THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 53 
(4) Relative Humidity. — The relative humidity was obtained from 
the records of the university observatory situated about one half 
mile from the campus. The values given represent the average from 
7 a.m. to 7 p.m. 
IV. Experimental Data 
A . Leaf Temperature under Winter Conditions 
The collection of data began early in January. Of the mass of 
data obtained only a part can be given. The results shown by the 
tables following in the text may be taken as representative. 
The readings, as a rule, were taken from one half to three minutes 
apart for a period of five to ten minutes. Then the air temperature, 
radiation, and the wind velocity were recorded for that period. To 
save space, the average of the readings for the period is given, as a 
rule. In a few cases all the readings are given to show the rapidity 
of temperature changes within the leaf. 
The readings of the black bulb in vacuum thermometer are the 
maxima for the period unless otherwise indicated. 
The wind velocity given in column 9 is the average for the period 
as recorded by the anemometer on the platform and has only general 
value. Since the tree, partly sheltered from direct winds by the 
neighboring buildings, was subject to sudden gusts and eddy currents, 
some of which were vertical and did not, therefore, affect the anemo- 
meter, the velocity given does not necessarily represent the true 
velocity at the moment the leaf temperatures were taken. 
Column 4 in the tables gives the differential temperature between 
the leaf and the shade temperature of the air. The values are obtained 
by multiplying the bridge readings by the factor (see figure 2) indicated 
by the shade temperature given in column 5. Column 8 gives the 
actual temperature of the leaf. The values are obtained by adding 
the differential temperature to the air temperature. Column 7 gives 
the difference between the shade temperature and that recorded by 
the black bulb in vacuum thermometer — a better standard for com- 
parison than the values in column 6. In general, the differential 
temperature between leaf and air increases as the radiation increases. 
When this is not the case, an explanation can usually be found by 
referring to the wind velocity recorded in column 9. 
The data presented here are given to show: (i) the effect of full 
