INVERSIONS OF TEMPERATURE. 41 



actually reach the valley or plain as a warm wind, if it has had 

 a descent of a few thousand feet. As a fall of sixty meters 

 gives rise to an increase of temperature of i F., the total 

 amount may be distinctly noticeable, although but few actual 

 observations of this phenomenon are recorded. If the cooling 

 slopes descend gradually to the plain, the heat lost by radiation 

 in the slow downward progress of the current may entirely 

 compensate for the adiabatic increase, and the breeze will 

 reach the lowlands with no marked change in temperature. 



A probable example of this adiabatic increase of temperature 

 of a descending current of air is to be seen in some of the ther- 

 mographic records made at Flagstaff, Arizona, by myself. This 

 place lies in the mouth of a great valley, enclosed on one side 

 by a mesa, on the other by a spur of the mountain, and on the 

 other the valley gradually ascends the long slopes of the San 

 Francisco Mountain, reaching the summit of Humphrey's Peak, 

 twelve kilometers distant, at a height of over 4000 meters. As 

 previously described, the cold air from the near-by mesa and 

 ridges pours into the valley, steadily reducing its temperature 

 through the night. The downward currents also form on the 

 long slopes of the mountain, but, descending very slowly, do 

 not show any great rise in temperature, and usually do not 

 occasion any marked disturbance in the cooling of the valley. 

 At certain times, however, the descending current gains such 

 strength and rapidity that its increase in temperature overbal- 

 ances radiation, and it reaches the valley in the latter part of 

 the night as a warm current which causes an upward bend of 

 the thermographic curve, which is steadily falling. The rise 

 due to this warm current may amount to 2 to 5 F. at some 

 time between 2 and 5 A.M. 



The heating of the layers of air in contact with the soil in 

 valleys and canons during the daytime generally results in 

 ascending currents, which, cooling at the normal adiabatic rate 

 as they ascend, blow over the adjacent mesas and hilltops as 

 cool winds. This reduces the maximum of the highlands, and 

 in the curves taken on the Observatory hill at Flagstaff it is 

 seen to be 2 to 6 lower than that of the valley below. It is 

 thus to be seen that the nocturnal inversions of temperature 



