Interesting Engineering: How artificial rain works

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Rapid Creek normally flows peacefully through Rapid City, fueled mostly by the runoff of rain and snow from the surrounding hill. But the creek became a killer on the night of June 9, 1972. The forecast called for “variable cloudiness with a chance of scattered showers and thunderstorms,” and the Institute of Atmospheric Sciences was conducting routine seeding experiments on some small clouds west of Rapid City. Previously, they had mostly used silver iodide as the freezing nuclei in the clouds, but this day they sprinkled three to four hundred pounds of finely ground table salt from the plane. The final seeding flight concluded about five o’clock.

Two hours later, before he left for home, IAS meteorologist Alex Koscielski noticed a powerful storm gathering in the Black Hills. Since the weather office at the airport had no radar, Koscielski called to warn them to be ready for the possibility of flooding. Watches were in effect, and the rain was already coming down in a 50 mile wide area between Belle Fourche and Rapid City. In the next six hours, a rainstorm that meteorologists described as “a monster that only hits once every one hundred years” dropped 10-15 inches of rain on the Black Hills in six hours. The dam at Canyon Lake gave way. An estimated 375,000 gallons of water poured into the city. The deluge turned Rapid Creek into a devastating rage. A newspaperman said it sounded like a freight train. Jerry Mashek said in the Rapid City Journal: “We could hear people trapped on houses on the other side of the creek calling plaintively, rather than desperately for help. It was pitch black, rain was falling in sheets and all we could do was listen to the please. How many people were swept to their deaths in that area alone, I’ll never know.”

When it was over, the flash flood had killed 236 people and injured 2,900 others, with 6,570 families sustaining losses. Only 40-50 households carried flood insurance, and the city was left with damage in excess of 150 million dollars. It was the worst natural disaster in the history of South Dakota, and the greatest loss of life from a single flood event in America for the past fifty years. Within a week, questions were being asked about the cloud seeding experiments that had been going on at the School of Mines. Could they have caused the storm?

The IAS suspended all seeding operations to consult with public officials about the future direction of the program. Following an investigation, both the Institute and the U.S. Bureau of Reclamation ruled that there was no link between the experiments and the flood. Experiments that had been conducted that day took place away from Rapid City, and were completed before the storm hit. Koscielski recalled, “Nobody believes we could have caused that storm, no scientific person anyway. Anyone who knows anything about clouds knows that flood couldn’t have been caused by salt.”

Three years later, relatives of five of the victims filed a class action lawsuit in U.S. District Court, charging that the School of Mines had “negligently, carelessly, and recklessly performed experiments on clouds under adverse weather conditions.” The lawsuit sought millions of dollars from the U.S. Interior Department, which had contracted with the School of Mines for the experiments. The lawsuit was supposed to be heard by Judge Fred Nichol, who set a hearing date. But Nichol said, “Nothing ever did happen. The lawyer never took the case to trial. He just dropped it.” But the Rapid City flood would later become emotional ammunition for those who opposed weather modification in the 1970s.

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