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600 WEATHER MODIFICATION The term weather modification encompasses all changes in atmospheric phenomena resulting from human activities. Inadvertent weather modification includes all changes in weather or climate resulting from activities carried out with no intention of modifying weather conditions. Deliberate weather modification includes all ef- forts to alter artificially the natural phenomena of the atmosphere. Many attempts at deliberate weather modification have involved some form of cloud seeding to increase rainfall or otherwise modify precipitation at the ground. The most commonly used cloud seeding agent is silver io- dide (AgI), but dry ice, various salts, and several organic compounds have also been introduced into clouds to influence their behavior. I. Early History Atmospheric processes involve such large en- ergy transfers that one could never hope to mod- ify the weather by a brute force approach. In- stead, experimenters have attempted to identify instabilities in the atmosphere so that small ex- penditures of energy can influence natural pro- cesses. As clouds are characterized by colloidal and sometimes phase instability, it is no acci- dent that nearly all weather modification experi- ments to date have involved some form of cloud seeding. [See CLOUD PHYSICS.] Clouds of water droplets form whenever air is cooled below its dew point. In the free atmo- sphere, cooling is most frequently produced by adiabatic decompression of rising air parcels. Cloud droplets form by heterogeneous nuclea- tion on relatively large, hygroscopic aerosol par- ticles called cloud condensation nuclei (CCN). Diameters of CCN are commonly in the range of 0.1 to I ILm. They are composed predominantly of ammonium sulfate [(NH4hS04], while sodium chloride (NaCl) from the oceans is only a minor constituent. Cloud condensation nuclei are more numerous over land than over the oceans, and this fact influences the characteristics of mari- time and continental clouds. Newly formed clouds over the oceans typically have droplet concentrations of 50 to 100/cm3 and median droplet diameters of about 20 p.m, while conti- nental clouds may have concentrations exceed- ing 500/cm3 and median droplet diameters of about 10 ILm. Even in maritime clouds, droplet diameters rarely exceed 30 ILm, while under ordinary con- ditions a raindrop must have a diameter exceed- ing 0.2 mm in order to fall to the ground without evaporating. Calculations show that a cloud droplet cannot grow into a raindrop by conden- sation alone in any reasonable period of time. In the early twentieth century, atmospheric scien- tists speculated on the possibility that collisions among cloud droplets lead to their coalescence and hence to formation of raindrops. It was rec- ognized that the initial collisions among cloud droplets would be rare events, but that the pro- cess would proceed quite rapidly by gravita- tional capture once a few drops exceeded about 200 p.m in diameter. Various processes, includ- ing turbulence and electrical charging of cloud droplets, were invoked to account for the initial cloud droplet collisions. A German physicist, Alfred Wegener, took note of the fact that many clouds at tempera- tures below ODC consist mostly of supercooled liquid droplets rather than ice particles. He showed that the few ice particles that do form in such clouds would grow into snowflakes in a few minutes by deposition of water vapor. This growth is accounted for by the fact that the equi- librium vapor pressure over supercooled water is greater than that over ice at the same tempera- ture. At an international scientific conference in Lisbon in 1933, a Swedish meteorologist, Tor Bergeron, presented evidence that much of the world's snow and rain is formed by this process, which still is referred to in many places as the Bergeron process. The scarcity of ice particles in many supercooled clouds is attributed to the fact that natural ice nuclei are both scarce and relatively inefficient. In general, the number of natural particles capable of nucleating ice in- creases by a factor of 10 for each 3.5 to 40C drop in temperature, and the concentration of parti- cles active at - 200C is typically one per liter. In 1938 a German meteorologist, Walter Fin- deisen, pointed out that snow might be produced in supercooled clouds by the addition of artificial ice nuclei. Even before the appearance of Fin- deisen's papers, atmospheric scientists were se- riously considering the possibility that artificial intervention in cloud processes could modify precipitation and other weather phenomena. Cloud seeding flights to test various seeding agents were carried out in the United States (Maryland) in 1924, in Holland in 1930, and in the Soviet Union in the 1930s. None of these flights led to any scientific breakthroughs. Also in the 1930s, Henry Houghton attempted to modify fog in Massachusetts by dispersing hy- groscopic sprays, especially calcium chloride (CaClz). '0