FORT LAUDERDALE, Fla. — On radar, it looks like rain.
But it's really clouds of aluminum-coated glass fibers called chaff. And this week, U.S. Navy jets have been dropping the stuff as part of military training exercises near Key West.
The problem for the National Weather Service in Miami: It can't always tell what's chaff and what's rain.
"If the winds aloft are strong enough, it can get as far as Palm Beach County," meteorologist Barry Baxter said. "When we do our precipitation totals, it does count as rain sometimes."
Jim Lushine, a retired weather service meteorologist, said it also can fool television weathercasters, whose forecasts you may see, and anyone else who monitors weather, including those who rely on phone apps. "The chaff does not behave like a ticker tape parade but gradually falls to the surface while dispersing as it spreads out," he said.
Trice Denny, spokeswoman for the Naval Air Station Key West, said the chaff is being used as part of a training exercise to confuse enemy radars in aircraft.
"We have a number of fighter jet squadrons in town for training," she said. "January and February are typically very busy for us as the weather in other parts of the country can prohibit fighter jet training."
Studies have shown the chaff, which is primarily dropped over water, does not hurt the environment or marine animals, said Ted Brown, Navy installations and environmental public affairs officer for the U.S. Fleet Forces Command.
"The materials in chaff are generally nontoxic in the marine environment," he said. "Fish that ingested high concentrations of chaff under experimental laboratory conditions showed no negative impacts."
Forecasters usually can tell the difference between the chaff and rain, thanks to Dual-Polarization Radar, which provides a vertical and horizontal view of showers. While chaff appears "thin and flaky," rain is more circular and concentrated on a radar screen, said meteorologist Robert Molleda.
But sometimes it takes up to 45 minutes before forecasters know for sure what they're dealing with, and the Navy rarely if ever gives the weather service advance notice when it performs its chaff exercises.
"Of course, when chaff is dropped or spreads over areas where it is actually raining, it can cause problems in interpreting the data, which we deal with from time to time," Molleda said.
Of some consolation, he said: "The chaff drops seem to be more common in the dry season than in the wet season."