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iodide, which encourage the transforma- tion from vapor to snowflake. The temperature factors in the orographic windows vary from region to region. From Skywater research, it has been learned that in the Jemez Mountains of Northern New Mexico a cloud-top temperature rangeof-100Cto-17.90C(140Fto 00 F) is a window. In the San Juan Mountains of southwestern Colorado, the range is believed to be within _80 C to -230 C (180 F and _90 F). In the Bridger Moun- tains of Montana, however, scientists have found that temperatures of -100 C to -220 C (140 F to _80 F) at 10,000 feet, rather than at cloud top, indicate a window in which seeding increases precipitation. Other factors related to the optimum oro- graphic seeding windows involve the num- ber and size of natural ice nuclei present, wind direction and velocity, and the differ- ence between temperatures at varying levels. The precise role of some of these, in both summer and winter situations, is being defined. Continuing research is placing their effect on precipitation in perspective. Most can be identified and verified by data obtained through the use of instrumented aircraft in the cloud and through other experiments. An operational technology, however, must have the ability to recognize seeding op- portunities inexpensively and quickly from afar. Research in this direction has cen- teredon the use of radar and the computer. In Project Skywater, radar has become the cornerstone for the remote sensing of seeding opportunities. Weather radar has the unique ability to resolve the internal structure of clouds. Thus, the radar mete- orologist can scan areas' in excess of 30,000 square miles and identify those clouds which appear to be seedable. Radar data, such as cloud size, height, internal structure, and cell duration, are now used for determining the location of proposed seeding activities and the eleva- 13 tion at which the seeding material should be injected into the cloud. , , COMPUTERS IANl1 .:~ RAl1AiR j . , I ' . SKYWATER SCIENTISTS HAVE also teamed computers with several radar sys-' tems to quantitatively and objectively evaluate radar returns. Although this marriage places new demands for preci- sion and stability on the radar, the poten- tial rewards of more accurate precipitation measurement will be very helpful. The initial results of this marriage have been computer-analyses and enhancement of radar data and improved, nearly instanta- neous, visual representations for prompt decisions. Skywater also uses the computer to learn the causes and effects of these criteria for seeding within the overall dynamic struc- ture of clouds. This is being accomplished primarily through the use of computer- based cloud models which simulate the internal processes mathematically. The mathematical formulations are based on the laws of physics and can be handled accurately and rapidly by modern digital computers. The various cloud components and atmos- pheric conditions are inter-related. Even a slight change in just one of these many conditions may have extensive effects on the others. For instance, cloud models have 3hown that, under certain atmos- pheric conditions, clouds can be stimulated to grow by as much as 15,000 feet in depth more than they would have under natural situations. This may result simply by causing cloud droplets to freeze earlier than they would have under natural conditions. The cloud models also predict how and when precipitation can be formed earlier