Why do clouds turn dark when it is going to rain even though water is colourless?

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Clouds consist of fine droplets of water - droplets having a size typically of the order of about several microns. A micron is one millionth of a metre or one thousandth of a millimetre. Visible light has a wavelength of around 0.5 micron. So compared with the wavelength of light, the size of a droplet is thus quite large. These droplets scatter light in all directions almost uniformly. Since the clouds contain large number of droplets, a beam of light passing through such a cloud will be subjected to multiple scattering. We see a cloud by means of the light so scattered. Sunlight appears white due to the presence of all components of the well-known rainbow colours, and since all these components are scattered almost equally, the colour quality of the scattered light is practically unchanged. Hence a cloud very often appears white most of the time of the day. That is not the case during sunset and sunrise as the sunlight is reddish in colour; the clouds in turn assume that colour.But scattering has also the effect that when light passes through a mass of cloud, its intensity drops down continuously. The intensity of scattered light at the top of the cloud that faces the sun is large compared with the intensity of light from the bottom end of the cloud that is seen from the ground. Despite the droplets scattering all wavelengths of light uniformly, a drop in intensity of the light towards the bottom end of the cloud prevents it from appearing uniformly white. Some portions of it appear bright, whereas others appear darker and greyish. In the case of a thin mass of cloud with low density of the water droplets, the effect is not very significant. But when sun's light has to travel through very thick and dense clouds then light reaching some parts of the clouds falls down almost to nil, and those parts appear dark grey. Rain-bearing clouds belong to this category. Usually we see the lower side of the overhead cloud, while the sun shines above it on the other side. Interestingly, the same clouds would appear bright white, when they are seen from an airplane flying above the cloud.

Source: thehindu.com