120 THE ROYAL SOCIETY OF CANADA 
Snowfall affects vegetation in at least two ways. In regions 
with extreme temperatures a thick covering of snow prevents frost 
from penetrating into the soil as far as it might otherwise do. In 
forests situated on mountain slopes the snow takes a considerable 
time to melt and thus furnishes a more permanent supply of moisture 
than if it fell in the form of rain with a much quicker rate of run-off. 
Some snowfall totals are given in Table VIII. 
Hail, if measured by the damage it sometimes causes in field 
crops is of importance; but as it produces no permanent effect or 
special botanical features in the native vegetation of a district it is not 
considered at further length here. 
EVAPORATION. 
Rainfall figures alone are of comparatively little value unless 
we know at the same time the amount of evaporation taking place. 
As the evaporation from the surface of the soil will depend to a great 
extent on the nature of the soil, its texture, colour, etc. and would 
at the same time be difficult to measure, it is much more satisfactory 
to determine the evaporation from a freely exposed water surface. 
The rate of evaporation will vary concurrently with a number of other 
factors such as temperature, sunshine, wind, and relative humidity 
of the atmosphere. It may be safely assumed that a high rate of 
evaporation from a water surface will indicate a high rate of trans- 
piration from most plants unless they have some special adaptation 
to resist drought. It is consequently a factor of great importance 
in plant life. Livingston and Shreve emphasize ‘‘the importance of 
evaporation as a compound climatic factor which approaches more 
nearly to furnishing a criterion of vegetational possibilities than any 
other which is readily measured.” Prolonged evaporation will result 
in reducing the plant’s activities to a dormant state, or may even 
result in its death. 
Broounoff!f in connection with the study of evaporation in Russia 
uses the term ‘‘dry ten-day periods’’ by which he understands periods 
of ten days during which the total rainfall does not exceed 5 mm. 
In the National Weather and Crop Bulletin of the United States!’ 
a drought is defined as a period of thirty consecutive days or more 
in which the precipitation does not amount to «25 in. in any 24 hours. 
16 Livingston, B. E. and Shreve, F. The Relation between climatic conditions 
and plant distribution in the United States. Johns Hopkins University, Cire. N.s. 
2: 19-20, 1912. 
16 Mon. Bul. Agr. Int. and Plant Dis. V: 325-9. Mar. 1914. 
17 No. 7 p. 6, 1915. 
