FOG

water dust suspended in the atmosphere close to the earth, depends on the solid matter in the air. If the air is free from dust the temperature may be below dew-point, but no drops of water will form, for as soon as water is present it will evaporate. But if particles of dust are present the water condenses on them as a thin film, from which re-evaporation does not take place readily. The water is probably produced in favorable sites by the movement of ma es of moist air at different temperatures. In the country, f. is usually white, and consists of minute water-globules which disperse the sunlight by repeated reflection, but it is fairly translucent. In the town, soot and smoke blacken it, so that light may be quite obscured. The presence of f. in the air decreases the velocity of sound,