Clouds, by checking radiation from the earth, and wind, by mixing the colder and warmer layers of air together, both prevent frosts that would otherwise occur. Artificial methods of protection include covering plants with screens of wood, paper, or cloth, building smudge fires to provide a blanket of smoke (a method of doubtful value), and, above all, heating by means of wood fires or various types of “orchard heater,” burning either oil or coal. An elaborate technique of orchard heating has been developed, having in view especially the most economical use of fuel and labor consistent with the object to be attained. In many cases orchards are provided with alarm thermometers, which ring a bell when the temperature approaches the danger point in the orchard. The local prediction of frost from the readings of meteorological instruments is a problem that has not been fully solved. The idea formerly prevailed that the temperature of the dew point, as determined from readings of the dry-bulb and wet-bulb thermometers in the early evening, was a safe guide to the fall of temperature to be expected during the night, but this belief has not stood the test of accurate observations. At the present writing certain formulas involving data of both temperature and humidity are being used experimentally by Weather Bureau specialists for predicting the lowest temperature of the night when the general conditions indicate that frost is possible. A comprehensive discussion of this subject has been published by the Bureau as Supplement No. 16 of the “Monthly Weather Review.” (Washington, 1920.) CHAPTER XVI COMMERCIAL METEOROLOGY It is a significant fact that the American Meteorological Society, which was organized in 1919, has a Committee on Commercial Meteorology. The appointment of this committee was one of the earliest tokens of the fact that the applications of meteorology to business, always recognized to be important and far-reaching, had at last been segregated as a distinct field of inquiry. The time is near at hand when this field will have its corps of specialists and its textbooks. Courses in commercial meteorology will be given in business colleges, and meteorologists will be attached to the staffs of large business enterprises. The chamber of commerce of a wide-awake western city already maintains a Department of Meteorology, with a former Weather Bureau official at its head, and “consulting meteorologists,” now practicing their novel profession in other parts of the country, find their principal clientele among business concerns.