ALPINE PLANTS IN THEIR NATIVE HAUNTS. 



69 



soil upon vegetation and flora locally, all calculations as to a plant's 

 requirements based upon the conditions of soil only are liable to be 

 overthrown by a change in the climatic factors of temperature and 

 atmospheric precipitation. 



To turn now to the conditions of the alpine climate. On ascending 

 a mountain, we have, with an increasing rarity of the atmosphere, a 

 fall in temperature, and up to a point an increase in atmospheric preci- 

 pitation. After that point, which, I believe, in the Alps is generally 

 estimated to be at an altitude of 7500 feet, atmospheric precipitation 

 becomes less as we ascend. " The fall in temperature as altitude in- 

 creases necessarily causes a reduction in the amount of aqueous vapour 

 in the atmosphere, and the intensity of atmospheric precipitation must, 

 therefore, at a certain altitude be so far diminished that even a greater 

 frequency of precipitation can no longer compensate for the reduction. 

 Thus increasing altitude is associated with an increase in rainfall, but 

 only up to a certain level, which varies according to the general climatic 

 conditions and local circumstances ; above the level at which the maxi- 

 mum rainfall occurs, atmospheric precipitation again rapidly diminishes. 

 Eainfall is generally associated in our minds with a moist atmosphere, 

 but this is not applicable in the case of high altitudes. As we ascend, 

 the diminution of aqueous vapour in the atmosphere proceeds at a much 

 greater pace than does the rarefaction of the atmosphere itself. Thus, 

 taking the aqueous vapour in the atmosphere and the density of the 

 atmosphere itself to be unity at sea level, it has been estimated by Hann 

 that at an altitude of 2000 metres, or, roughly, 6500 feet, the amount 

 of aqueous vapour is represented as .49, and the density of the atmo- 

 sphere as .78. That is to say that while the density of the atmosphere 

 has decreased by, roughly, 25 per cent., the water vapour in the 

 atmosphere has been practically halved, f ' ' Everything, ' ' says Hann, 

 "dries much more rapidly at great altitudes; animals that have been 

 shot, or killed by falling, become mummies without undergoing decay, 

 perspiration evaporates rapidly, the skin becomes hard and dry, and 

 one's thirst increases. The reduced atmospheric pressure renders pos- 

 sible a much more rapid dispersal of aqueous vapour, and consequently 

 accelerates evaporation. ' ' | 



With the increasing rarefaction of the air there is also an increase 

 of heat radiation. Objects exposed to the sun's rays become heated 

 more rapidly than in the lowlands, but they also cool with equal rapidity 

 when the sun's rays are withdrawn. Thus the more rarefied the 

 atmosphere the greater the heating by day, and also the cooling by 

 night. At the same time, the ground in the High Alps is relatively 

 much warmer than the air. Keeneb has ascertained by numerous ob- 

 servations at different heights in the central Tyrolese Alps that the mean 

 temperature of the soil exceeds that of the air by the following amounts : 



* Hann, Handbuch der KUmatologie, Bd. 1, p. 299. 



t This refers only to the absolute humidity of the atmosphere, tlie relative 

 humidity being a constantly varying factor. 

 t Hann, op. cit. Bd. 1, p. 283. 



