[Celt.] When the vapor in the air reaches the point of saturation (see Dew) it condenses, and assumes the form of very small drops, which constitute fog if they are present in the lower regions of the atmosphere, and cloud if in the higher. Fogs are therefore of the same nature as clouds {q.v.). Fogs may be caused by the flow of a current of warm moist air over masses of ice, such as are sometimes encountered in the Atlantic, and are often seen on the Banks of Newfoundland. Fog Signals. Signals to prevent the collision of vessels in foggy weather. Many methods of signalling have been tried, the best being the whistle and the trumpet. The most powerful is the siren trumpet, whose sound can be heard for more than 20 miles.