Frost does not necessarily form from air that has fallen to the freezing point, as many suppose. On the contrary, the air ten feet or less above the vegetation may be several degrees above freezing when there is a heavy and destructive frost upon vegetation. The fact is that vegetation radiates heat towards a clear sky faster than does the air and may fall to the freezing point or below; while the air, except the molecules actually in contact with the vegetation, is considerably warmer. Frost is not frozen dew. The water vapor is precipitated, or rather congealed, upon the vegetation without passing through the liquid state at all. Frost is spoken of as light, heavy, and killing. Tomato plants are killed by only a light touch of frost, while fruit blossoms will stand several degrees of cold below freezing. Therefore the tomato grower would consider as killing a frost that to the fruit grower would only appear as light. The radiation of heat from the earth is continuous both day and night when there are no clouds to obstruct the passage of the heat rays. The amount received from the sun during the day is greater than the loss by radiation from the earth and the temperature of the air rises. After the setting of the sun the radiation of the earth goes on but there is no incoming heat from the sun to offset the loss and the temperature of the air falls. As previously stated, the soil and vegetation radiate faster than the air and the air in immediate contact with the soil is cooled by conduction to it. Thus over a level plain on a clear calm night there is found a relatively thin layer of cold air near the ground, which increases in temperature up to two hundred or three hundred feet, or which may be only five or ten feet deep. Over sloping ground the force of gravity tends to cause this thin surface layer of cold air to move down the slope and to gather in depressions in somewhat the same manner as water would move. Such movement is called Air Drainage. Of course this air is slowly gaining heat by compression as it passes to lower levels, but it is hugging closely to the cold earth and losing by conduction much or all that it thus gains by compression. After a study of the contour of the region with respect to air drainage the writer purchased a considerable tract of land near Rockville, Montgomery County, Maryland, and planted extensive orchards thereon, with the result of harvesting nine successful crops of fruit in a period of ten years after the trees became large enough to bear. With the composition and the surface covering of the soil the same, the low places in a field are always the ones that suffer most when frost is possible. Figure 8 shows a minimum temperature of 25° to have occurred at the base of a steep hillside when on the higher ground at an elevation of but fifty feet the lowest temperature was 44°, and at two hundred and twenty-five feet up the mountainside the minimum was 52°.