In addition, cloud varieties are discussed according to arrangement and transparency, and there is a classification based on the heights at which the various genera are usually found. The height classification is divided into high, middle, and low, giving approximate ranges of the genera based on their observed average altitudes in temperate regions. The height classifications are not rigid and overlap each other. Because of variations in air density according to temperature, clouds tend to be lowest in polar regions, and in the tropics, they tend to be highest. And of course there are clouds with vertical development.
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.