254 The Principles of Fruit-growing 



"Preceding weather. The character of the preceding weather is 

 important because damaging frosts often follow an abnormally 

 warm period. The weather moves over the country from west to 

 east in somewhat irregular, but nevertheless well-defined waves. 

 Hence, there is a strong tendency for extremes to follow each other, 

 and such extremes are often disastrous in their effects. A light 

 frost, particularly in spring, following an unusually warm period 

 often will cause more damage than a severer frost preceded by 

 cooler weather, because the warm weather forces out the blossoms 

 prematurely and renders them more susceptible to injury. While 

 an unusually warm period does not mean always that frost will 

 follow, yet the fact that most frosts do follow such periods should 

 be regarded as sufficient warning that frost is likely to occur. 



"State of the sky. Frost is not likely to occur when the sky is 

 overcast, because the heat given off by the earth at night does not 

 penetrate the clouds easily and is practically all retained in the air 

 below them, which therefore remains at a comparatively high tem- 

 perature. Even a hazy condition of the sky, or the thinnest cirrus 

 clouds, have an appreciable effect in retarding the fall of tem- 

 perature at the surface. But on clear nights the heat escaping from 

 the earth passes away quickly, almost without hindrance, far beyond 

 the limits of the atmosphere. Hence, the fall of temperature at 

 the surface is rapid and, unless the earth has a vast store of heat, 

 frost is likely to occur. 



"Direction and force of the wind. The direction of the wind is 

 a reliable indication of the approach of colder weather. If, after a 

 day or two of warm southerly winds and possibly rain, the wind 

 changes to the southwest or west, it is an almost unfailing indication 

 that the warm spell is over, and it is well to watch closely the con- 

 ditions that follow, particularly if there are signs of clearing weather. 

 Occasionally, a frost follows when the wind backs from east or north- 

 east to north or northwest. In either case, the conditions to be 

 looked for as soon as the wind changes are falling temperature, 

 decreasing wind, and a clear sky. Frost is not likely to occur unless 

 the air becomes quiet and the sky clear, for wind prevents the 

 accumulation of the colder air at the surface. 



"Trend of the temperature. The rate of fall in temperature 

 during the late afternoon or early evening is a good indication of 

 the lowest temperature that will be reached during the night. For 

 example, a temperature of 40 at about 6 P. M., with a clear sky 



