Weather: Fooling the Clouds

Fooling the Clouds

Now that we have a handle on the processes nature uses in delivering a rainy day, what about those artificial rainmaking efforts? Do they really work?

In days of old, prior to the twentieth century, rainmaking efforts involved the ringing of bells, the explosive firing of cannons, ritual drum beating, and dancing. Sometimes the rain came—just often enough for people to keep trying. Hail cannons were exploded in Europe, and even earlier, Native American tribes made rain dances famous, but these rainmaking efforts had already been going on for centuries by groups of people from all over the world. Armies have fallen and civilizations have risen based on the success or failure of these efforts.

In modern times, weather modification methods haven't been quite so explosive. Based on the concept that "ice is nice," in the early 1940s, Vincent Schaefer, an atmospheric physicist, tried to produce more ice crystals in the clouds. How did he do that?

Well, dry ice, which is frozen carbon dioxide, has a temperature of more than 100 degrees below zero. So if dry ice is put into a cloud, the cloud's temperature will be lowered enough to cause ice crystals to appear even if ice nuclei aren't present in abundant quantities. At first, big chunks of dry ice were tossed out of airplanes. Predictably, they just crashed to the ground—sometimes through roofs. Later, the ice was crushed into small pieces and sprayed into the cloud. It worked! (Sort of.) At first, the planes would fly figure-8 formations through the cloud, and the droplets would fall, leaving the shape of an eight.

On the very day after Schaefer's first dry-ice experiment in a cloud, Bernard Vonnegut, brother of famed writer Kurt Vonnegut, figured out that a certain chemical, silver iodide, has crystal faces similar to real ice. He proposed that if silver iodide were sprayed into a cloud, the cloud would think that ice is present, and the tiny water vapor molecules would quickly attach themselves to the crystals. He was right. Even if extremely low temperatures were not present, ice would quickly form on the silver iodide, and then the natural precipitation process could do its thing. Coalescence would become possible as the ice crystal and droplet size rapidly increased. The silver iodide served as a magnet for water vapor, just as if ice were present. The cloud was "fooled" into raining!

Silver iodide could be sprayed into a cloud from generators at ground level or injected into clouds from planes. It was easier to work with than dry ice and became widely used in experiments through the 1950s and 1960s. But there is one problem with silver iodide: If the crystal faces are exposed to sunlight, they lose their magical attracting properties.

Age of Experimentation

The concept of fooling a cloud into raining caught on very quickly, and numerous rainmaking experiments were conducted for more than 25 years. Although the theory was solid, the reality was a little different. Sometimes, rain did occur. Changes always seemed to happen, but the changes were temporary. The question always remained whether that rain would have occurred naturally. The most optimistic experiments showed a 10 to 30 percent increase in rainfall following cloud seeding. That could be significant, especially in areas short of water. But overall, the results have been mixed. While scientists can modify clouds, their ability to make it rain at will leaves a lot to be desired.

Cloud-seeding experiments have included more than rainmaking. Hail, which generates about one billion dollars' worth of damage each year in the United States, has been the object of considerable study. The idea has been to generate numerous smaller hailstones rather than the fewer large ones that cause the damage. In theory, more ice crystals would mean less water vapor for the bigger, chunky hailstones, but in practice, after seeding, clouds sometimes generated more and bigger hail.

Hurricane seeding experiments have also had mixed results. The idea is to generate rain outside the steamy core of the storm so that rain generation would release heat and cause the pressure distribution of the storm to become less organized. Without a central hot core, the storm should weaken. But again, the results haven't been clear. Sometimes, the storm diminished in intensity. Other times, it actually increased. After decades of experimentation, the hurricane modification work was discontinued. The project is discussed further in "Hurricanes: The Greatest Storms on Earth."

Raining on Our Parade

Although early rainmaking experiments offered some promise, they yielded confusing and disappointing results. We're a long way off in making it rain, or not rain, on somebody's parade. And in modification experiments, the socioeconomic questions always lead to plenty of debate. During a severe drought in New York during the 1960s, resort owners in the Catskill Mountains sued New York City for its rainmaking efforts. The city needed more water in the storage areas of upstate New York, but in that region resort owners wanted those sunny days! In Pennsylvania, weather modification experiments have been banned. More than half the states in the United States have laws controlling the weather controllers.