Note 79 — IDEAS FOR A PHYSIOGNOMY OF PLANTS.

p. 217—“Precipitation from the clouds.”

The vertical ascent of currents of air is one of the principal causes of the most important meteorological phenomena. Where a desert or a sandy surface devoid of vegetation is surrounded by a high mountain-chain, the sea-wind may be observed driving a dense cloud over the desert, without any precipitation of vapour taking place before it reaches the crest of the mountains. This phenomenon was formerly very unsatisfactorily referred to an attraction supposed to be exercised by the mountain-chain on the clouds. The true cause appears to lie in the ascent from the sandy plain of a column of warm air, which prevents the condensation of the vesicles of vapour. The more barren the surface, and the greater the degree of heat acquired by the sand, the higher will be the ascent of the clouds, and the less readily will the vapour be precipitated. Over the declivities of mountains these causes cease. The play of the vertical column of air is there weaker; the clouds sink, and their disintegration is effected by a cooler stratum of air. Thus deficiency of rain and absence of vegetation in the desert stand in a reciprocal action to one another. It does not rain because the barren and bare surface of sand becomes more strongly heated and radiates more heat; and the desert is not converted into a steppe or grassy plain because without water no organic development is possible.