Lightning Research Is Charged With Finding A Rain Gauge In Space

December 09, 1996

If you are deep in the Amazon jungle, far north in the Arctic wastes or afloat in the mid Pacific Ocean, how can you know what the weather might be? Will it rain, will it snow, or will it be dry? In the absence of weather stations in remote areas of the globe, monitoring rainfall would seem to be impossible. But the solution may be lighting up the sky.

Lightning research was once the stepchild of atmospheric science because of the belief that it had no connection with climate study. Now, thanks to new research at the University of Washington in Seattle, and to recent data from NASA's space-based lightning detector, scientists believe that lightning frequency might be a reliable surrogate for tracking precipitation in those regions where direct, ground measurements are not possible."Lightning research could become very important in climate studies," says Marcia Baker, professor of geophysics at the UW.

The key to understanding lightning's connection with weather monitoring seems to be, curiously enough, ice. A great puzzle in cloud studies was once: where does the electric charge that creates lightning come from? Today it is widely believed that the charging occurs when fast-moving ice particles collide in clouds.

Small ice particles, produced in the updraft of moisture from the ground, crash into soft hail as it falls downward, and in the process transfer a negative charge of electricity. It is, says Baker's fellow researcher, UW physics professor emeritus Gregory Dash, like "traveling at 25 miles an hour into a thick cloud of sleet and hail." The trillions upon trillions of ice-on-ice collisions, he says, rapidly build up a charge until the electric field gets high enough and the air breaks down into lightning.

Much of that soft hail carrying the negative charge ultimately reaches the ground as precipitation. Thus, says Baker, "if we could relate lightning frequency to charging frequency, and then calculate how long it's going to take those ice particles to fall to the ground, we would have some sort of predictor of rainfall." In other words, the more lightning there is, the more ice particles there are that will ultimately reach the ground as rain, sleet or snow.

The association between lightning and rainfall has already received some confirmation from the Optical Transient Detector, a sensor that was launched aboard a National Aeronautics and Space Administration satellite in April last year to detect and locate lightning flashes. Early results from the sensor suggest a strong correlation between lightning flashes and heavy rainfall.

To make use of the lighting flash data, though, will require a greater understanding of the physical mechanism of lightning charges. Why, asks Baker, do the ice particles produce the electric charge in the first place? "You can take two rocks or two pieces of glass or anything else and collide them and charging won't happen on a reproducible basis," she says.

It is known, says Dash, that the charge transfer between two ice particles mainly occurs at temperatures between minus 5 C and minus 20 C (23 F and minus 4 F) and at an altitude of about six kilometers (3.72 miles) in temperate regions of the globe. As the ice breaks apart after the collision, the small particles tend to carry a positive charge, and the hail a negative charge. What isn't known is why. Baker and Dash theorize that in the collision, a thin layer of liquid carrying the electric charge is transferred from the small ice particle to the hail. To test the charge transfer theory, Dash's doctoral student Brian Mason has for the past 3 1/2 years been creating artificial ice collisions in the laboratory. Inside a copper cylinder, cooled to minus 20 C, Mason has created the kinds of conditions believed to occur in clouds, including temperature, pressure and humidity. He doesn't have hailstones. Instead, he has an assembly consisting of a quartz crystal (the same as in a watch) and gold electrodes, or terminals. This is placed inside the cylinder, and an identical assembly is placed upside down facing it, so that the gold terminals are a hundredth of an inch apart.

Ice is grown on both sets of terminals, and then a brief soundwave from a tiny loudspeaker causes one terminal to shudder and briefly tap the terminal on the assembly above it.

Mason is then able to measure both the mass transfer -- of ice or liquid --and the charge transfer between the two electrodes. He expects to have significant results in the next six months. Says Mason: "I am detecting charging. And I am finding appreciable amounts of liquid or ice moving from one terminal to the other."

He is, says Baker, the first researcher to look at single ice collisions in such carefully controlled surroundings. And from this may come one answer to the many puzzles about ice. "Look," says Baker, "it's 1996 and we still don't fully understand everything that happens when water freezes in your freezer ice cube tray." For additional information, contact:

Marcia Baker at (206)-685-3799 or at {marcia@geophys.washington.edu}

Gregory Dash at (206)-543-2785 or at {dash@phys.washington.edu}
-end-


University of Washington

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