Fog is merely a cloud at ground level. Smog is any visible urban air pollution. Another type of cloud can be formed from the vapor contained in the exhaust of a jet engine of an airplane when they are flying at high enough altitudes where cold temperatures cause the vapor to turn into ice crystals like cirrus clouds. The 10 main cloud families are divided into three groups on the basis of altitude. A cirrus cloud appears in delicate, featherlike detached bands, sometimes in tufts, and is usually white with no shading.
Cirrocumulus clouds look like very small round balls or flakes.
Sometimes cirrocumulus clouds form the pattern of the buttermilk or mackerel sky. Cirrostratus clouds sometimes form tangled webs or thin whitish sheets. When cirrostratus clouds cover the sky, a large ring or halo is sometimes seen around the sun or moon. This is caused by the natural bending of rays of sunlight or moonlight as they pass through the ice particles. Altocumulus clouds are rounded puffs of cloud larger than cirrocumulus.