why are rain clouds dark even though water is colorless

"As the light, that falls on the top surface of a cloud passes through the cloud and reaches the bottom of the cloud, the intensity of light decreases considerably and hence, depending on the thickness of the cloud, the underside of the cloud appears dark"

       Clouds are formed when water vapour in the atmosphere condenses on dust particles to form water droplets and ice particles. These droplets of water have a size in the range of microns(1 micron = 1 millionth of a meter). As visible light has a wavelength of 0.5 micron, the size of a droplet is comparatively much bigger. These droplets of water scatter almost equally all colours of light, unlike dust particles which mostly scatter blue light giving the sky its blue colour. A light ray incident on a cloud is subjected to multiple scattering, as the cloud consists of a large number of water droplets. And since a water droplet scatters all components or colours of light almost equally, most of the time clouds appear as white.

       However the intensity of light varies as the sunlight which falls on the top surface of the cloud passes through the cloud and appears at the bottom surface. The intensity of the sunlight is high on the top surface and the intensity is reduced at the bottom surface of the cloud in the case of thick clouds. In spite of the the water droplets scattering light uniformly, drop in intensity of light gives the cloud its dark shade. However in the case of thin clouds the drop in intensity of light is not significant and hence appears as white. In the case of thick clouds, the intensity of light reaching the base of the cloud is almost nil and usually we see the base or the under side of the cloud with the sun shining on the other side of the cloud and hence we see the cloud as dark.

References


[1] http://www.livescience.com/39069-why-are-rain-clouds-dark.html

[2] http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech-and-agri/dark-clouds/article3225486.ece

[3] http://en.wikipedia.org/wiki/Nimbus_cloud