Why are clouds white and fluffy? Why are they not a different colour and flat?
Well, not all clouds are white, some are pretty grey...
When warm air rises from the ground, it carries water vapor with it. When the water vapor meets the cold air found high in the sky, the gas condenses to liquid and forms cumulus clouds. While these fluffy-white clouds look like soft pillows of cotton, they are actually composed of small water droplets.
Thus, clouds are really made of frozen crystals or very tiny droplets of water that float in the Earth's atmosphere. These tiny frozen crystals, as well as the water droplets, are excellent reflectors of light.
When the sun shines, its electromagnetic spectrum emits light at a wide range of wavelengths, including the visible spectrum of red, orange, yellow, green, blue and violet light. The combination of these colours creates white light.
As the sun's light hits the clouds' small water droplets, each droplet scatters the light. As the entire visible spectrum is scattered, the clouds appear white. In fact, the colour of a cloud can suggest what it actually contains!
When the size of the water droplets and frozen crystals is small, light is easily reflected, and the cloud appears white. However, when the drops of water turn larger, then light can penetrate inside the cloud. This light is not reflected back and it remains absorbed (the light in this case may only be reflected upwards.) Therefore, the colour of the cloud then appears of a different shades such as grey or even black when seen from the Earth below. This itself creates an impression of an overcast sky and weather forecasts then would obviously predict rain.
Apart from these colours, clouds are also seen in shades of red or even orange especially on days when one can witness a gorgeous sunset. This again, is a result of the scattering of light: as the sun rays approach from a small angle, the red part of the spectrum, which is less scattered than the blue one, is more pronounced making the clouds appear reddish.
A cloud becomes a fluffy cumulus cloud instead of a wispy or thin or curly type of cloud when the water droplets that are joining together are in patches of moist air with breezes moving upwards. This makes them grow tall and puff up!