The Botany of Iceland - 01.12.1930, Side 175

STUDIES ON THE VEGETATION OF ICELAND
165
APPENDIX.
Temperature Conditions in the Upper Soil Strata.
Apart from volcanic regions, where the upper soil strata receive
heat from the interior of the earth, temperature conditions at the
surface are practically determined by insolation. According to the
extent of the cloud-covering, a greater or Iess amount of heat will
reach the surface of the earth where part of it will be used for
heating the air, another part for evaporation of the water in the
s°il, and a third part, finally, will heat the upper soil strata.
Investigations on the temperature conditions in the upper strata
°t the soil have been made at a series of stations in tlie most
different climates. An accessible presentation of the questions re-
lating to this subject will be found in Ramann, Bodenkunde 1911
and Hann, Lehrbuch der Meteorologie 1926, to which the reader is
referred. A series of these investigations are, however, of such great
phytogeographical interest in their bearing on the investigations
descrihed in this treatise that a brief abstract of the main results
'will be given in the following.
The investigations referred to originate partly, and especiallj’’,
írom Finland, and partly from Ilussia, and were made by Th. Homén
(1894, 1896, 1897) J. Keranen (1920), and H. Wild (1897). Tlie
investigations comprise the daily and annual variations in tempe-
rature in snow and sandy soil, the temperature of the surface with
and without snow-covering, and the daily variations in temperature
111 different kinds of soil, different in regard to structure, water-
content, and plant-covering.
As an example of the daily variation in temperature
111 snow and sandy soil may be mentioned J. Keránen’s investi-
gations from Sodankylá of whicli an abstract is given in table 321—2.
I he temperature was measured every second hour throughout the
24 hours in the surface of the soil or the snow at different depths,
111 the case of the snow at depths of 4, 14, 24, and 44 cm. and in
Ihe case of the sandy soil at 10, 25, 40, 80, and 120 cm. Tlie
temperature of the air is given for each investigation.
The temperature of the surface of the snow or the sandy soil
ls determined by the proportion of insolation and radiation. Ra-
diation is greatest in the night, lience the temperature decreases so
Ihat the lowest degrees of temperature occur just before sunrise; in