54 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 4 
weaker, with distance down the slope, owing to surface friction and to 
the rapidly increasing cross section of the inflow, as indicated in 
fig. 3. 
Exact mathematical and quantitative solutions of this and kindred 
problems, involving viscosity and turbulence, would, no doubt, be 
highly instructive, but such solutions have not been obtained. True, 
important progress has been made, but this progress, which is found 
chiefly in the turbulence papers of Rayleigh, Reynolds, Taylor, and 
Richardson, is not adapted to a brief and inclusive summation. 
Return current—When there is an appreciable wind from the 
mountain, there often is a lighter surface wind in the opposite direction 
up portions of the mountain itself, as indicated in fig. 4, and up the 
lee side of any paralleling ridge in the valley. This, too, has no 
necessary relation to the gathering of clouds and the onset of precipi- 
tation, since it applies equally to winds crossing the mountain in 
Fig. 4. Return current, and sound crossing mount 
either direction. It differs from the’opposing wind near the top of the 
mountain, just explained, in that it extends much farther down from 
the crest. ° 
In this case the valleys contain but little if any stagnant air; the 
wind flows across the mountains in undulations more or less parallel 
to their sides, and the change of temperature with change of elevation 
is everywhere, owing to turbulence, in keeping with the corresponding 
change of pressure. That is, the change of temperature is due entirely 
to change of pressure, and not at all to loss of heat to, or gain of heat 
from, anything extraneous to the rising or falling mass of air. 
In general the explanation of the return current is precisely the 
same as that given above of the opposing winds at the crest, except 
that as its cross section remains small (the parent wind being only 
a little above the surface) its velocity is correspondingly retained over 
a considerable distance down the mountain side. 
During clear days the return current is strengthened by thermal 
convection; during clear nights, on the other hand, it is weakened if 
