Cumulus clouds are the fluffy, white cotton ball or cauliflower-looking clouds with sharp outlines. They are "fair weather clouds" and they are fun to watch as they grow and change in shape and size. Cumulus clouds make beautiful sunsets.
Cumulonimbus clouds are a sure sign of bad weather to come--at least bad if you don't like rain and hail! These clouds grow on hot days when warm, wet air rises very high into the sky. Up and down winds within the cloud may push water droplets up to very cold parts of the atmosphere, where they freeze. When the ice drops come back down, they get another coating of water and are pushed back up to freeze again. Finally, they get too heavy to stay in the cloud and fall to the Earth as hail stones.
Altocumulus clouds have white or gray patches or layers, and seem to be made up of round shapes. They are lower than cirrus clouds, but still quite high. They are made of liquid water, but seldom make rain.