370 FUNDAMENTALS OF FRUIT PRODUCTION 



its effects may be made. In a general way frost may be looked for on a 

 clear, still night; clear, because it favors radiation, still, because the cooled 

 air is not mixed with the warmer air. These conditions are associated 

 with high barometric pressure. However, they do not always produce 

 frost and a closer estimate is desirable. 



Relation of Dewpoint to Minimum Temperature. — Until recently the 

 dewpoint as determined in late afternoon or early evening has been 

 considered to mark the minimum temperature for the following morning. 

 Air contains varying percentages of moisture; the higher the tempera- 

 ture the more it can carry as vapor. If any given sample of air is cooled 

 the point is reached ultimately where some of the moisture is deposited. 

 This is the dewpoint. The condensation of moisture releases heat to the 

 air and it was thought that the heat thus released was sufficient to prevent 

 any further drop in temperature and that the evening dewpoint therefore 

 marked the minimum for the following morning. 



Careful comparison of indicated and actual temperatures has shown, 

 however, that the afternoon or evening dewpoint alone is not a suffi- 

 ciently reliable indicator to be of any great value in predicting the 

 minimum for the following morning. In fact Cox^^ records a slight degree 

 of frost with the humidity at 100 per cent. Ordinarily, however, it may 

 be assumed that when the evening relative humidity is from 40 to 50 

 per cent., the ensuing minimum temperature on a characteristic radiation 

 night will be very close to the evening dewpoint; when the evening relative 

 humidity is below 40 per cent the minimum will average 5° above the 

 evening dewpoint; with evening relative humidities above 50 per cent 

 the minimum temperatures will be below the evening dewpoint. 



Little reliance can be placed on the afternoon maximum alone as an 

 indicator unless it is very high indeed. No maximum below 75 or 

 76°F. should be regarded as a guarantee against frost the following morning. 



Weather Bureau Methods. — At present no one method of predicting 

 minimum temperatures is in use by Weather Bureau officials throughout 

 the country. Local conditions apparently make a certain method fit 

 closely at one point while at another point it gives less satisfactory results. 

 It seems probable that observations extending over at least 2 years for 

 each section should be accumulated and the data studied to determine 

 which method will give the closest approximation in future predictions. 



Smith^*- has devised several methods and applied them to data from different 

 points. The simplest, perhaps, is the so-called median temperature method. 

 This is based on the assumption that, in weather characteristic of most spring 

 frosts, the "radiation nights," clear and rather still, the temperature falls practi- 

 cally at a uniform rate from a maximum in the afternoon to a minimum in the 

 morning and that the times of maximum and minimum temperatures will be 

 the same for all such days. The average time of the median, half way between 

 the times of the maximum and of the minimum, is ascertained from previous 



