Atmospheric Transparency.—Gulf Stream Cloud Bank. 389 
which almost exceeds the display on the brightest and coldest 
nights of a northern winter. It seems singular to find a climate 
80 moistened by the Gulf Stream, still glorying in the starriest 
nights. Association had made a lavish display of the starry 
hosts seem the peculiar prerogative of clear, cold, winter nights, 
and yet here they came forth, amid moisture-laden tropic airs, 
with a magnificence and profusion I had never seen excelled. It 
needed no long acquaintance with the equable climate, the nearly 
unvarying temperature and the steady trade winds, to see that 
the reason of this phenomenon is to be found in the prevalent 
tranquillity of the atmosphere, where it is so little influenced by 
contrasts of land and sea. These small keys scarcely vary the 
ocean conditions. I have known the thermometer at Boston 
pass through a longer range in one day, than in the whole year 
at Key West. The winds are mostly gentle and steady in diree- 
tion. There are usually no conditions of great contrast and no 
regular admixtures between upper and lower strata. The 
Tequisites for developing visible vapor are rarely prevalent, and 
have only twice known positive fogs at Key West. However 
moist the air may be, if the atmosphere lacks the conditions of 
contrast and intermixture to make that moisture visible as vapor, 
the sky should seem habitually clear. Such is the obvious fact 
at Key West. With a climate never, even after the severest 
northers, below 45°, rarely down to 55°, and seldom rising to 90° 
in the shade, it is not to be expected that the admixture of con- 
‘asted currents should often cool to the dew point portions of 
this moist warm air. The equability of atmospheric conditions 
is thus the real reason of the rare beauty of the sky and the 
ich display of starry splendors, so attractive amid the soft and 
balmy airs of this locality, which lacks but one degree of being 
tropical. There is much in the quality of these nights to suggest 
Hat the astronomer would find his ise here, but the sum- 
Mer mosquitoes, rain and yellow fever are rebutting facts. Fi 
_ Winter observations, the conditions are truly admirable, 
8. Gulf Stream Cloud Bank.—Among the striking local phe- 
nomena of Key West, is the formation, shortly before and after 
_ Sunset, of a grand bank of clouds above the Gulf Stream, rising 
_ Some 200 to 500 feet in prevailing height. In running along 
the Gulf Stream or its margins, this bank is habitually seen 
: Tey sunset hours, and a profuse atmospheric moisture is 
felt while sailing in the evening over the warm-water belt. Key 
a West being about 12 miles north of the regular Gulf Stream 
_ Waters, this cloud bank rises gradually along the southern hori- 
_ 200, stretching from E. to W. in massive and irregular fleeces, 
_ dark below and silver gilt above, under the rays of the settin 
_ San. When the prevailing S.E. wind is brisk, this cloud 
_ AMt Jour. Scr—Skconp Surims, Vou. XXXV, No. 105.—Mar, 1863. | 
