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process:' "cold-cloud process:' or the "Bergeron Process." (Tor Bergeron w~s a Swedish'meteorol- ogist who contributed greatly to our knowledge of precipitation formation mechanisms in clouds containing ice particles.) . Cloud droplets.beil1g relatively clean. do not freeze immediately when a cloud top rises above the freezing level. In fact. pure water can be cooled to a bout -40 0 C (-40 0 F) before it freezes. A cloud consisting of liquid water droplets colder than 0 0 C is called "supercooled". Freezing at temperatures warmer than -40 0 C is attributed to very small foreign particles called ice nuclei. Natural ice nuclei include certain types of soil. dust. and decayed vegetation. . . An ice crystal in the presence of supercooled droplets will grow rapidly at the expense of the droplets. Water will evaporate from the drop- lets. diffuse as vapor to the ice crystal. and deposit as additional ice on its surface. When the ice crystal gets large enough by the diffusion process. it will start to fall through the cloud and can start to grow by accretion; that is. by collision with the droplets. The droplets freeze instantly onto the crystal when they touch. forming piles of ice beads called rime. This process of riming some- times builds up frozen droplet deposits so thick 8 .J6 fI) ::I E ... 14 !i: --. S2 ~2 0' NO SEEOING bl HYGROSCOPIC SEEDING J E ... 14 I- 5 ~2 01 ICE PHASE SEEOING (STATIC EFFECTI dl ICE PHASE SEEDING (OYNAMIC EFFECTI Figure 1.1 -Basic cloud seeding concepts applied to typical cumulus clouds of the northern High Plains. 20 -10 ~ I Q: o ~ I- -lOP I II: ::I o 1&1 I- that the interior ice crystal cannot be seen. The resulting lump of snow is called an ice pellet or . graupeL If ice crystals bump into each other and stick. the result isan aggregate snowflake composed of a few to hundreds of ice crystals. Thus. the presence of ice in a supercooled cloud can speed the production of precipitation by the . d iffu~ion. accretion. a nd/or aggregation processes. . Most modern cloud seeding projects have been based on the assumption that some supercooled clouds do not contain enough ice crystals to make the precipitation process efficient. Many clouds are relatively free of ice until their tops reach the -20 0 C (":'4 0 F) level. Additional ice particles are induced at warmer temperatures by seeding with an efficient ice nucleant or by chilling the air tem- porarily below -40 0 C (-40 0 F)(fig. 1.1). The most common artificial ice nucleating agent is Agl (silver iodide); a commonly used chilling agent is solid CO2 (carbon dioxide). which is usually called dry ice. Sometimes natural clouds produce more ice crystals than the available ice nuclei could create directly. The additional crystals are attributed to "ice multiplication." Several different mecha- nisms have been proposed. One process of ice multiplication initiates extra ice crystals at temper- atures near -5 0 C (23 0 F) when graupel and large cloud droplets are simultaneously present. Clouds supporting an ice multiplication process fre- quently produce showers at warmer cloud-top temperatures than do clouds producing ice crys- tals only by direct action of ice nuclei. In choosing clouds for seeding experiments and in evaluating results. it is important to determine which of the natural precipitation processes are at work and the efficiency of that process. The need to do that implies a need to measure cloud- top temperatures. cloud droplet sizes. and so on. Acquiring such measurements was. in fact. one of the major HIPLEX activities. The possible effects of cloud seeding are not limited to changes in the efficiency of the precip- itation processes. Many scientists have written about the possibility of changes in cloud size or cloud lifetime by seeding (fig. 1.1). These postu- lated dynamic effects of seeding are even harder to document than the physical effects mentioned above. Nevertheless. their potential impact on rainfall from convective clouds is so great that they cannot be ignored in any consideration of the future of cloud seeding technology on the High Plains. 4