It may be a dense white blanket over the sea, Over a city, it nay be a thick curtain dark enough to turn noon into midnight. fogs come in various qualities and they are formed by various weather conditions.
But all of them are made from vapor, the gaseous form of water in the air.
The particles which form a cloudy fog are microscopic droplets of water. They measure from one hundredth part to one tenth of a millimeter,and droplets of all sizes may be present in the same fog. The tiny particles are widely separated by the air in which they float but countless trillions of them cast a veil over the scenery or shut out the view altogether.
A clean white or greyish fog is made only from vapor in the air which has been condensed into liquid droplets. You often form a miniature fog when you breathe out warm, moist air on a frosty morning. When nature's fogs tangle with the soot and grime of a busy city we get a dark peasoup mixture of fog particles and floating grime.
The air is always thirsty and ready to gather a load of vapor by evaporating the moisture on the face of the earth. But the air can hold so much vapor and no more and when its quota is filled, it reaches saturation point. Warm air can hold more vapor than cool air and when saturated air is cooled, some of its surplus vapor must be turned back again into liquid moisture.
Fogs and clouds form when warm, moist air is cooled and there are several weather conditions which can cause this to happen.
On a clear night, the ground soon loses the heat it gained during the day and the air temperature falls. The air may now contain more than its quota of vapor which is then turned into the tiny droplets which form a fog.
The dense, white fogs off Newfoundland are caused by a clash between the warm air over the Gulf Stream and the cold air over the Labrador Current. Fogs also form when warm moist air blows over a chilly surface. Rising air tends to cool and form foggy patches on the mountain slopes. All these fogs are fouled because the air has been cooled and left with a surplus of vapor which must be turned into tiny droplets of liquid water.
Meteorlogists have scientific terms for the causes of different fogs. The dense mists off Newfoundland are advection fogs, caused by the chilling of warm air as it blows over a cool ocean surface. A low morning mist is a radiation fog, caused by loss of the earth's heat by radiation. Mountain mists are usually adiabatic fogs, caused by the loss of heat which occurs when air expands and rises