THE VARIABLE DESERT 45
A ekiin of ice may form over one's aluminum drinking cup at breakfast before be breaks camp to collect the earliest flowering winter annuals. Topographic irregularities greatly complicate temperature relation- ships. SoQthem species may have their northernmost limits of distri- bution on the southwestern slopes of the rocky hills. Here the tem-
��Fro. 2. NartbeBstemmoit limit of the Orsan Caetiu, or PltaharB, in a pro- tected Taller of tba CababI Hlllf about leTenty-BTe mile* aoathweat at Taeton. opening towards tbe Mexican desert. Here It la ktowIds among the cDlmniiai cltDt Met] and a thick atand of balf ahrnba which ace chacacterlitlc of the cocky ilopea.
perature is far higher than over the crests where cold winds sweep against their northern slopes. It is in such locaUtieB that the splendid organ cactus, or pitahaya, flourishes, apparently beyond its reproductiTe limits, io the Cababi hills.
It is in these deserts that the temperature environments, which one might predict, from commonly accepted rules, are apt to be modifled by the phenomenon which MacDougal has designated as Cold Air Drainage. During the night the air from higher levels, becoming cooled, flows down the rocky slopes through the canyone, where it may form true aereal rivers, and into the valleys where it lowers the night temperatures of the plant organisms.
The naturalist, trained in a region where there is not a great diurnal beating of the earth's surface and a rapid nocturnal radiation of heat from a rocky dry soil, relatively unprotected by vegetation, is apt to think of this factor as one that might be demonstrated to exist only by long series of exact instrumental observations. On the contrary, the ' phenomenon is readily appreciable. A beautiful demonstration is to he seen from Tnmamoc Hill. In the early morning, the broad valley of the Santa Cruz seems filled with fog. This is really the mesquite smoke of Tucson carried down by the cold air drainage from the higher-