Mat 11, 1906.] 



SCIENCE. 



, 725 



cloudiness are consistent with those of tem- 

 perature, and are hence of some signifi- 

 cance. The total precipitation of the 

 period comprised 0.11 inch on the morn- 

 ing of August 9 and 0.01 inch on the morn- 

 ing of August 27, as measured at the camp 

 within a few yards of the instrument 

 shelter. On both occasions the amount 

 varied greatly in both vertical and hori- 

 zontal directions, the zones of precipitation 

 being narrow and the gTeater part of the 

 initial volume being evaporated in the 

 lower atmospheric strata — indeed, but a 

 small part of the rain precipitated from 

 the dark bases of the cumulus clouds ever 

 reached the ground. In the heavier pre- 

 cipitation of August 9 the rain diminished 

 to a trace within 300 yards down the arroyo 

 and some 60 feet below the level of the 

 , station, while on the gorge walls 300 feet 

 south of and 200 feet above the station the 

 fall was probably between 0.20 and 0.30 

 inch.° 



The value of the cloud observations was 

 impaired by the smallness of the visible 

 sky, only about 50 per cent, of the total 

 being in sight at the station. From E. to 

 N. 30° E. the horizon was broken only by 

 Sierra de la Cabeza Prieta some ten miles 

 away, and the still remoter Mohawk Range ; 

 thence by north to west the skyline aver- 

 aged some 20° above the horizontal, while 

 the mean altitiide of the southerly sky line 

 was over 30°. Accordingly, stratus and 

 other clouds toward the western and south- 

 ern horizons were seldom seen. Some 90 

 per cent, of the clouds observed gathered or 

 drifted in northerly directions over Cabeza 

 Prieta and other ranges east of the station ; 



° This rainfall, especially on the rocks above the 

 level of the station, served to partly refill the 

 tinajas and so prolong the maintenance of the 

 station; the evening before its occurrence the 

 total quantity of water remaining was estimated 

 at five gallons, or four days' supply, with an extra 

 gallon for the walk to Wellton. 



and but few cirrus and still fewer cumulus 

 clouds formed or floated over Tinajas Altas 

 and the neighboring portions of Sierra Gila. 

 In general the cloudiness diminished some- 

 what from the middle of May to near the 

 end of June and then increased during 

 July and August (as indicated in Table 

 II.) ; in general, too, the cloudiness was less 

 in the evening during the earlier part of 

 the period, with a tendency toward becom- 

 ing greater in the evening in the later part. 

 Concurrently with the cloud and rain ob- 

 servations a record was kept of thunder, 

 lightning and haze, which were lacking in 

 May and June, fairly frequent in later 

 July, and quite frequent in August — in- 

 deed during the later half of August more 

 or less distant thunder was heard almost 

 every afternoon and sheet or flash light- 

 ning was seen during most evenings. By 

 far the greater portion of the located 

 thunder and over 75 per cent, of the visible 

 lightning occurred in the east, i. e., over 

 Cabeza Prieta and other ranges; and at 

 least a dozen— perhaps a score— of rainfalls 

 were seen in the same quarter. Occasional 

 views were had of the entire horizon from 

 the crests and peaks of Sierra Gila; and 

 while clouds were sometimes seen over the 

 valley of Rio Colorado and the Cocopa 

 Mountains beyond, these only confirmed 

 the observations made at the station which 

 served to locate a zone of precipitation 

 passing in a northerly direction over Cabeza 

 Prieta and neighboring ranges, i. e., from 

 some ten to fifty or one hundred miles 

 further eastward than Tinajas Altas and 

 Sierra Gila. There were no observations 

 on dew, for the sufficient reason that 

 throughout the entire period there was: 

 none; the greatest relative humidity at the 

 hours of observation was but 75 per cent. 

 and the maximum probably fell below 90 

 per cent, even in the feeble rains — during' 

 which it was evident that evaporation from 



