Why Texas Weather Radar Shows a Thunderstorm That Is Actually Bats

On a clear night in Central Texas, a weather radar screen can fill with a huge blob of the kind that usually means a storm is on the way. It is not rain. It is millions of bats, streaming out of a cave and fanning across the sky.

A blob bigger than a city

The radar station near New Braunfels, Texas, spends its time scanning the horizon for approaching thunderheads. On a summer evening it can pick up something else: a mass of flying animals larger than the city of San Antonio. A guide at Bracken Cave held up a phone showing the live radar feed, and there it was, spread across the screen. Weather apps usually change their default settings so that scenes like this do not appear, the guide said. If they did not, people checking the radar on a clear night would reach for their umbrellas, convinced a downpour was about to hit.

Three and a half hours of wings

Bracken Cave sits in the hills of Central Texas, and for at least 8,500 years its summer population has swelled in the same way. Up to 10 million free-tailed bats fly 700 miles north each spring from smaller roosts in and around central Mexico. After the mothers give birth, the numbers double again, to nearly 20 million — the largest gathering of nonhuman mammals on Earth. By midsummer the nightly departure, known simply as "the emergence," is so crowded that it takes more than three and a half hours for every bat to get out through the cave's bottleneck entrance. The result is a plume of small flapping wings that looks like a serpent reaching into the evening sky. The bats climb as high as 10,000 feet, travel between 60 and 100 miles in a night, and eat more than 150 tons of moths, beetles and other insects between them before returning home just before daybreak.

Counting bats on a screen meant for rain

Lee Mackenzie, a co-founder of the Austin Bat Refuge with Dianne Odegard, has watched weather radar every single day for more than a decade to follow these departures. He signed up for a training course at the National Severe Storms Laboratory in Norman, Oklahoma, a course designed for Air Force pilots and other military professionals who read weather radar in real time. Then he went further. Using raw data from weather stations in San Angelo, Granger, New Braunfels and Brackettville — the four corners of what he calls the "Significant Bat Area of Central Texas" — the laboratory built him a set of mathematical instructions that turns the blobs on the screen into an estimate of how many bats are in the air. He called it "crazy complicated." He gathers ninety different points from the radar data. Dianne Odegard said he probably has more data about bat emergences in Central Texas than anybody on Earth.