This video that CNN aired on Sunday of 20 million Mexican free-tailed bats exiting the Bracken Bat Cave between Austin and San Antonio, Texas) has been making the rounds on the Internet.

Since bats can often be spied on weather radar, I thought I'd see what the local NEXRAD radar in San Antonio had to say. An in fact you've been able to see the bats leave, disperse, and return to the cave all week. Here's what it looked like Monday night (you can download* high-res radar loops here):

http://vortex.accuweather.com/video/video_inline_launchv4.asp?video=200909020806_VBLOG_FERRELL

This is not new news by the way, the bats in this cave has been there for more than 10,000 years. But this was arguably the first time that it had gotten wide media exposure and the video is pretty incredible (they didn't say how they counted the bats). I had done a previous blog entry about these bats on radar before, though I didn't realize the significance of the Bracken colony. There is evidence evidence that bats have gotten accidentally sucked into thunderstorm updrafts as well.

Here's the breakdown of what I believe was happening (I'm making some assumptions here on what is bats and what is not, and I'm not a radar expert so I could be wrong):

At 8:04 PM, around sunset, a heavy radar signature (equivalent to moderate rain) begins to grow from the cave. To the east is the hole in the data where the radar is located (radars can't see directly above themselves, it's called the "cone of silence"). Around that hole is normal "clutter" that appears on the radar every night, as the beam hits nearby trees and buildings.

Near 9:00, they have expanded over several miles and additional heavy radar returns (that may be additional bat caves) have shown up to the north. Some of these are expanding rings, which can be birds taking off from a roosting place, but probably not at sunset.

By 1:10 AM, the Bracken Cave bats (or possibly a combination of them and the others to the north) have expanded up the interstate to past Austin. Note that the overall light blue color has expanded across this entire area, but this is something called "Radar Bloom" or "SuperRefaction" that happens every Summer night. Some people say this is showing all birds or bats; others (quoted in the previous link) believe that they are only a small part of what the superrefraction of the radar beam is picking up.

By 4:17 AM they have shrunk back to the area around the cave and the size of the colony is steady, though it's possible some have flown out of radar range.^

Just before 6:00 the area of bats starts shrinking rapidly as they race back to the cave, and by 6:40 they are all but gone as the sun rises.

^This article from 1996 says that "Radar studies of the bats from Bracken Cave in Texas show that they fly as high as 10,000 feet (3,000 m), with the densest aggregations at altitudes of 600 to 3,200 feet (200 to 1,000 m) above the ground." Depending on the range and height, bats could fly out of the range of the "slice" of the atmosphere that the radar can see, though I am using the Composite Reflectivity product above, instead of the standard "R1" radar product that you would see on AccuWeather.com. While R1 is only one slice, CR takes all tilts into account and gives a more true picture of what's going on in the atmosphere.

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