Since clouds are made of water droplets that are denser than air, why do they float?
The water droplets in clouds aren't actually floating like a log on water. They are constantly falling. However, they are so tiny that they have a large surface area compared to their mass. That creates a lot of friction as they fall through the air and really slows them down. They are also falling through an air mass that is moving upwards. So, they are moving up faster than they are falling down. If the tiny water droplets collide with enough other droplets, they may become large enough to overcome this effect and fall as precipitation. Read more here and here. Why are clouds sometimes different colors? White clouds - Small water droplets in clouds scatter all the wavelengths in white light equally so the clouds appear white to us. Gray clouds - Very thick, dense clouds full of large raindrops, block and absorb sunlight so they appear dark to us. Red clouds at sunset/sunrise - At sunset/sunrise light has to travel farther through the atmosphere before it reaches us. Most of the short wavelengths from the blue end of the spectrum have already scattered away leaving mostly light waves from the red end of the spectrum. If thin clouds are present, they can reflect these red/orange light waves down to us. Iridescent clouds (rainbow colored) - Rarely, sunlight will travel through a thin cloud with very tiny ice crystals or water droplets and be diffracted into different wavelengths creating a rainbow effect. Read more here and here and here.
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