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and the coalescence process to initiate rain well within the lifetime of the cloud. Conversely, continental regions are char- acterized by much smaller and more nu~ merous condensation nuclei. Medium- sized clouds formed in these areas normally dissipate before the coalescence mechanism has had a chance to initiate rain. Likewise, many regions have a dearth of ice nuclei which cuts down the efficiency of the cold rain process. Man can assist nature by furnishing ap- propriate nuclei through <<<seeding" the cloud at the proper time and place, and in the proper amounts, when he determines through scientific measurement that nature has provided too few nuclei to trigger the precipitation process. Seeding with very large condensation nuclei (hygroscopic particles) can be done to accelerate the warm rain process. And seeding with proper ice nuclei (such as silver iodide) to supply naturally deficient clouds with an optimum concentration of ice crystals will increase rainfall through the cold rain process. In many cold clouds a combination of hygroscopic and ice nuclei seeding can be done, since, as has been previously mentioned, both warm and cold rain mechanisms operate in these clouds. The preceding section has shown t lm- portant role played by the sizes ang.eon- centration of cloud elementsintheinitia- tion and efficiency of precipitation. Of even greater significance in the, case of supercooled cumulus clouds are the dy- namic effects due to ice-phase seeding. When natural environmental conditions are right, cumulus clouds can be stimu- lated to grow larger and last longer. The introduction of silver iodide into the super- cooled part of the cloud causes the cloud droplets to freeze into ice crystals. This conversion process, multiplied millions of times over within the cloud, releases a large amount of heat, called the heat of fusion. This phenomena makes the cloud more buoyant and causes it to grow larger, thereby to process more water longer and more efficiently than would have occurred naturally. I I 'I SEEDING WINTER STORM I SYSTEII1S COLD-CLOUD SEEDING OF winter oro- graphic storms is fairly well established and understood. Clouds form as moist air is lifted and cooled during its west- to-east course across the mountains. Left to na- ture's devices, many are highly inefficient precipitators, content to keep aloft more than 90 percent of their moisture burden. By treating certain of these cold clouds with silver iodide, their precipita.tion.~re*ffi.. ciency can be greatly improved. As, has been already stated, the microscopic crys- tals of silver iodide act as artIficial ice nuclei that form ice crystals whi<:h attract mO~~~r~ifFo:m. surrounding droplets and grow large , enough to fall to the ground as snow. . TREA TI/fIG CUMUL CLOUDS THE MODIFICATION OF cutm,llus clouds is considerably more lcomplex. These large, billowy giants form when warm, moist air fairly close to the earth's surface is lifted and cooled to the point where water vapor condense into cloud droplets. Lifting can be ..c;lu d by the heating of the earth's su~!eI the sun, .,};:~~~~:.e: . ,~ ," ;; .;' ,.:, .,.,'", ' ..',', ""'i::i:~'."l~'J.i i-~ ..~ . .' , ' \w'llI~ .. ':r ~ i..,. . 61...a... -ro'. :WI