PROTECTION AGAINST FROST 373 



total of 11° which, it is evident from the table, was realized frequently. Though 

 it is unsafe to generalize from a few observations, it is interesting to note that 

 for the lower temperatures at Station A the departures for Station 7 averaged 

 greater than they did for the higher temperatures at Station A ; in other words it 

 would seem that as the temperature at Station A came nearer to the freezing 

 point the temperature at Station 7 was in even greater measure more likely 

 to drop below that point. Evidently a strawberrj^ grower at Station 7 should 

 deduct at least 11° from the minimum indicated for Station A to forecast the 

 probable temperature at his own place; if apples were the crop at the same point 

 the deduction would be somewhat less. 



Even greater differences are reported by Cox''" between minima on the bog 

 at Mather, Wis., and the minima at the "key station" La Crosse, 55 miles away. 

 Shelter minimum temperatures on the upland at Mather for May, 1907, averaged 

 3.8° below those at La Crosse with ranges from —14° to +8°; minima at 5 

 inches above the bog at Mather averaged —8.5° below those for La Crosse, 

 with ranges from —20° to +5°. Cox states that the average difference when the 

 weather is clear and the pressure high is about 18°, so that in such weather a 

 minimum of 50° for La Crosse signifies a bog minimum at Mather of about 32°. 



The grower who wishes to prophesy with accuracy what the minimum 

 will be in his own orchard, bog or field must rely on the Weather Bureau 

 to furnish information as to the probable minimum at some fixed point 

 and he must rely on himself to adapt these indications to the spot where 

 his own crop is located. To do this it will be necessary to keep accurate 

 records of minima at his own orchard on all clear nights during the spring 

 for 2 or 3 years, to compare them with the records of the Weather Bureau 

 and from these data to determine the probable and the safe corrections 

 to be made. 



FROST FIGHTING 



The data already discussed show that much can be accomplished in 

 combatting frost by selection of site, fruit and variety and in some cases 

 by cultural practices. All these measures may be regarded as preventive. 

 There remain for consideration the palliative measures. 



Smoke Screens to Reduce Radiation. — In view of the emphasis placed 

 on radiation as a factor under frost conditions, efforts to prevent heat 

 loss through radiation might be expected to be fruitful. In fact it is 

 rather generally assmned that a dense smoke will so retard radiation 

 losses that frost damage will be checked or prevented. Such cases have 

 been recorded. However, quantitative data available for comparison of 

 temperatures in smudged areas where the heating factor is eliminated 

 with those in unsmudged and unheated areas do not indicate a sufficient 

 saving of heat to make the smudge in itself of any great value. Table 73 

 shows temperatures in a smudged area and in an unsmudged area adja- 

 cent, in a German vineyard. The averages include some figures not 

 presented here. 



