Rainmaking with Soot?

Most clouds and fogs are made of water droplets that are too small to fall. Nature has various methods of making the droplets grow big enough to fall as rain, but they are not always in operation. Often great clouds heavy with water float across a thirsty land without dropping rain, or fog hangs for hours over an airport.

The well-known silver iodide and Dry Ice methods of cloud precipitation work only on clouds that are well below freezing. Warm clouds are common,too. and Dr. Florence W. van Straten of the Naval Weather Service reasoned that they should precipitate if some of their droplets could be made warmer than the others.

The problem was how to heat the droplets differentially. Dr. van Straten solved it with carbon black, which is a fluffy kind of soot whose intensely black particles, about 500.00 in diameter, accumulate radiant heat just like a blacktop road. When these particles are released in a cloud, she reasoned, the water droplets that capture one or more of them should grow warmer by absorbing sunlight, and should lose their moisture by evaporation to droplets that have stayed comparatively cool because they have captured no particles. Then the cool, fattened-up droplets should fall slowly through the cloud, growing gradually bigger by jostling small droplets and combining with them. Eventually they should grow big enough to fall from the cloud.

This system, worked out theoretically, worked like a charm in actual fact, the Navy announced last week. It was tested last July over the coast of Georgia. The usual tactic was to attach a package of 1-2 Ibs. of carbon black to a static line and toss it out of an airplane flying through the top of a cloud. When the slack snapped out of the line, the package broke open, releasing the carbon black. Seven clouds out of seven tested dissipated entirely in 2 1/2 to 20 minutes.

When carbon black is released in moist, cloudless air the effect is opposite but no less magical. Its black particles catch sunlight and heat the air between them. The heated air rises, expands and grows colder. Some of its moisture condenses, and a new, white cloud appears in the sky. This system will not form clouds in dry air, but when the air is moist enough, it works almost every time. The official Navy attitude is that the action of carbon black is "an interesting effect" that will have to be studied a great deal more before it can be rated as a promising rainmaking agent.

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