RYDBERG: Puy APHICAL NOTES 683 
FORM OF PRECIPITATION 
Another important factor influencing the timber line is the 
form of precipitation. In high altitudes the air is too rare to hold 
much moisture and the rain falls at the least lowering of the tem- 
perature. The rain falls therefore either in the form of mists or in 
light showers, which only wet the surface of the ground. It may 
be sufficient to keep alive the low rosettes or cushions of the alpine 
vegetation, but it is not sufficient for the deep-rooted trees. 
Furthermore, if a little heavier rain should come, the water would 
rush down the steep slopes of the mountains, not having time to 
sink down into the ground. The tops of the mountains are there- 
fore arid, because the air is too rare to hold much moisture and 
quickly gives it up in light showers. Nowhere in the Rockies proper 
is the moisture very great. In the foot-hill regions and on the 
surrounding plains the temperature is too high in the summer to 
allow any precipitation. These zones are therefore also arid. 
It is at middle elevations that the precipitation is the greatest. 
The air here is dense enough to hold more moisture and the tem- 
perature low enough to allow precipitation. It is also at middle 
altitudes that we find the forest areas in the Rocky Mountain 
region. 
LARGE MOUNTAIN MASSES 
In the Swiss Alps, observations have been made that in regions 
of large mountain masses, as for instance in the Monte Rosa 
region and the Engadine and others, the timber line is higher 
than on isolated mountains. I have not seen any satisfactory 
explanation of this fact. It may be due partly to the fact that 
the central mountains of such massed groups are more or less pro- 
tected from the desiccating winds. It may be due also to the 
circumstances that in the winter more snow lodges between the 
mountains, the melting of it is more retarded, and the water is 
more arrested in its downward course by the trees and their roots. 
The air in the summer time would be therefore, from the evapo- 
ration, more loaded with moisture, which would naturally also 
benefit the mountain tops. Whatever the real cause may be, it 
seems as if the observations made in Switzerland hold good in the 
Rockies. From my own experience, I know that the timber line 
in the isolated Belt Mountains and Crazy Mountains in Montana 
