Sea-fog occurs mostly in the spring or summer, and there are two reasons why it may develop. Warm air, heated by the land, may drift out over relatively cold sea-water; or warm "tropical" air may approach Ireland from the south and be cooled by contact with the relatively cold seas around our coast. Either way, the lower atmosphere gradually adopts the temperature of the surface underneath it, and if this temperature happens to be below the temperature at which condensation must take place, then fog will form.
In contrast to the typical inland "radiation" fogs, sea-fog can occur at any time of the day or night, and is not restricted to conditions of light winds and clear skies. Indeed it is more common when there is a noticeable breeze, because its very existence depends on a movement of the air over a cold surface. And it is very much a coastal phenomenon; summertime sea-fog usually dissipates quickly if it is blown inland, because the warm ground underneath heats the air and allows it to re-absorb the moisture.
As with showers, it is easy to predict the conditions in which sea-fog is likely to occur, but hard to specify exactly when and where. Satellites, however, make the task a little easier, and two types of satellite picture are used in combination for this purpose - visible and infrared. The visible picture records more or less what might be seen by an ordinary black-and-white camera. The clouds are a brilliant white, because they reflect back towards the satellite a large proportion of the sunlight which hits them, and fog shows up in the same way - and for this very reason it is often difficult to distinguish it from white cirrus cloud some 20,000 ft above the ground, a feature of no immediate interest as far as the weather forecast is concerned.
But in the case of infra-red pictures, the sensor reacts to temperature rather than to visible light; cold objects appear white, and those at higher temperatures black. On an infra-red picture, fog can hardly be seen at all, because there is little or no contrast between its temperature and that of the land or sea a few feet below it. By comparing the two pictures - visible and infra-red - the forecaster can identify the fog on the visible image for what it is, easily distinguishing it from cloud. Then having pin-pointed the areas where sea-fog is present, and analysed the local winds, it is possible to forecast with considerable accuracy the areas into which the fog is likely to be carried during the next few hours.