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612 WEATHER MODIFICATION F. RESULTS OF LARGE-SCALE EXPERIMENTS The operational projects discussed above have been concerned only with the increasing of precipitation from individual convective clouds or from a relatively small portion of a more ex- tensive cloud system. However, it is possible that some dynamic effects occurred on a scale as large as tens of kilometers. Numerical simula- tions have indicated such effects, even in strati- form clouds formed in weakly stable air masses. Many workers, notably Langmuir, have spec- ulated on the possibility of deliberately modify- ing larger scale weather systems so that effects could be produced over large areas at one time. Langmuir arranged for a periodic seeding exper- iment with seeding conducted 1 day per week in New Mexico beginning in 1949. Although a 7- day periodicity appeared subsequently in rain- fall patterns in the central and eastern United States, scientists generally did not attribute it to the periodic seeding. Project Scud was conducted in the early 1950s to influence cyclonic developments over the eastern United States. Newly developing frontal cyclones along the east coast were seeded with dry ice to see if the release of latent heat could influence their subsequent development. Statis- tical analyses have indicated that the project was not sensitive enough to detect the minor results that could have been expected. The pro- ject failed to detect either a change in the rate of storm intensification, as measured by a cy- clone's central pressure, or in the precipitation produced along the Atlantic seaboard. Although latent heat releases are thought to play some part in the intensification of winter storms, it appears that most of the energy driv- ing them is derived from the large-scale thermal gradients existing in the atmosphere. With re- lease of latent heat playing only a secondary role in cyclone intensification, and the changes pro- duced by artificial seeding perhaps an order of magnitude less than the latent heat released by natural glaciation processes, it is not surprising that no results were detected from Project Scud. Possibilities to be considered for large-scale weather modification, other than cloud seeding, include deliberate changes in sea surface tem- perature, to be induced by mixing surface and subsurface layers, and changes in the radiative properties of clouds or of the earth's surface (e.g., by spreading carbon black on snow) to in- fluence the tracks of cyclonic storms. Work re- lated to the EI Nino phenomenon in the Pacific Ocean over recent years has established a clear link between major precipitation patterns and sea surface temperatures. However, no projects are under way at this time to exploit the estab- lished relationships between surface tempera- tures and large-scale weather patterns. G. POSSIBILITY OF ALLEVIATING DROUGHT If one rules out the manipulation of weather processes on the large scale, drought relief by weather modification is restricted to the possi- bility of stimulating local rains from such cloud systems as do appear during the drought period. Climatological studies to investigate the fre- quency of occurrence of potentially seedable clouds during drought situations have indicated that clouds do appear during droughts with suffi- cient frequency to justify further investigations. A point of concern is whether or not the clouds that appear during drought periods are seedable. If cloud systems that appear during droughts are marginal, that is, consisting of shallow strati- form decks or cumulus congestus clouds rather than mature storm systems, then it appears that some beneficial results might be obtained. It has even been suggested that the clouds that appear in drought areas are supercontinental clouds in the sense that their cloud droplet concentrations are unusually high because of the large numbers of CCN available to them. If that were the case, some artificial intervention might be required to initiate precipitation processes. However, this consideration relates principally to rain forma- tion by coalescence and should not be so impor- tant for cases where clouds rise above the OOC level and the Bergeron process becomes opera- tive. ~ IV. Suppression of Weather Hazards A. DISSIPATION OF SUPERCOOLED FOG The dissipation of supercooled fog at airports has been one of the most successful applications of weather modification technology. Super- cooled fogs are often capped by an inversion and can persist for days in the absence of a strong wind to break the inversion. Supercooled fog can be cleared by releasing any chilling agent to bring the temperature briefly below -40oC. Liq- uid propane has been used at a number of air- ports, for example, Anchorage, Alaska, for this purpose, but the most common method is the