PROTECTION AGAINST FROST 379 



Concerning the influence of wind velocity Humphreys says: "Of course a 

 greater wind velocity than 2K miles per hour, the velocity above assumed, would 

 appear to necessitate a correspondingly larger consumption of fuel for the border 

 or entrance heating. But this, presumably, is not true in practice, since probably 

 even this velocity, certainly a greater one, would considerably mix the surface- 

 cooled air with the warmer air above, and thereby decrease the amount of 

 necessary heating. During a perfect calm the required border heating is zero; 

 it is also zero when there is a fairly good breeze and hence has its maximum value 

 at some quite moderate intermediate velocity." 



It should be noted that Humphreys is stating that the higher the 

 velocity of the air movement the higher the air temperature is likely to be. 

 This is quite different from the case of high wind at a dangerous tem- 

 perature, for here the heating required increases with the wind velocity 

 and too many times becomes impossible. 



The choice of heater types depends on the nature of the service 

 required. In some sections where dangerous temperatures are of 

 short duration the simple 1-gallon heaters will be adequate; in other 

 sections longer burning may be required. Young- '^ points out that the 

 size of the temperature inversion characteristic of many of the California 

 frosts permits the use of stack heaters which, perhaps, could not be 

 employed in sections where the temperature inversion is weaker. No 

 one type is best for all sections or for all occasions in one section. 



Conditions Determining Practicability. — No general discussion can 

 decide the question whether orchard heating is profitable. The con- 

 tinuance of the practice in certain sections over a long period is rather 

 good evidence that with conditions as they are in those sections it is 

 either profitable or necessary or both. The necessity of the practice, 

 if fruit is to be grown in a certain spot, may mean that it is desirable or it 

 may mean that the spot should be devoted to some other crop. If the 

 value per acre of the crop is high, as with oranges, heating may be 

 economically sound; if the value per acre of the crop is low, heating is 

 of doubtful wisdom. If a given spot is exposed to several frosts a year 

 heating is likely to pay as compared with no heating but it may be that 

 fruit growing should be abandoned at that spot. 



The installation of an orchard heating equipment involves a heavy 

 overhead expense. Each year heaters and fuel must be distributed and 

 made ready. The chief difference in expense between a frosty and a 

 frostless spring so far as heating is concerned is in the oil consumed and a 

 reduction or increase in the labor charge. The profits of the frostless 

 season are taxed only somewhat less than those of the frosty season. 

 Frequently the yearlj^ expense has amounted to $20 per acre; it has 

 reached $40. In many, if not in most, fruit growing sections, $40 per 

 acre added to the initial price of the land will secure sites located advan- 

 tageously enough to escape this tax. 



