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of this amount. The number of years needed to get 100 cases will vary from place to place, with an average of 4 years. Increases from some individual orographic storms can be up to 200 percent, but only a few such storms occur. Recently another variation of evaluatipn procedures has become avail- able - the use of numerical model predictions as the control. The model predicts the amount of precipitation to be expected from the seeded clouds and the amount that would have been produced in the absence of seeding. The observed precipitation can be compared with the model predictions and the results of seeding can be determined. Because every seedable event is seeded, reliable results should be available in fewer years. The models, while still rather crude, may soon yield results at least comparable to the more standard proce- dures for some high mountain target areas. Convective Clouds Convective clouds are short lived when compared to orographic clouds, they are rarely stationary, and techniques for seeding them are some- what more complex than for orographic clouds. Furthermore, convective clouds can be seeded with hygroscopic m~terials or with ice-forming nuclei, depending on the existing atmospheric characteristics. Recognition. - The recognition of seedable convective clouds almost invariably requires forecasting the occurrence of clouds that do not yet exist. Suitable conditions for convective clouds i~~~ude ~ deep layer of warm, moist air from the ground up to at l~ast several thou- sand feet, and cool, relatively dry air above about 18,000 fe~t. In the absence of some sort of mechanical lifting, such as flow over a mountain or a front, there must be en04gh convergence or heating and turbulence at the s4rface to lift, and therefore cool, the moist air. To be seedable, a convective cloud should be at least 4,OQO feet thick, its top should be warmer than about minus 230 C and be com- posed primarily of water droplets rather than ice crystals, and it should be i~ the growing stage. If the cloud is much less than 4,000 feet thick, any precipitation that does form will be cp~osed of drops so small they will evaporate before reachi~g the grouhd~ Clouds with tops colder than about min~s 230 C usually have enough natural ice nuclei active to st~rt the precipitation process. Unless the clouds are in the growing stage, where water is being brought into them, seeding will have little or no positive effect on precipi- tation. Other factors are also important to the efficiency of the precipitation process. Cloud depth, updraft speed, and the lifetime ~ 6