HUMPHREYS: HOLES IN THE AIR 243 



and frequent mixing of the lower atmosphere, from the surface 

 of the earth up to a level that averages 3 to 4 kilometers, there 

 must in general be considerably more dust up to this same level 

 than there is at greater elevations. 



The vertical temperature gradient thru the first 3 kilometers 

 or so, general^ is much less than the adiabatic, and hence ordi- 

 nary convection, cumulus clouds, and, of course, the correspond- 

 ing dust layer, usually are all restricted to comparatively low 

 levels. Strong cyclonic storms, however, produce convections 

 that overcome the temperature gradient of the lower atmosphere 

 and extend quite to the undersurface of the isothermal region, 

 beyond which level vertical convection obviously can not greatly 

 extend. Dust, therefore, in greater or less extent is distributed, on 

 such occasions, thruout the convective atmosphere, or up to 

 the under surface of the isothermal region, 11 kilometers or there- 

 abouts above sea level. 



This then is the third and last possible dust layer of the 

 atmosphere, each layer in turn being of increasing thickness and 

 decreasing density, and all three, but not more, are essential 

 to the physical interpretation of observations on the neutral 

 points of sky polarization. 



METEOROLOGY.— Holes in the air. W. J. Humphreys. To 

 appear in Popular Science Monthly. 



There are, of course, no holes in the ordinary sense of the term 

 in the atmosphere, no vacuous regions, but for all that the pic- 

 turesque phrase "Holes in the air," is likely to become a perma- 

 nent acquisition to the language of aeronautics since it is both 

 brief and elegantly expressive of the fact that occasionally there 

 are conditions in the atmosphere which, so far as flying is con- 

 cerned, are quite like unto holes — conditions that cause sudden 

 drops and disastrous falls. 



These may be classified as follows: / 



1. Aerial fountains. More or less vigorous uprushes of air 

 occur over dry heated ground, and especially above barren conical 

 hills, during warm summer afternoons. When the vertical velo- 

 city of this heated air is 10 feet per second or more, as it often 



