FEB. 19, 1923 HUMPHREYS: THE MURMUR OF THE FOREST 63 
passing wind and driven, as flying scud, down the mountain to quick 
oblivion—rapid disappearance through evaporation, owing to -the 
increase of temperature that results from the gain of pressure always 
incident to loss of level. 
Isolated cloud billow.—As the scud fleeces grow in size and number, 
a detached cloud bank of the roll-cumulus type forms over the leeward 
valley parallel to the mountain crest. At first this cloud is narrow 
and often broken, but it soon grows wider, deeper, and darker. It is 
more or less agitated, but as a whole remains fixed in position. 
The inertia of the air, as it sweeps up and over the mountain, carries 
it to an elevation more or less above its level of equilibrium, from which 
elevation it drops back farther on, and again, like the swinging pendu- 
lum, passes beyond its point of rest. In this way a few rapidly damped 
billows are set up parallel to the mountain crest just crossed. That is, 
the air rises first above the mountain, then falls to a lower level, rises 
Fig. 6. Stationery clouds on mountain and over valley 
again (affording the soaring bird easy toboggan sailing), and so on in 
rapidly decreasing amplitude. Hence, as the humidity of the air 
increases, first a slight cloud forms a little above and to the leeward 
of the mountain ridge along the crest of the topping billow, while 
the crest of the next billow beyond, being at a lower level and therefore 
warmer, is entirely cloudless; then, with further increase of humidity, 
this initial cloud thickens until it rests on the mountain top and gives 
off continuously avalanches of seud—secud that evaporates in its first 
descent, condenses on rising to the next billow crest, and again evapo- 
rates as it passes on and down the farther side of this aerial wave, all 
as schematically illustrated in fig. 6. 
It is this convectionally-recondensed scud at the crest of the first 
air wave beyond the mountain ridge that constitutes the isolated roll 
of cumulus cloud along the leeward valley, a cloud that is continuously 
formed and as continuously evaporated, a permanent cloud of fleeting 
particles, and stationary because the crest of the billow is fixed in 
position. 
