WEATHER AND CLIMATE IX THE TIAN SIIAX. 71 
Jul\- 8-9. Rain fell not infrequently between noon and night, for the most part from 
the mountain-made, overgrown cumulus or nimbus clouds, which drifted slowh- 
eastward, their cirro-stratus cover far outreachiiig the main cloud mass. Main- of the 
showers fell only on the mountains, leaving the open, inter-range depressions, such as 
the Alabuga and Narin valleys and the Issik Kul basin, dr)- and of subarid appear- 
ance. Thunder showers swept b\- while we were in the (western) Kugart \'alley, 
June 30, and while we were crossing the Kugart and Oi-Kain passes, July i and 2; 
hea\y rain and hail showers drifted over us at Son Kul, July 10. We wore long, 
black woolen waterproof cloaks (burkas) of the Caucasus, that protected us admira- 
bly while riding in the rain. The Chaar Tash range, ending eastsvard in the angle 
between the Alabuga and Narin rivers, fed a series of floating cumuli f July 6), which 
slowly dissolved as they drifted bejond the mountains. We saw a number of distant 
thunder storms o\-er the mountains by Issik Kul. The fair-weather days on this 
lake were characterized by clear sky over the water and by long rows of cumuli 
over the snowy Kungei and Terskei Alatau to the north and south. We were 
troubled with high wind only on July 17, when a dr}' gale from the west swept over 
the plain bj' Issik Kul ; and for a short time in the afternoon of August 2, when a 
furious dust squall from the west beset us as we rode into Semipalatinsk. 
The only climatic feature which our short excursion brought clearly forth 
is the contrast between the mountains and the deeper valleys as to rainfall and 
relative aridity. As already noted in the Kopet Dagli, a difference of elevation of 
a few thousand feet produced a marked difference in the appearance of the surface. 
\'egetation was scanty in June in the deeper interior \-alleys or basins of the Tian 
Shan at elevations of 7,000 feet or less ; it was abundant in the higher valleys 
above 8,000 feet. The cause of this contrast did not seem to reside mereh- 
in increase of rainfall with altitude, and in the protection of the inner valleys 
from the rain-bringing winds by the inclosing mountain barriers, but also in the 
direct excitement of rain-making processes on the mountain ranges and in the 
cessation or perhaps e\-en the reversal of these processes in the large, open valle\-s. 
The preceding paragraph tells of several examples in which the growth of thunder- 
shower clouds was intimately associated with moimtain ranges, thus suggesting 
their dependence on the ascending diurnal breezes on the mountain sides, as has 
often been noted elsewhere. In contrast with the mountain cloud masses was the 
prevailingly clear sky o\-er the open depressions, as noted in the Alabuga and Narin 
valle)-s and over Issik Kul ; and here a descending component of atmospheric 
movement should prevail to compensate for the ascending component where the 
cloud masses occur. Hence the open valle\s not only receive ver>- little summer 
rainfall, but they are swept over by air whose dryness has been increased b}- the 
descending component of its motion. Their descending component is not merelv 
that by which a wind should, after crossing a range, tuni do^^^lward into a basin. 
The descending component of this general origin must be largely increased by the 
local con\ectional circulation that is excited by the mountains. Thus the basins 
not onh- get little rainfall, but are parched by evaporation into the dr}-ing winds 
that settle upon them. The seasonal migration of the Kirghiz, with their herds 
