STUDIES ON THE VEGETATION OF ICELAND 169 
temperature of 5.3° and an annual amplitude of 19.4. Thus, while 
a natural surface is on the whole similar to a bare sandy surface, 
the presence of a covering of vegetation or snow will cause a re- 
duction of the annual amplitude of c. 10° and an increase of the 
mean annual temperature of no less than 1.30. 
This applies to the surface of the soil, but the deeper-lying 
strata too are affected by a covering of snow or vegetation, and in 
the same direction. There will be a decrease in the annual amplitude 
and an increase in the mean annual temperature. A comparison 
between the two sides of table 33 will show the numerical values. 
A covering of vegetation will, in the summer, cause a lower 
average monthly temperature of the surface below it than the na- 
tural surface or a bare sandy surface would have done. This difference 
will, however, only amount to a couple of degrees. In the winter, 
however, a covering of snow will mean an immense increase of the 
surface temperature under the snow compared with that above the 
snow. According to Wild, a snow-covering of 30—45 cm.s’ depth 
will cause a temperature difference of 8—9° between the surface of 
the soil and that of the snow. Thus a sandy surface has a January- 
February temperature of c. + 10°, while the surface below a snow- 
covering of 30—40 cm. has only a January-February temperature 
of + 2°; at a depth of 40 cm the values are + 6° and + 1° re- 
spectively. 
Very convincing and thorough investigations on temperature 
conditions in snow have been made by J. Keränen (1920). The 
investigations comprise two snow-periods 1915—16 and 1916—17, 
the temperature in the surface of the snow at various depths and 
at the surface of the soil being measured 3 times daily, at 7 o'clock, 
at 13, and at 21 o’clock. By means of these figures the average 
monthly temperature for the respective depths has been calculated. 
The mean values for the two periods have been given in table 34, 
where likewise the depth of the snow-covering is given, besides the 
temperature of the air, and the temperature at various distances from 
the surface for the year and for each month. 
The snow-covering during the months November—April has an 
average depth of 47.8 cm and due to this an average surface tem- 
perature of + 14.2° can only cause the temperature at the bottom, 
i.e. at the surface of the soil to drop to -- 2.7°. 
An investigation of temperature conditions in the snow in the 
‚arious months will: also show the varying isolation power of the 
