24 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



The number of hours with temperatures 32° or below. 

 The number of hours with temperatures 33° to 41°. 

 The number of hours with temperatures 42° or above. 



Frosts. 



Since the actual freezing of foliage may have some very definite 

 effects on plant functions apart from the temperature effect, the 

 occurrence of frost on the ground in the morning should be noted 

 and recorded at the time of regular observations. This notation is 

 especially important in cases where air temperatures near the 

 ground are not being recorded. If the latter are available, records 

 of occurrences of 30° F. or below may be taken in lieu of frost ob- 

 servations. This record should also be tabulated on the "Air 

 Temperature Record." It may be of value in determining at least 

 a normal growing season for some types of vegetation. 



Mean Temperatures. 



It is generally accepted by climatologists that the mean tempera- 

 ture for the day is sufficiently well represented by the sum of the 

 maximum and minimum divided by two. This is probably less 

 satisfactory in the forest, however, than in most open situations 

 where insolation and radiation are not interfered with. Where a 

 thermograph is in use, a nearer approach to the true mean may be 

 obtained from the sum of the hourly temperatures divided by 24. 

 Means for the day should be entered to the nearest tenth degree 

 Fahrenheit. Some of the problems connected with computations 

 of means are described by Hartzell (22). 



The means for the decades and the whole month, in all tempera- 

 ture columns, should be obtained and entered to the nearest tenth 

 degree. The following computations are suggested for valuable com- 

 parisons of growing conditions. They should be made and entered 

 at the foot of Form 5. All should be taken from the maximum 

 and minimum temperature records, since it is likely that some of the 

 stations to be compared will have no hourly records. 



Mean temperature for days with snow on ground. 



Mean temperature for days with no snow on ground. 



Xumber freezing days without thawing (maximum below 32°). 



Number freezing days with thawing (mean 32° or below, but maximum 

 above 32° F.). 



Xumber cold days (mean 32.1° to 41.0°). 



Number cool days (mean 41.1° to 50.0°). 



Xumber moderate days (mean 50.1° to 60°). 



Number warm days (mean 60.1° to 72.0°). 



Number hot days (mean 72.1° or above). 



