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616 WEATHER MODIFICATION fect hurricane intensities and paths by cloud seeding. After a series of devastating hurricanes in the 1950s, the U.S. Government undertook an in- vestigation of Langmuir's optimistic predic- tions. Following some preliminary seeding trials in 1961, which gave promising indications, the U.S. Navy and the Department of Commerce jointly established Project Stormfury in 1962. The objective of Stormfury was to investigate methods to reduce the intensity of hurricanes by cloud seeding. Various suggestions were offered on ways to do this. All shared one common thread, namely, that the desired changes would be brought about by seeding to produce dynamic effects in convective clouds within the hurricane itself. The final Stormfury hypothesis, devel- oped around 1968, held that seeding of clouds in the hurricane rain bands spiraling inward toward the eyewall cloud could lead to their intensifica- tion and growth upward toward the tropopause. The increased inflow to the rain band clouds would lead to a collapse of the eyewall clouds and the reformation of the hurricane eye with a larger radius. Because it was observed that hur- ricane winds are strongest when the eye is small, it was reasoned that moving the region of most rapidly rotating air outward from the center would diminish the peak winds. Equipment deployed to test the Stormfury hy- pothesis included a fleet 00 to 15 aircraft, many of them equipped with Doppler radars for mea- suring the hurricane winds and other advanced instrumentation, and new AgI pyrotechnics. It was assumed that it would be necessary to freeze cloud water rapidly, and thus huge pyro- technic devices were manufactured containing as much as 30 kg of AgI each. As is now well known, such devices are grossly inefficient in terms of ice nuclei produced per gram of AgI consumed. By the time Stormfury ended, atten- tion had turned to the use of small pyrotechnics 250 mm long and containing about 17 g each of AgI. Project Stormfury was based in Florida and the West Indies and dealt with hurricanes over the Atlantic Ocean. Restrictions were built into the project to avoid seeding a hurricane that might reach land soon after seeding. Two seed- ing experiments were conducted on Hurricane Debbie in August 1969, and each was followed by a decrease in wind speed. However, the num- ber of hurricanes occurring in the Atlantic be- yond reach of land but within aircraft range proved disappointingly small. Some years went by with no seeding experiments at all. In the 1970s the U.S. Government explored with the governments of Mexico, Australia, and other countries bordering the Pacific Ocean the possi- bility of moving Stormfury to the western or southwestern Pacific. These negotiations did not lead to any final agreements. Meanwhile, experimental evidence was accu- mulating that the Stormfury hypothesis was seri- ously flawed. Flights through the convective clouds constituting hurricane rainbands showed much lower concentrations of supercooled wa- ter than had been assumed, which indicated that dynamic effects would be less than anticipated. It was also found that some of the eyewall and rain band clouds contained high ice crystal con- centrations in the - 5 to -100C level, about 50 to 100/liter. Because these concentrations were equivalent to those expected from seeding, it be- came difficult to understand how any pro- nounced dynamic effects could be produced. A second line of investigation weakening the Stormfury hypothesis was the finding that hurri- cane eyes can reform naturally with attendant temporary decreases in wind speed. It became obvious that the encouraging results observed following seeding in 1961 and 1969 could have been the result of such a natural reformation of the hurricane eye. In view of the evidence that the microphysical conditions in hurricanes were often unsuitable for seeding for dynamic effects and that the ef- fects expected from seeding would be hard to distinguish from natural hurricane variations, Project Stormfury was officially abandoned in 1983. At this time there are no active hurricane modification projects anywhere in the world. V. Inadvertent Weather Modification A. EFFECTS OF URBAN AREAS It has been recognized for over a century that the climates of large cities differ from those of their surrounding countrysides. Apart from the reduced visibility due to urban pollution, there is a heat island associated with each large city in which temperatures, and especially minimum temperatures, tend to be higher than over sur- rounding regions. The effects of a large city on climate are not limited to the heat island. A comprehensive in- vestigation of such effects was undertaken in the