HOLES IN THE AIR HUMPHREYS. 261 



AERIAL CASCADES. 



The term ''aerial cascade" may, with some propriety, be applied 

 to the wiiid as, followuig somewhat closely the surface contour, it 

 sweeps down to the lee of a hill or mountaui. Ordmarily it does not 

 come very near to the ground, where indeeed there frequently is a 

 counter current, but remains at a considerable elevation. Other 

 things being equal, it is always most pronounced when the wind is at 

 right angles to the direction of the ridge and when the mountain is 

 rather high and steep. The swift downward sweep of the air when 

 the wind is strong may carry the aeroplane with it and lead observers, 

 if not the pilot, to fancy that another hole has been encountered, 

 where, of course, there is nothing of the kmd. Indeed, such cascades 

 should be entirely harmless so long as the aeronaut keeps his machine 

 well above the surface and therefore out of the treacherous eddies, 

 presently to be discussed. 



WIXD LAYERS. 



It is a common thing to see two or more layers of clouds moving in 

 different directions and at different velocities. Judgment of both 

 the actual and the relative velocities of the cloud layers may be badly 

 in error — the lower seems to be moving faster, and the higher slower, 

 than is actually the case. Accurate measurements, however, are 

 possible and have often been made. 



These differences in direction and velocity of the winds are not 

 confined to cloud layers, nor even to cloudy weather, as both pUot 

 and manned balloons have often shown. Occasionally balloons float 

 for long intervals with a wind in the basket, showing that the top 

 and the bottom of the balloon are in currents of different velocities. 

 Another evidence of wind layers moving with dift'erent velocities is 

 the waves or billows so often seen in a cloud layer. 



A beautiful example of the long, regular cloud waves produced by 

 winds that have the same direction but different velocities is shown 

 in figure 1, plate 2, whUe figure 2, plate 2, shows an equally good 

 example of irregular or choppy waves produced hj currents that are 

 more or less crossed. Both kinds of waves may, and in fact often do, 

 exist in close proximity to each other. Thus, for instance, the accom- 

 panying figures on plate 2 are, indeed, from adjacent portions of but a 

 single negative, plate 1, taken by Prof. A. J. Henry, of the United 

 States Weather Bureau, and kindly lent for these illustrations. 



It was explained by Ilelmholtz as far back as 1889 that layers of 

 air differing in density arc of frequent occurrence, and that they glide, 

 sharply divided and with but little intermingling, the one over another, 

 in much the same manner that air flows over water, and watli the 

 same general wave-producing effect. These air waves are ''seen" 

 85.360°— SM 1012 18 



