The Botany of Iceland - 01.12.1930, Side 177

Sl’UDIES ON THK VKGETATION OF ICELAND
167
the day, on the other hand, insolation is greatest, and Ihe tenipe-
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
»t the surface of the earth has again dropped below that of tlie
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 resull will llien be that shown in table 621—2. The
daily variation in temperature is greatest at the surface, thence it
hecreases 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.
Tlius, 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
°f 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
grealer 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 l'rom table 32, viz. the displacement
°t 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., tliis will not occur until
16—lg 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
írom the surlace of the soil into the deeper-lying strata, in the
winter the current will pass in the opposite direction. The annual
araplitude will likewise decrease strongly with the distance from the
surface, just as a displacement of the moment of incidence of the