How to distinguish the usual cloud from rain? Just look at his color: if it is white or a little gray - it is unlikely to rain, but if the cloud of black or dark blue - precipitation is not avail. Why do the rain clouds differ so much in color from ordinary?
The clouds are formed when warm wet air rises to height and condenses around the microparticles into the smallest drops of water or water vapor. The light coming from the Sun is a combination of light waves of different lengths, which together form white color (more in the article "What color is the sun actually?"). Gas molecules that are in the atmosphere, have the ability to dispel short-wave light waves corresponding to blue and blue color - that is why we see the sky blue.
When the light falls on the clouds, numerous water drops and ice crystals dispel all the light waves, making the cloud white. The greater the density of the particles, the less light waves pass through it to the ground, because of which the cloud seems to be dark on earth. But it is worth climbing the plane even over the dark wasting, how can you see that the upper layers, where most of the world do not have time to dispel, absolutely white.
This is interesting: This amazing feature of light makes many actually transparent things white for our perception. For example, the hair on the skin of a polar bear is not really white, but transparent. Snowflake also seems white to us, although individual crystals, of which it consists - transparent.