FROST 



FROST 



1283 



Varieties as well as individual plants differ greatly 

 in their ability to withstand cold, and a temperature 

 that may destroy a plant at one time may cause little 

 injury at any other time. Hence, no general statement 

 can be made with regard to injurious temperatures 

 that will apply in all cases. 



The following table indicates the temperatures (Fahr.) 

 that have been found injurious to tree fruits during the 

 blossoming period: 



General atmospheric cotidUions that cause frost. 



The loss of heat that brings the temperature of 

 plants to the freezing-point occurs in two ways, — (1) 

 loss by conduction, and (2) loss by radiation. Loss of 

 heat by conduction occurs when the air in contact with 

 the plant is cokler than the plant itself. This allows 

 the heat to flow directly from the plant into the colder 

 air about it. Frosts due to this cause alone result 

 almost invariably from the importation of large masses 

 of cold air, brought down from the upper atmosphere 

 by descending currents, or from higher latitudes by 

 northerly winds, both of which movements usually 

 are active when the weather clears after a storm 



Frosts, particularly in the late spring or early fall, 

 result also from loss of heat by radiation. Plants radi- 

 ate heat continuously. During the day more heat is 

 received by them from the sun than they give off, and 

 the plant becoriies warmer; at night plants pour more 

 heat into the atmosphere than they receive from it, 

 hence they become colder. 



Radiation proceeds most rapidly when the sky is 

 clear and the atmosphere is quiet. Clouds check radia- 

 tion, because the heat given off from the earth does 

 not penetrate them easily. Much of it is reflected 

 back toward the earth, warming the whole stratum 

 of air between the earth and the clouds. 



A quiet atmosphere allows the colder, therefore 

 heavier, particles of air to settle to the surface of the 

 earth. Thus, on quiet, clear nights, when frost is likely 

 to occur, the air near the ground may be 10° colder 

 than the air 10 or 15 feet above the ground. The wind, 

 by stiring up the atmosphere, prevents the settling 

 of the cold air, and in this way maintains the stratum 

 of air near the surface at a more nearly uniform tem- 

 perature. 



Frost results seldom from conduction or radiation 

 alone. Both usually are active when frost occurs. 



Forecasting frost from weather maps. 



Frosts that injure vegetation are a part of the regu- 

 lar weather sequences. The weather comes to us in 

 what may be termed waves that travel with more or 

 less regularity in the middle latitudes from west to 

 east. The weather map is a survey of the atmosphere. 

 It charts the daily progress of these weather waves. 

 By the aid of such a chart it is possible to foresee in a 

 measure the coming of a frost, and to judge its probable 

 extent and severity. 



The weather map is based on observations of pres- 

 sure, temperature, cloudiness, wind, and precipitation, 

 made at many places scattered over a large area. Low 

 atmospheric pressure indicates the trough of the 

 weather wave, and high pressure its crest. The low- 

 pressure area is called the cyclone, because the winds 

 whirl or eddy about its center, the direction of rota- 

 tion being counter clock-wise in the northern hemi- 

 sphere. In approaching the center, the winds have an 



ascending as well as an inward component of motion. 

 The cyclone also is calleil a storm, because it is attended 

 on its eastern side by southerly or easterly winds, 

 cloudiness, rain or snow, and comparatively high tem- 

 perature, and on its western side by northerly winds, 

 clearing weather, and a decided fall of temperature. 



1584. Weather map, 8 a.m., June 7, 1913. 



The crest of the wave is indicated by increased pres- 

 sure, and is called the anti-cyclone. It is so named 

 because its structure is exactly opposite to that of the 

 cyclone. The winds of the anti-cyclone blow outward 

 frorn its center, and have a downward component of 

 motion; the sky is clear; the precipitation is scanty or 

 absent; and the temperature is comparatively low. 



Frosts are most Ukely to occur in the rear (western 

 side) of the cyclone, and just in front (eastern side) of 

 the crest or center of the anti-cyclone. Here is found 

 the mass of cold air, imported from the north by the 

 northerly winds, and augmented by the cold brought 

 down from above by the gently descending currents; 

 the sky is clear; and as night comes on the air becomes 

 quiet. Thus, the conditions that cause frost are ful- 

 filled. 



A weather wave, in which frost was the most pro- 

 nounced characteristic, moved from North Dakota to 

 the Atlantic coast from June 7 to 10, 191.3 (Fig. 1584). 

 The general conditions on the morning of the 7th and 

 8th (Fig. 1585) are shown by the accompanying weather 

 maps for those dates. The eastward progress of frost 

 from day to day is indicated by the dotted hnes on the 

 map of the 7th. The relative position of the cyclone 



1585. Weather map, 8 a.m., June 8, 1913. 



(low) and the anti-cyclone (high), on the 7th, indicates 

 clearly that frosts will occur over the Upper Mississippi 

 Valley and the Upper Lakes on the following morning, 

 while the conditions shown by the map of the 8th 

 make it certain that the frosts will spread eastward 

 over New York and the New England states by the 

 morning of the 9th. 



The Weather Bureau issues frost warnings when 

 frosts are indicated for any part of the United States. 



