AUDIE CORNISH, HOST:
This past Tuesday, in Huntsville, Alabama, something unusual appeared on the local weather radar.
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UNIDENTIFIED WOMAN: Well, it is the blob that ate Huntsville. On radar, at least.
UNIDENTIFIED MAN: A giant angry-looking red spot. It caught the attention of all...
CORNISH: That blob hung around on the radar for nine hours and inspired a lot of theories - some practical, some less so. But like many strange occurrences, the Huntsville blob has a scientific explanation. Joining me now to talk about it is atmospheric science professor Kevin Knupp, from the University of Alabama, Huntsville. Welcome to the program.
KEVIN KNUPP: Thank you.
CORNISH: So when this image first came to your attention on the radar, help us understand, what did it look like? I know the weather folks thought at first maybe it might be rain or something weather-related.
KNUPP: It was brought to my attention shortly after it appeared. One of my graduate students had been watching radar and saw this very intense echo to our west, southwest about five miles.
CORNISH: And maybe for folks thinking about this at home, we're talking about one of those kind of - it looks red and yellow, you know, sort of the hotspot colors you see on radar when there's something there.
KNUPP: Yeah, when you watch the TV, anything red is usually intense and this had a lot of red in it.
CORNISH: Now, what did you think it might be?
KNUPP: Well, when I was made aware of this, we were walking down the hallway on our way to the outdoors where our instrumentation is and my immediate thought was, it's an unexpected shower. But it seemed to be awfully intense for a shower, so once we got outside, we looked in that direction and saw nothing but a few puffs of clouds and a lot of blue sky.
CORNISH: Now, I heard, to get to the bottom of this, you ended not looking to the sky, but to the ground, right? I mean, what's your latest theory on this?
KNUPP: Well, the theory was that this radar return was produced by chaff, which is very fine fibers that are coded usually with aluminum and they're very reflective. They're designed so that they maximize the signal intensity that a radar sees.
CORNISH: Help us understand what chaff is used for, especially given that you have a military facility nearby, the Red Stone Arsenal, which does some aviation and missile testing.
KNUPP: Well, chaff has most of the applications for military operations and it's used to confuse potential enemy radars. It can be used to deflect any missiles that might be based on radar return and it can hide aircraft.
CORNISH: Now, the Red Stone Arsenal has issued a press release, saying essentially that they can't comment about the specifics of any of their testing activities. But what was it like for you seeing all the speculation? I mean, you were at home and all the meteorologists are going nuts, saying what is that dark red spot on our screens?
KNUPP: When we see things interesting as part of our daily research activities, we usually put a tweet out. And this made it out amongst the TV meteorologists and other people interested in weather really quickly. And theories were floating around on what this might be. Not everybody believed us that it was chaff, but we eventually proved that it was by finding the chaff fibers the next day.
CORNISH: Well, Professor Knupp, thank you for helping us get to the bottom of it.
KNUPP: My pleasure.
CORNISH: That's Kevin Knupp, professor of atmospheric science at the University of Alabama, Huntsville. Transcript provided by NPR, Copyright NPR.