COLD AIR DRAINAGE 115 



tude between localities of the same topographic situation. Com- 

 paring the 6000 feet station with its corresponding canon tempera- 

 ture we have a difference of 3000 feet, but this is due to the in- 

 creasing divergence of the gradients with descending altitude, 

 which — as already stated — is probably not normal. , 



The influence of cold air drainage might be expected to affect 

 both the upward limitation of lowland species and the downward 

 occurrence of montane species. As a matter of fact the down- 

 ward limitation of the forest and chaparral vegetation of the desert 

 mountain ranges is due to the operation of the factors of soil and 

 atmospheric aridity, and not to the chimenal factors. The 

 limitation of the upward distribution of desert species appears to 

 be attributable to chimenal factors, as the writer has shown for 

 Carnegiea gigantea. 3 The writer has observed that a number of 

 the most conspicuous desert species range to much higher alti- 

 tudes on ridges and the higher slopes of canons than they do in 

 the bottoms and lower slopes of canons. Samples secured by the 

 writer indicate that there is no essential difference between the 

 soil moisture of ridges and the bottoms of canons during the driest 

 portions of the year. Neither is there any evidence that desert 

 species would fail to survive in the canon bottoms if they were 

 somewhat higher in soil moisture content. An explanation of 

 the absence of the desert species from canon bottoms and their 

 occurrence at higher elevations on ridges must be sought in some 

 operation of the chimenal factors rather than in the factors of 

 soil and atmospheric moisture. 



An analysis of the operation of the chimenal factors will be 

 sure to discover that cold air drainage plays an important role 

 in determining not only the lowness of the minimum, but also 

 the still more important features of the duration of low tempera- 

 ture conditions. 



3 Shreve, Forrest, The influence of low temperatures on the distribution of the 

 giant cactus. The Plant World, 14: 136-146. 1911. 



