December, 1920 



BETTER FRUIT 



Page 5 



Smoke and Direct Radiation in Frost Protection 



By Floyd D. Young, Meteorologist U. S. Weather Bureau 



SMUDGING, or the creation of dense 

 blankets of smoke over fields and 

 orchards on frosty nights through 

 the burning of damp straw or other ma- 

 terial, has been practiced since the be- 

 ginning of the Christian era. In his "Na- 

 tural History", published A. D. 77, Pliny 

 the Elder advised farmers of his day 

 when frost threatened to "make bon- 

 fires in the fields and vineyards of cut- 

 tings or heaps of chaff, or else of weeds 

 that have been rooted up; the smoke 

 will act as a good preservative". The 

 degree of faith in the efficacy of this 

 method of protection that prevailed in 

 Germany near the end of the 17th cen- 

 tury is shown by the fact that smudg- 

 ing was compulsory in one part of that 

 country. 



Only comparatively recently have 

 some orchardists come to believe that 

 actual warming of the surface air is of 

 greater importance than creating a 

 smoke cover, and the belief is still held 

 by many that the smoke and other pro- 

 ducts of combustion from the fires 

 check the loss of heat from the ground 

 and prevent a further fall in tempera- 

 ture. 



In order to determine accurately just 

 how effective the smoke cover is in di- 

 minishing the rate at which heat is lost 

 to the sky, instruments for measuring 

 the rate of nocturnal radiation were in- 

 stalled in the Pomona Valley, Cal., in 

 the fall of 1918. Observations were 

 made every half hour from sunset to 

 sunrise during two clear nights when 

 there was no firing, in order to find out 

 whether there was much variation in 

 the rate at which heat was radiated to 

 the sky. These radiations, reduced to a 

 common temperature basis, showed the 

 rate of radiation to be relatively uni- 

 form throughout both these nights. 



The 1918-19 frost season proved to be 

 the most severe in years. Firing was 

 general over the entire valley and was 

 continued for from nine to ten hours 

 on four different nights. Radiation ob- 

 servations were begun before the heat- 

 ers were lighted, to obtain the rate at 

 which heat was being lost to the clear 

 sky, and were continued all night under 

 a smoke cover that increased in density 

 as the night progressed. Since the rate 

 of radiation was fairly uniform during 

 clear nights when there was no smoke 

 cover, any decrease in the rate after the 

 heaters were lighted could be attributed 

 to the influence of the smoke. 



As the smoke often remains near the 

 ground, the first observations were 

 made from the top of a fourteen-foot 

 tower, at about the height of the tops 

 of the trees in an orange grove, in order 

 to find out whether the lower portion 

 of the trees does not receive more bene- 

 fit from diminished radiation than the 

 top. (See figure 1.) 



During the two nights on which ob- 

 servations were made from this tower, 

 very little smoke rose above the top of 

 the instrument shelter and the rate of 

 radiation showed no appreciable de- 

 decrease as a result of the firing. 



The radiation instruments were then 

 moved to a location on the ground in a 

 small open space in the same grove, and 

 observations were made on two nights 

 during which the smoke was probably 

 as heavy as will ever be experienced 

 anywhere. Despite this heavy smoke, 

 the rate of radiation was diminished by 



Tower shelter for radiation instruments. It 

 was found that practically all the smoke re- 

 mained below the top of the shelter and 

 showed practically no influence on the rad- 

 iation. 



only about 10 per cent during any con- 

 siderable period of time, although in- 

 dividual readings showed a decrease 

 amounting to as much as 25 per cent. 

 (See figure 2). 



Additional observations of the same 

 kind were made at Medford, Oregon, 

 during the spring of 1920, outside and 

 under a dense blanket of smoke pro- 

 duced by open lard-pail heaters. The 

 average decrease in the rate of radia- 





Kadilinn in.str 



tion due to the smoke was 9 per cent, 

 with an individual reading which 

 showed a decrease of 26 per cent. 



The rate at which heat is radiated 

 from the ground decreases very rapidly 

 as the temperature falls. As the orchard 

 heaters are not lighted until the tem- 

 perature has reached a comparatively 

 low point, a reduction of even 25 per 

 cent in the radiation at this time is not 

 very important; in order to prevent 

 damage through the use of the smoke 

 cover alone, the rate of radiation would 

 have to be cut down at least 80 per 

 cent. It is evident that the smoke would 

 be of far greater value in protection 

 against frost damage if it were com- 

 pletely consumed in the heaters. 



The smoke may often be of indirect 

 benefit in preventing a too rapid thaw- 

 ing of frozen fruit or blossoms at sun- 

 rise, but from the data at hand it ap- 

 pears that neither in this connection 

 nor in its influence in reducing the rate 

 of radiation of heat to the sky, is dam- 

 age from a moderately severe frost to 

 be prevented by a smoke cover alone. 

 When the lowest temperature during 

 the night is only slightly below the dan- 

 ger point, a heavy smoke from smudge 

 fires may lessen or prevent damage, but 

 to combat a frost which would other- 

 wise cause widespread damage, it is 

 necessary to supply great quantities of 

 heat to the surface air to replace that 

 which has been lost by radiation to the 

 sky. In most cases the straw or manure 

 burned to create a smoke over the or- 

 chard would do more good if used for 

 fertilizer, making the trees more vigor- 

 ous and therefore better able to resist 

 damage by low temperature. 



Radiation From Heaters to Trees. 

 Some careful observers of orchard 

 heating operations have noted for years 

 that when heaters are placed in alter- 

 nate rows between the trees, the great- 

 est damage from low temperatures is 

 always found in the rows in which no 

 heaters are placed; in other words, "the 

 dark rows are the cold rows." One fruit 

 grower of Pomona, Cal., estimated he 

 had fully 50 per cent more damage dur- 

 ing the severe 1918-19 season in rows 



ground. 



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