62 THE DATE PALM. 



feet, and at Tucson, at the University weather station, at an altitude 

 of 2,230 feet, the temperature fell to 11 F. in 1891, while at Dragoon 

 Summit, at about 4,611 feet altitude, some 60 miles to the east of Tucson 

 and 80 miles southwest of San Carlos, the temperature is not recorded 

 even as low as 15 F. in 1891. a In January, 1891, the temperature did 

 not fall below 32 F. at Dragoon Summit, while at Wilcox, only 20 miles 

 northeast, and nearly 500 feet lower, the temperature fell to 9 F. A 

 still more striking example is shown by a comparison of the temperatures 

 at Parker, on the Colorado River, at an altitude of about 500 feet, and at 

 Supai, nearly 7,000 feet above sea level, some 120 miles to the north- 

 east. In the winter of 1899 and 1900 the temperature did not fall 

 below 26 F. at Supai, while the imperfect record at Parker shows a 

 minimum of 23 F., that is to say, that although Supai is nearly 6,500 

 feet higher than Parker and is about 65 miles farther north, the min- 

 imum temperature was actually higher at Supai in winter. Numerous 

 similar instances could be cited in California, and the "thermal belt" 

 along the foothills of the Sierra Nevada Mountains, adjoining the 

 interior valley region, offers some of the most striking examples that 

 are known of inversion of temperature. 



All of these anomalies are the result of a drainage of cold air to 

 lower levels. During the night, if radiation is unhindered by clouds, 

 as is usually the case in arid regions, the air next the ground is cooled 

 rapidly and flows from the higher levels into the valleys below, much 

 as water would. As the cold air flows into the plains it 'doubtless 

 tends to flow under and to lift up the warm air; at any rate, all eleva- 

 ted points where there is a good drainage of air show relatively high 

 temperatures during the night, while points located in the valley floor 

 frequently show very low temperatures, constituting an exception to 

 the general rule that the lower the altitude the higher is the tempera- 

 ture. It will frequently be possible to grow date palms along the 

 foothills where it would be impossible for them to succeed in the 

 plains a few hundred feet below. 



However, high summer temperatures are essential to the proper 

 fruiting of the date palm, as will be shown in the next paragraph, and 

 the upper limit in altitude of its culture is more likely to be set by 

 the insufficient heat of summer than by the severit}" of cold in winter. 

 At points situated at high altitudes, whence there is a good drainage 

 of air, the fluctuations of temperature are less than in the plains 

 below, and consequently the winters are warmer and the summers are 

 cooler. In order to grow date palms at high altitudes, it will usually 

 be necessary to search for canyons or ravines with a southern expo- 

 sure, where the air is heated by reflection from mountain cliffs as well 

 as by direct insolation. 



All the-data as to temperature at: the various points named are from the reports 

 of the Weather Bureau and of the State weather services of California and Arizona. 



