200 
FORREST SHREVE 
showing the lowest absolute minimum for the mountain to occur not 
at the highest station, but at the highest station below the forested 
elevations. The absolute minimum in the heavy timber at Marshall 
Gulch was 4° higher than it was 1,700 ft. below in an open stand of 
manzanitas and oaks, a phenomenon in which cold air drainage was not 
concerned, since the higher station was in the timber and the lower on 
a ridge. 
The strongly defined character of the cold air drainage of the Santa 
Cruz valley can be appreciated from the fact that the absolute mini- 
mum recorded in the valley was 1°, only three degrees higher than the 
absolute minimum 5,400 ft. above in the forested region of the Santa 
Catalina Mountains. 
The mean gradient of fall in temperature with increase of altitude, 
averaged for a large number of mountains in different latitudes and 
climates, indicates a fall of 3.46° per 1,000 ft. This gradient happens 
to be identical with that for Pikes Peak. The steepest gradient pre- 
viously recorded is 4.12° per 1,000 ft., for the western slope of the 
Sierra Nevada. The gradient for the Santa Catalina Mountains has 
been determined by comparing the daily mean temperatures of the 
Desert Laboratory with the daily means secured from observations 
at the Montane Plantation of the Laboratory and at Mt. Lemmon 
(alt. 9,150 ft.). This comparison evades the influence of cold air 
drainage by reason of the fact that both the laboratory and the forested 
altitudes are beyond its effects. The value of the gradient thus deter- 
mined is 5.2°, or more than one degree higher than the highest gradient 
previously reported, and differing from the maximum reported 
gradient more than the said maximum differs from the mean of all 
reported gradients. 
The gradient derived from the absolute winter minima of the 
Desert Laboratory and of the ridge stations at 4,000 and 6,000 ft. is 
6.6° per 1,000 ft. The gradient above the commencement of timber 
is 2.5° per 1,000 ft. The gradient of temperature fall in the free air 
has been determined at the Blue Hill Observatory, for low altitudes, 
as 2.16° per 1,000 ft. 
The streams of cold air which flow from the mountain canons are 
shallow, never exceeding 75 ft. in depth, and often being less than 50 
ft. Below elevations of 6,000 ft., by virtue of these streams, the 
minimum temperature conditions of cafions and other topographic 
depressions are equivalent to those of ridges and slopes which lie 
