Orchard Heaters in Operation. The economical use of this method of frost protection depends upon accurate forecasts of the right time to “fire” the orchard. (Courtesy of Hamilton Orchard Heater Co.)
The United States Weather Bureau has been a branch of the Department of Agriculture since 1890, and a very large share of its routine work is devoted to the agricultural interests of the country. The climatological statistics that it has assembled are indispensable in many departments of agricultural research, besides furnishing varied information of practical value to farmers. The Bureau has developed a number of special types of forecasts for the rural industries; such as predictions, three or four days in advance, of favorable weather for cutting alfalfa; forecasts of weather unfavorable for sheep-shearing; notices to fruit growers of dry-weather periods in which fruit trees should be sprayed; and warnings of the occasional summer showers that would do so much damage to the great raisin-drying industry of California but for the vigilance of the forecasters and the efficient arrangements made by the industry itself for disseminating and acting upon the warnings. Of course, the ordinary daily weather forecasts, storm warnings, and cold-wave warnings are valuable in many ways to agriculturists, and the Bureau has made great efforts to give such information prompt and general distribution in the rural districts. The forecasts are generally displayed in post offices, and in many cases the rural telephone exchanges are pressed into service to distribute weather information regularly to all their subscribers. Some exchanges sound a signal every morning when the forecast is ready for distribution. Lastly, the wireless telegraph and the wireless telephone, which, in the immediate future, will form part of the equipment of every up-to-date farm, afford ideal channels for the dissemination of weather news and are already extensively used for this purpose.
A Snow Surveyor at Work. Note the cylindrical snow sampler, with its serrated cutting edge, and spring balance for weighing the sample of snow (Photographed by J. C. Aller.)
Snow Rollers, or Wind-Blown Snowballs on a Lawn at Potsdam, N. Y. (Photographed by T. J. Moon.)
There remain to be mentioned the various steps the Weather Bureau has taken to protect the rural industries from the night frosts of spring and autumn, in the shape of special forecasting arrangements, the publication of frost charts, and a wide range of scientific investigations. The Bureau’s undertakings in this line are merely a part, though a leading one, of a great campaign of frost protection that is being carried on by scientific and official agencies in this country on a larger scale than anywhere else in the world.
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.