520 Bulletin 316 



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 temperature. Even a hazy condition of the sky, 

 or the thinnest cirrus clouds, have an appreciable effect in retarding 

 the fall of temperature at the surface. But on clear nights the heat escap- 

 ing 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 v/est, it is an almost 

 unfailing indication that the warm spell is over, and it is well to watch 

 closely the conditions that follow, particularly if there are signs of clear- 

 ing weather. Occasionally, a frost follows when the wind backs from 

 east or northeast 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 tem.perature during the late afternoon or early even- 

 ing is a good indication of the lowest temperature that will be reached 



