What is Fog and How is it Formed?
A fog is a cloud that has formed near the ground or over a body of water, instead of higher above the earth. Clouds in the sky and fogs lower down form when water vapor in the air condenses in the form of water droplets or tiny particles of ice or snow. If so much water vapor condenses that the air can not hold it in the form of fog or cloud, then it falls to earth as rain or snow, or lies on the ground as dew.
Fogs and clouds form when cool air strikes warmer air or a warm mass of ground or sea. On a summer day when there is much moisture in the air, mist forms on your glass of cold water. The warm air, striking the cooler glass, condenses. If there is ice in the glass, so that it is quite cold, the water vapor in the air condenses in drops of water on the outside of the glass.
Hot water evaporating out of the spout of the teakettle strikes the comparatively cool air of the kitchen, and becomes steam, in a similar way. Hold a cold plate near the steam from the kettle and it will condense in water on the plate. Your breath on a cold day sometimes makes a little fog in the same manner as the teakettle steam. You breathe out a warm current of air against colder air.
Fogs are frequently seen over the sea, over lakes and rivers and swamps, because the air here usually contains water vapor which the sun has drawn up, or evaporated (turned to vapor). Over some parts of the ocean, fogs are present much of the time. One such place is off the coast of Newfoundland. There a cold current flowing south from the Arctic meets warm air blowing northwest from the Gulf Stream.
In cities there is sometimes fog colored by smoke particles from chimneys. Some people give the name smog to this combination of smoke and fog. Mist is thin fog.
