The Botany of Iceland - 01.12.1930, Blaðsíða 179

STUDIES ON THE VEGETATION OF ICELAND
169
temperature of 5.3° and an annual amplitude of 19.4. Thus, wliile
a natural surfaee is on the whole similar to a bare sandj' 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.
Tliis 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 temperatui-e. 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, liowever, onlj^ 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 soii and that of the snow. Thus a sandy surface has a January-
February temperature of c. -h100, 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 tlie values are -r- 6° and +1° re-
spectively.
Veiy convincing and thorough investigations on temperature
conditions in snow have been made by J. Keránen (1920). The
investigations comprise two snow-periods 1915—16 and 1916—17,
the teinperature in the surface of the snow at various depths and
at the surface of tlie soil being measured 3 times daily, at 7 o’clock,
at 13, and at 21 o’clock. By means of these figures tlie average
monthly temperature for the respective depths has been calculated.
The mean values l'or the two periods have been given in table 34,
"where likewise the deptli of the snow-covering is given, besides the
temperature of the air, and Ihe temperature at various distanc.es 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 -í- 14.2° can only cause the temperature at tlie bottom,
i- e. at the surface of the soil to drop to -h2.7°.
An investigation of temperature conditions in the snow in the
various months will also show the varying isolation power of the