82 VEGETATION OF A DESERT MOUNTAIN RANGE. 



the same latitude. The radiation of the desert valleys and the diurnal 

 convection currents of warm air are not without a strong ameliorating 

 influence on the climate of elevated but small masses of land. 



DEPARTURES FROM THE NORMAL TEMPERATURE GRADIENT DUE TO 



COLD-AIR DRAINAGE. 



The ideal conditions of uniform decrease of temperature with increase 

 of altitude are seldom actually encountered in nature, at least not in 

 mountains of small size and rugged topography. The principal factor 

 which brings about departures from the normal or ideal gradient of 

 fall is the operation of cold-air drainage, or inversion of temperatures. 

 This is a phenomenon which has long been known and has frequently 

 been discussed with respect to its influence on vegetation. In an 

 earlier paper * the writer has described some observations of tempera- 

 ture inversions at the Desert Laboratory and in the Santa Catalinas, 

 and has pointed out the causes involved in making cold-air drainage 

 much more pronounced in deserts than it is in humid and forested regions. 



The scanty vegetation of the desert subjects its soil, rocks, and sands 

 to full insolation and to a pronounced heating throughout the day. 

 The dark rocks of Tumamoc and other volcanic hills in its vicinity 

 become so hot during the long clear days of May and June that it is 

 impossible to hold one's hand on them without pain. During the day 

 there is a constant and active radiation of heat from the rocks and soil, 

 which warms the lowest layers of air and causes a convectional heating 

 of the lowest portion of the atmosphere. Immediately after sunset 

 the warmed surfaces become rapidly cooler and the rate of radiation 

 is quickly reduced. The air nearest the cooling rocks and soil becomes 

 cooler than the air above it, and consequently begins to fall by gravity 

 before there is opportunity for it to mix with the warmer air above. 

 This cooled air descends from hillsides and even from gentle slopes 

 and soon collects in valleys and depressions, where it results in a slowly 

 or rapidly moving mass of air which is appreciably cooler to the senses 

 than is the air of the slopes or hillsides. The inversion of temperature 

 thus caused usually reaches its maximum during the first half of the 

 night, although this is determined in great measure by the size of the 

 drainage area. 



It is only on clear and still nights that cold-air drainage operates 

 in the most pronounced manner. A high wind will disturb the flow 

 or completely eliminate it. Heavy cloudiness will cause the rate of 

 radiation to lag so that there is time for an admixture of cool and warm 

 air, thereby preventing the flow of cold air or greatly reducing it. 



On clear nights which follow heavy rains the inversion of temperature 

 will be reduced to a negligible amount, because of the increase of the 

 specific heat of the soil brought about by its becoming wet. 



* Shreve, Forrest. Cold Air Drainage. The Plant World, 15: 110-115, 1912. 



