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 less 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 
soil, and a third part, finally, will heat the upper soil strata. 
Investigations on the temperature conditions in the upper strata 
of the soil have been made at a series of stations in the 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 
described in this treatise that a brief abstract of the main results 
will be given in the following. 
The investigations referred to originate partly, and especially, 
from Finland, and partly from Russia, and were made by Th. Homén 
(1894, 1896, 1897) J. Keränen (1920), and H. Wild (1897). The 
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 
in different kinds of soil, different in regard to structure, water- 
content, and plant-covering. 
As an example of the daily variation in temperature 
in snow and sandy soil may be mentioned J. Keranen’s investi- 
gations from Sodankylä of which an abstract is given in table 321—2. 
The temperature was measured every second hour throughout the 
24 hours in the surface of the soil or the snow at different depths, 
in the case of the snow at depths of 4, 14, 24, and 44 cm. and in 
the case of the sandy soil at 10, 25, 40, 80, and 120 cm. The 
temperature of the air is given for each investigation. 
The temperature of the surface of the snow or the sandy soil 
is determined by the proportion of insolation and radiation. Ra- 
diation is greatest in the night, hence the temperature decreases so 
that the lowest degrees of temperature occur just before sunrise; in 
