COLD AIR DRAINAGE 111 



temperature gradient of the Santa Catalina mountains, in south- 

 ern Arizona. In this work it has been necessary to know the 

 extent to which cold air drainage modifies and locally reverses the 

 normal fall of temperature encountered with increase of altitude. 

 The results secured afford fresh evidence of the magnitude of 

 this chimenal factor, which is capable of causing a variation in the 

 altitudinal limits of species and in the chronology of their seasonal 

 activities, in relation to the topographic site which they occupy. 

 It is the hope of the writer that similar observations in other 

 regions will be made, and that other workers will bring forth 

 any evidence they may have as to the influence of cold air drain- 

 age conditions on the distribution of plants. 



The temperature data used consist of a series of fortnightly 

 readings of the minimum at 4000, 6000, and 7000 feet (1220, 1830, 

 and 2135 m.) in the Santa Catalina mountains, taken from April 

 to October. The writer has also used a series of daily readings 

 of the minimum at 8000 feet (2438 m.) for June, July and August, 

 for which he is indebted to the hearty cooperation of Prof. J. G. 

 Brown of the University of Arizona. As a basis of comparison 

 for these mountain data, use has been made of the continuous 

 thermograph record of the Desert Laboratory, the daily readings 

 of minimum temperature taken at the Acclimatisation Labora- 

 tory by Mr. J. K. Breitenbecher in connection with Dr. W. L. 

 Tower's work on the Leptinotarsid beetles, and kindly furnished 

 by them, and also the record of the Arizona Experiment Station, 

 furnished through the courtesy of Prof. Robert H. Forbes. 



The mountain stations at 4000, 6000, and 7000 feet occupied the 

 summits of ridges, while that at 8000 feet was in the bottom of a 

 deep canon. The Desert Laboratory occupies the narrow slope 

 of a hill, 335 feet (102 m.) above the Santa Cruz flood plain, and 

 the Acclimatisation Laboratory stands at the edge of this plain 

 about 1000 yards (915 m.) away. The location of the two labor- 

 atories is comparable, then, to ridge and valley in the mountains, 

 except that the mountain valleys are all much smaller than that of 

 the Santa Cruz river, which is about 90 miles (145 km.) long and 

 from 10 to 30 miles (16 to 49 km.) wide. It is to be expected that 

 this large valley would give greater depressions of temperature 



