Frosts, classified according to their severity as “light,” “heavy,” and “killing,” are most likely to occur in spring and autumn, when an extensive area of high barometric pressure brings its usual accompaniment of clear skies and calm nights. They are predicted on a general scale from the weather map, and locally from indications of temperature and humidity and a knowledge of important topographic influences, such as those due to hills and valleys and neighboring bodies of water. In agricultural usage the term “frost” is applied to the occurrence of a temperature low enough to kill or injure tender vegetation, such as growing vegetables or the buds, blossoms, and fruit of fruit trees. The occurrence of a frost, in this sense, is not necessarily identical with the deposit of ice crystals known as “hoarfrost.” Different species and varieties of plants are, of course, susceptible in very different degrees to the effects of low temperature; i. e., they differ greatly in “hardiness.” In the case of fruits and vegetables the danger point generally lies a little below the freezing point of water (32 degrees F.). The occurrence of frost is favored by the rapid cooling, by radiation, of the earth and its plant covering, which goes on at night under a clear sky and in still air. Under these conditions a layer of stagnant, cold air forms close to the ground, with warmer air lying above it. The difference in temperature at different levels is often so pronounced that fruit on the lower branches of a tree is killed while that growing on the higher branches remains uninjured. Similarly, frost will occur in the bottom of an inclosed valley but not on the surrounding slopes. In the case of a valley the layer of cold air that forms at the bottom is commonly deepened by additional cold air draining down from the hills. Many large orchards have their “warm spots” and their “cold islands” or “north poles,” well known to the orchardist; due in some cases to the nature of the soil rather than to topography. Certain mountain regions in North Carolina are famous for their “thermal belts” or “verdant zones”; i. e., areas part way up the slopes that escape the frosts occurring both above and below them. These frostless belts, which have been the subject of numerous investigations for three-quarters of a century, seem to mark the upper level of the pool of cold air that collects in the valley by drainage from the mountainsides. A detailed temperature survey of the thermal belt region of North Carolina was made during the years 1912-1916 by the United States Weather Bureau and the North Carolina State Board of Agriculture. In some places the minimum temperature at night was found to be 15 or 20 degrees higher in the thermal belt than at the bottom of the valley, a few hundred feet below.