STUDIES ON THE VEGETATION OF ICELAND 167 
the day, on the other hand, insolation is greatest, and the tempe- 
rature increases until, simultaneously with or some time after the 
maximum of insolation, it attains its maximum value. In the course 
of the afternoon, evening, and night, insolation decreases and with 
it the temperature until it again attains its minimum at sunrise the 
next morning. Deviations from these typical relations occur with 
irregular conditions of cloud-covering. 
From the heated surface of the earth a current of heat now 
passes partly upward to the air, partly downward to the deeper-lying 
strata, and this current of heat is not arrested until the temperature 
at the surface of the earth has again dropped below that of the 
surroundings, at which juncture the current of heat begins to change 
its direction. The surface of the earth now receives heat from the 
lower strata and, though to a less extent, from the lower layers of 
the air. The result will then be that shown in table 321—2. The 
daily variation in temperature is greatest at the surface, thence it 
decreases upwards as well as downwards until it becomes practi- 
cally zero, which value is attained at different depths, varying ac- 
cording to the heat-conducting power of the different kinds of soil. 
Thus, it appears from table 32 that the daily variation in tempe- 
rature penetrates far deeper into sandy soil than into snow. In 
snow the daily variation will as a rule be imperceptible at a depth 
of 10—20 cm., whereas in sandy soil it is not imperceptible until at 
a depth of c. 30—50 cm. This agrees well with the much smaller 
heat-conducting power of the snow. Other kinds of soil show still 
greater deviations, thus table 35 shows that granite rock has a con- 
siderably greater heat-conducting power, boggy soil on the other 
hand, considerably less heat-conducting power than loose sandy soil. 
Another fact will appear from table 32, viz. the displacement 
of the moment of incidence of the maximum and the minimum at 
the various depths. While the temperature at the surface of the 
snow attains its maximum at 14 o’cl., this will not occur until 
16—18 o'clock at 14 cm.s depth, and not until 24 o’clock at 24 cm.s 
depth. The temperature minimum varies in the same way. 
The annual variation in temperature is as the daily 
variation. In the summer an unbroken current of heat will pass 
from the surface of the soil into the deeper-lying strata, in the 
winter the current will pass in the opposite direction. The annual 
amplitude will likewise decrease strongly with the distance from the 
surface, just as a displacement of the moment of incidence of the 
