Weather radar maps are pretty neat. Using just colors, the maps can indicate whether rain, snow, or sleet is passing over. Every once in a while, though, even experts aren’t quite sure what a particular big blob is.

Such was the case at around 1 p.m. on the afternoon of Tuesday, October 3. At that point, someone from the National Weather Service in Boulder, Colorado, looked at their ZDR radar, which indicates shape as well as size, and saw a strange pink cloud swooping over the Denver metro area.

The pink color indicated large, round objects. Based on previous experience, the NWS offered up a diagnosis. They included a GIF of the visitors, and helpfully labeled it, “Birds.”

Then the controversy began. Jeff Wells, tweeting under the authority-burgeoning username @Bird_Wells, pointed out that most bird species actually migrate at night, not at 1 p.m. “Thanks for the insight, Jeff,” the NWS responded.

Another, Phil Stepanian (aka @RadarAndStuff) then presented his guess: insects. But the NWS shot it down, saying that when bugs show up on the radar, they’re usually more randomly dispersed.

The NWS doubled down on their interpretation, telling a local news station they were “confident it is local bird migration.” But then the reports started coming in from the ground. “Downtown #Denver is covered in butterflies right now, mostly Painted Ladies!” tweeted Joe Szuszwalak.

A solo painted lady.
A solo painted lady. Pix/CC0

By the next day, the NWS was singing a different tune. “We believe migrating butterflies are the cause of yesterday’s radar signature,” they wrote.

They had been tricked, they explained, by the sheer size of the impression: “Things with big wings need to fly together in the same direction with the wind to generate that signature.”

One mystery remained. Painted ladies normally migrate south for the winter, but this blob appeared to be going north instead. Our old pal Stepanian provided an explanation for this off-kilter path: The butterflies were trying to go south, but had been buffeted by headwinds, which pushed them northwest.

Properly identified at last, the butterflies then continued on their way, and the Boulder NWS went back to tweeting about clouds.

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