374 



FUNDAMENTALS OF FRUIT PRODUCTION 



Table 73. — Temperatures in Smudged and Unsmudged Areas^^s 

 (Degrees Centigrade) 



The differences are at the most too small to be of practical impor- 

 tance. It was suggested that the small difference was due to air move- 

 ment and the investigator appears not to have been convinced that 

 greater differences might not be found under other conditions. 



Kimball and Young/"^ using a pja-ogeometer, measured the radiation 

 in smudged and in unsmudged areas in California and Oregon, finding 

 decreases by smudging from 0.110 and 0.115 calories per minute per 

 square centimeter to averages of 0.098 and 0.103 respectively in Cali- 

 fornia and in Medford, Oregon, from 0.109 to an average of 0.099. Con- 

 siderable fluctuation under the smoke occurred, the maximum decrease 

 amounting to 28 per cent, with averages respectively of 11, 10 and 9 

 per cent. They conclude from their investigations that "the retardation 

 of nocturnal radiation by the smoke cloud plays an insignificant part in 

 frost protection." 



The reflection of heat from smoke clouds is evidently very small. 

 Miiller-Thurgau^^^ points out that smoke differs in its composition from 

 clouds. It should be recalled that radiation is constantly occurring, 

 clouds or no clouds, and that they do not prevent radiation but only 

 reflect heat, and since outgoing and incoming heat approach equal value 

 on cloudy nights the net loss by radiation is small. Smoke differs from 

 water vapor in being relatively transparent to long heat waves. There 

 is a relatively large difference in the way violet (or blue) and yellow (or 

 red) are transmitted through dust in the air^ — for example, the sun is 

 yellow or red at horizon, the short waves not being transmitted as readily 

 as the longer yellow and red waves. The sun looks red through smoke, 

 showing the same effect. The smoke screen appears opaque because the 

 eye uses the shorter waves but it must be very much less opaque to the 

 long waves which the earth radiates. 



