24 BULLETIN 1059, TJ. 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 espe- 
cially 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 observations. 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 satis- 
factory in the forest, however, than in most open situations where 
insolation and radiation are not interfered w 7 ith. Where a thermo- 
graph 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 temperature 
columns, should be obtained and entered to the nearest tenth degree. 
The following computations are suggested for valuable comparisons 
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 clays with snow on ground. 
Mean temperature for days with no snow on ground. 
Number freezing days without thawing (maximum below 32°). 
Number freezing days with thawing (mean 32° or below, but maximum 
above 32° F.). 
Number cold days (mean 32.1° to 41.0°). 
Number cool clays (mean 41.1° to 50.0°). 
Number moderate days (mean 50.1° to 60°). 
Number warm days (mean 60.1° to 72.0°). 
Number hot days (mean 72.1° or above). 
