20 BULLETIN NO. 161 



of the fruit buds and telegraph these readings at his expense to 

 Salt Lake City. About two hours later the Weather Bureau 

 will telegraph back, at the government's expense, the forecast 

 for the coming night and if the signs indicate a decided frost, he 

 will probably telegraph that evening 1 to the orchardist giving 

 him a more accurate forecast based on later data. (1) . 



A farmer should have several minimum thermometers on his 

 farm and determine the places where it is naturally coldest, 

 i. e., where the frost hits hardest; he should also compare the 

 readings taken in his own orchard with those taken on corres- 

 ponding nights in the key station in order to be able to determine 

 approximately how many degrees colder or warmer his orchard 

 is than the key station, so that when the key station receives 

 the forecast for that station he will know what to expect in his 

 own orchard. 



The difference between the maximum and the minimum tem- 

 perature readings for the twenty-four hours is called the daily 

 variation. The farmer should study his readings and determ- 

 ine the average daily variation for each week while the buds 

 are in bloom. If then the average daily variation for the week 

 be subtracted from the maximum temperature experienced in 

 the afternoon it will give approximately the minimum tempera- 

 lure that will be experienced for the night. Clouds or winds 

 usually decrease the daily variation. The minimum tempera- 

 ture for the twenty-four hours is a function of the dew point 

 and the relative humidity determined the evening before, pro- 

 vided the weather map indicates a frost for that night, the 

 constants for the imperical formula being 1 determined for each 

 locality. 



The following frost indications given by Humphreys are 

 helpful : 



"1. An evening orchard temperature of about 40o F. 



2. Clear skies, to permit rapid radiation ifrom the surface 

 covering. 



3. No wind, or very light, to avoid mixing the warm at- 

 mosphere above with the cold surface air. 



4. Wind movement, so far as there is any, from northwest- 

 erly direction. 



5. Dew point below 32 F. to avoid the formation of a fog 



(1). This is accomplished through a study of the weather map. Ex- 

 planations of how this is done are found in Milham's Meteorology, pp. 

 213-216. 



