304 THE FUTURE OF ARID LANDS 
much of the cloud shifts to false cirrus streamers of ice crystals, 
which often dominate the afternoon and evening sky. 
11. During periods of dust storm activity, clouds are commonly 
seen to shift completely to ice crystals when still quite small. At 
other times clouds in one region of the visible sky will show 
profound modification, while clouds in another region will develop 
in a normal manner. In some instances these effects are probably 
due to tongues of air containing natural ice nuclei. In some cases 
such effects may be attributed to the effect of silver iodide ground 
generators. 
Precipitation Pattern Due to Orographic Clouds in New Mexico 
Since the bases of most large cumulus clouds that form in New 
Mexico are at altitudes several thousand feet above the highest 
mountain summits, naturally occurring precipitation generally 
falls on the down wind side of the mountains. The cultivated areas 
and range lands which depend on this natural rainfall delineate 
this precipitation pattern in considerable detail. An excellent 
example of this effect is to be seen in the bean-growing area of 
New Mexico, situated east of the Sandia and Manzano Moun- 
tains, to the southeast of Albuquerque. In direct contrast, the 
orographic effects of mountains in subtropical regions and in 
places where the mountain summits rise higher than the cloud 
bases normally produce precipitation patterns on the upwind side 
of the mountain slopes. In such regions the area on the lee side 
of the mountains often shows regions of rainfall deficiency and 
actual ‘“‘rain shadows.’’ Good examples of this pattern are to be 
seen in the rain forest west of Hilo in the Hawaiian Islands and 
the rain forest on the western slopes of the Olympic Mountains 
in northwestern Washington. 
If methods are developed which effectively modify the “nat- 
ural” rainfall from orographic cumulus in places like New Mexico, 
attention should be directed to this phenomenon since the end 
result might cause trouble. If the precipitation pattern from 
seeded clouds tends to appear further upwind than the ‘‘natural” 
rain, the end result might be of little benefit to anyone. This 
danger may occur in any region of marginal rainfall. 
