There are several ways in which the free air may be cooled to the point at which condensation occurs. The commonest is dynamic cooling, due to the rise of a mass of moist air and its expansion under the reduced pressure that prevails at the higher levels. This process is beautifully illustrated in the formation of the roundish masses of fleecy cloud known as _cumulus_, on a warm summer day. Each of these clouds marks the summit of a column of air that is rising after having been heated at the surface of the earth. When the process goes on very actively, the cloud may tower up to enormous heights, forming a thundercloud. Some clouds are formed by the mixing of air of different temperatures. Fog, which is merely cloud at the surface of the earth, is often formed by the cooling of the air in contact with cold land or water. The persistent fogs of the Newfoundland Banks are due to the passage of warm moist air from the Gulf Stream region over the cold Labrador Current. On the other hand, a cold wind blowing over warm water will also often produce a fog by lowering the temperature of the moist air overlying the water. A common cause of land fog is the cooling of the air adjacent to the ground in consequence of nocturnal radiation. The moister the air, the more readily fog forms, and hence the frequent formation of fog by night along rivers and over marshes and damp valleys. Town fogs, such as the famous “London particular” and the fogs of Lyons, usually consist partly of smoke. Dense fogs of this sort occur when the conditions of the atmosphere are such as to cause the smoke to hang low over the city, instead of being dispersed. These fogs constitute a serious economic problem. Thus it is estimated that they cost the people of London upwards of half a million dollars a year, due to extra lighting, damage to vehicles, loss of business, etc. Since marine fog is also a source of enormous loss, through causing delays and accidents, and since fog along air routes is the greatest of all obstacles to successful aerial navigation, it is no wonder that much ingenuity has been devoted to the attempt to disperse fog artificially. Electric discharges have been successfully used for this purpose on a small scale. The depth of a fog may be anything from inches to miles. Measurements made by the United States Coast Guard during the international ice patrol of the North Atlantic show that the fogs on the Newfoundland Banks are very commonly so shallow that the mastheads of vessels rise above them, though in some cases they were found, from observations with kites, to be from 2,500 to 3,000 feet thick. Observations on the mountains of the California coast show that the upper level of fog in that region rarely exceeds 4,000 feet. On the other hand, aviators flying between London and Paris have encountered fog more than 10,000 feet deep.