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convective activity tends to be imbedded in a mass of layer clouds, and its contribution to the total precipitation of the orographic system is often masked by the general precipitation that accompanies such systems. Because they form over mountain barriers, orographic layer clouds are essentially stationary. Convective bands, however, frequently move through the more extensive layer clouds. Orographic precipitation management technology can be separated into three components: recognition of seedable events, treatment of these events, and evaluation of the results of seeding. " Recognition. - The recognition of seedable clouds involves either -observing or forecasting the occurrence of particular character- istics. In general, seedable orographic clouds will be at least 1,000 feet thick, will last for at least 3 hours, and will have a cloud-top temperature warmer than minus 230 C but colder than about minus 80 C. One of the more significant findings of recent research is the temperature dependence of seeding effectiveness. At temperatures colder than minus 230 C, the naturally occurring ice nuclei are active in great enough numbers to effectively seed the cloud and start the precipitation process. At warmer temperatures, however, many orographic clouds are deficient in ice-forming nuclei. The minus 230 C temperature is not an absolute number, but instead varies with other factors, such as windspeed. At temperatures warmer than about minus 80 C, the commonly used seeding materials in the concentrations normally supplied by the generators do not provide enough ice nuclei to effectively seed the cloud, and the ice crystals that are formed may not have time to grow appreciably. The dimensions of an orographic cloud are also important. They, along with the windspeed, determine how long a particle can remain in the cloud, which in turn is a major factor in whether or not the particle can grow big enough to fallout of the cloud as precipi- tation. The crosswind extent of an orographic cloud is determined by the characteristics of the orographic barrier. ~ An extensive supply of moist air is needed for appreciable preCIpI- tation to fall from orographic clouds~ Storms of short duration do not produce much precipitation, and the additional precipita- tion provided by seeding these brief storms is usually not worth the effort to run the seeding operation. If the number of ice crystals already present in the clouds and the rate at which mois- ture is being carried to the cloud are known, it is possible to 9 2