The Causes of Fog
Fog is simply cloud, the base of which rests on the land or the sea. It consists of water droplets which are far too small to be seen individually but can be so numerous that objects close at hand become obscured. For fog to form, there has to be sufficient water vapour in the air and it must fall below the temperature of the dewpoint (the temperature to which unsaturated air must be cooled in order to become saturated). Further cooling usually results in condensation of the water vapour. Dew-point varies according to the temperature and the water content of the air. The higher the temperature of the air the more water vapour it can hold before it becomes saturated. Moist air can therefore become saturated by either cooling it down or by causing more water to evaporate into it. Dew-point can be calculated (using a psychrometer) and when the temperature falls below this fog will form. There are four different types of fog and three of these are caused by factors causing the air temperature to be reduced below its dew-point. The fourth type, Arctic sea smoke, is caused by cold air absorbing more moisture.
Radiation fog is caused by the air radiating its heat into space until its temperature falls below that of its dew-point. This happens on clear nights when there are no clouds to trap the heat. Radiation fog will only occur where there is rapidly cooling land, moist air which will probably have travelled over water, and very little wind so that the air cannot be heated up by being mixed with the air above. It generally occurs in high pressure areas where there is little wind and clear skies, and tends to form in valleys where the mixing of air of different levels is least likely. During the early hours of the morning it may spread out to sea for several miles. However, it seldom extends further than about five miles and is normally dispersed by about noon. This is because, as we have already seen, it usually occurs in anticyclones and the fair weather gives the air a chance to warm up. It may persist during the morning if extensive cloud cover has moved in. Radiation fog is most common during spring and autumn and can cause problems in busy estuaries.
Advection fog Advection fog is caused by air being carried over a surface whose temperature is below the dewpoint of the air. Advection fog which occurs at sea is known as sea fog. It can occur in a number of different circumstances. In spring and early summer, when the temperature of the sea is still cold, warm land air may move over cold sea. Evaporation takes place, the dew-point rises, the temperature of the air falls and fog forms. Air moving from over a warm sea to a colder sea can be another cause of sea fog. This occurs most frequently on the Grand Banks of Newfoundland where the warm air from the Gulf Stream moves over the colder sea of the Labrador Current. A third cause of sea fog is the movement of air over a sea which becomes gradually colder. A warm, humid air mass moving into higher latitudes and over cold water is cooled, becomes saturated and fog forms. Sea fog can be very thick and irritatingly persistent. Often a complete change in the weather pattern is required for it to disperse.
The third type of fog is frontal fog. As its name implies, it occurs where a warm, moist front meets a colder polar front. The temperature of the former airmass is cooled to below its dewpoint and fog forms. This type of fog is usually experienced as low cloud which sometimes falls to sea level. The primary danger associated with this fog is that, although it is often clear at sea 1 level, it is misty at higher levels, so land masses, lighthouses and other crucial landmarks may be M obscured when the air around your boat is quite clear. This form of fog, by its very nature, exists 10 as a thin belt along a front.
Arctic sea smoke Arctic sea smoke is the fourth and last type of fog. As its name implies, it usually forms inside the Arctic circle. Unlike the fogs which we have already outlined it is not caused by warm moist air being cooled, but by cold air absorbing moisture through the evaporation of warmer water. Because the air in this part of the world 1 is very cold, its dew-point is correspondingly very low. Therefore any moisture which is absorbed is almost immediately transformed into fog. However, almost as soon as the fog has formed the air is warmed by the sea, the dewpoint rises and the fog immediately above the surface of the sea disperses. This warm air which then rises is cooled again by the cold air above so that fog forms again higher up. It is this continual process of fog forming, dispersing and reforming which creates the strange effects by which it acquires its name. Sea smoke lasts only a short time for the conditions bringing it about are quickly balanced out. The cold air becoming sufficiently warm to reduce the likelihood of fog.