ABSTRACT
Weather modification, or cloud seeding, is the treatment of individual clouds or storm systems with various inorganic and organic materials in the hope of achieving an increase in rainfall. Introduction of such material into a cloud that contains supercooled water, that is, liquid water colder than zero degrees of Celsius, has the aim of inducing freezing, with the consequent ice particles growing at the expense of liquid droplets and becoming heavy enough to fall as rain from clouds that otherwise would produce none. The data shown in Table 5.1 were collected in the summer of 1975 from an experiment to investigate the use of massive amounts of silver iodide (100 to 1000 grams per cloud) in cloud seeding to increase rainfall (Woodley et al., 1977). In the experiment, which was conducted in an area of Florida, 24 days were judged suitable for seeding on the basis that a measured suitability criterion, denoted S-Ne, was not less than 1.5. Here S is the ‘seedability’, the difference between the maximum height of a cloud if seeded and the same cloud if not seeded predicted by a suitable cloud model, and Ne is the number of hours between 1300 and 1600 G.M.T. with 10 centimetre echoes in the target; this quantity biases the decision for experimentation against naturally rainy days. Consequently, optimal days for seeding are those on which seedability is large and the natural rainfall early in the day is small. On suitable days, a decision was taken at random as to whether to seed or
not. For each day the following variables were measured:
seeding: a factor indicating whether seeding action occured (yes or no),
time: number of days after the first day of the experiment,
cloudcover: the percentage cloud cover in the experimental area, measured using radar,
prewetness: the total rainfall in the target area one hour before seeding (in cubic metres ×107),
echomotion: a factor showing whether the radar echo was moving or stationary,
rainfall: the amount of rain in cubic metres ×107, sne: suitability criterion, see above.
