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<br />A U G MEN TAT ION <br />T H R 0 UGH W EAT HER <br /> <br />POT E N T I A L <br />MOD I F I CAT ION <br /> <br />PRINCIPLES OF THE PRECIPITATION ~DDIFICATION PROCESS <br /> <br />Introduction <br /> <br />There are two general approaches to precipitation management today'. The <br />first modifies the precipitation processes directly so that their effi- <br />ciency is improved and more rain or snow reaches the groWld during the <br />life of the cloud. The second stimulates the growth of the cloud so that <br />it 'grows taller and lasts longer, with the result that more water is proc- <br />essed by the cloud and converted into precipitation. When desirable, <br />the two approaches can be used together. <br /> <br />Today's precipitation management technology is focused on two general <br />cloud forms - convective clouds (cumulus) and orographic clouds (clouds <br />that form when moist air is lifted over mountains). <br /> <br />Convective clouds provide much of the summer precipitation that falls in <br />the Western and Central United States and, for the area east of the Rocky <br />MOWltains, this summer rain is a large portion of the tota,l precipitation <br />for the year. The amount and timing of precipitation during the growing <br />season are critical in agricul tural areas where irrigation. is not <br />practiced. <br /> <br />That portion of the United States from the Rocky MOWltains west to the <br />Pacific Coast receives much of its annual precipitation during the winter. <br />Essentially all the rivers headwater in the mOWltains and derive their <br />water from snowmelt. If the clouds that form over the mountains can be <br />made to precipitate more of the water passing through them, the water sup- <br />plies of the West would be improved. Something like 20 seedable storms <br />can be expected during an average winter season in the southern regions <br />of the West, increasing to 60 or more in the Cascade Range of Oregon and <br />Washington. In general, the goal of orographic cloud seeding is to keep <br />the annual snowpack close to the historic long-term average in years of <br />below normal snowfall. This augmentation will supply the existing water <br />needs and have only minor effects on the environment. <br /> <br />Orographic Clouds <br /> <br />Orographic clouds can form whenever moist air is lifted by a mowlItain <br />barrier of any size. Furthermore, the orographic clouds can be e:i ther <br />layer or convective, depending on the state of the atmosphere. In winter, <br /> <br />I <br />