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 isin 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. 
