MESA AND FOOTHILL VEGETATION 



2 9 



mountain valleys usually have a higher temperature on clear nights 

 than the plain below. 1 This is well shown in the present study where 

 at times a difference of ten degrees occurred in the minimum of the two 

 stations (Table VI). During the month of May these higher minima 

 of the mesa brought up the mean two degrees above that of the campus. 

 It. may be well to point out that the campus station is not in a gulch or 

 near the creek where night 

 temperatures are expected 

 to be cool, but on a sloping 

 hillside about a quarter of a 

 mile to the south of the 

 creek and two hundred feet 

 higher. Late frosts of the 

 spring which injure vegeta- 

 tion of the plain do not reach 

 the mesa. In the present 

 year the last frost was six- 

 teen days later on the plain 

 than on the mesa (Table 

 VIII). In Table IX a com- 

 parison between University 

 campus and Horse Mesa is 

 made with regard to daily 

 means for May, 1908. Fig- 

 ures 3 and 4 illustrate graph- 

 ically the 'differences in 

 maxima and minima at the two stations for certain selected weeks. 2 

 Severity of the Plains Climate. — Since the University campus is 

 close to the mesas it does not show as great a difference in climate as 

 would be found on the plains to the east. Many cultivated shrubs which 

 are hardy at Boulder will not live even five miles out on the plains. 

 Hence the figures in the tables do not, by any means, represent extreme 

 conditions of mesas and plains. The figures tell too little instead of 



1 Hann, Handbook of Climatology (Ward's translation), p. 252, 1903. 



1 See in this connection a note by the present writer in Science, N. S., Vol. XXVIII, p. 695, 1908. 



Fig. 3. — Maximum and Minimum Tempera- 

 tures of Campus and Mesa for the Week 

 March 8 to March 14, 1908. — The campus maxi- 

 mum is about two to three degrees higher each day 

 and the campus minimum lower than the corre- 

 sponding readings at the mesa. The campus, rep- 

 resenting plains conditions, has a more severe 

 climate than the mesa. 



