INVERSIONS OF TEMPERATURE. 



39 



vation of about 2300 meters. I found the nocturnal inversions 

 of temperature very marked and more or less constant a fact 

 which must exercise a very notable influence upon the zonal 

 boundaries which cross this region. As will be shown presently, 

 northern plants would go farther south on a plain subject to 

 inversions of temperature than one free from such influences. 



Quite early in the course of my investigations attention was 

 drawn to the fact that crops of oats, wheat, and vegetables in 

 the valleys around the San Francisco Mountains were frosted, 

 while those on uplands and ridges were unharmed. In order 



July 18-23. 



FIG. i. Temperature curves obtained at Flagstaff, Arizona, at elevations of about 2300 meters. 

 The dotted line is the therm ographic record for the hilltop, and the unbroken line is the 

 thermographic record for the valley, 100 meters lower. The valley is warmer than the 

 hilltop during a period of only about three or four hours during the middle of the day, but 

 its temperature is lower than that of the hill during almost the entire night of twelve hours. 

 The daily average temperature of the valley is thus much lower than that of the hill. 



to determine the temperature relations thus indicated, record- 

 ing thermometers were placed in the valley at Flagstaff and at 

 the Lowell Astronomical Observatory, 100 meters above it, and 

 continuous records of the temperatures were made through June 

 and a part of July. The record of a few days is shown in fig- 

 ure I. From the data thus obtained it is to be seen that the 

 valleys and canons are subject to much greater extremes of 

 temperature than the adjoining hills and mesas, and that the 

 average daily temperature of the valley is lower than that of 

 minor elevations near it. The temperatures of the hill and 



