They are always more than three miles up where the temperature is below freezing, even in summer. Wind currents twist and spread the ice crystals into wispy strands. Contrails are made by high-flying jet airplanes. They are still clouds, though, because they are made of water droplets condensed from the water vapor in the exhaust of the jet engines. Cumulus clouds are the fluffy, white cotton ball or cauliflower-looking clouds with sharp outlines.
Cumulus clouds make beautiful sunsets.
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 made up of round shapes.