In a weird twist, the swarm itself resembles a huge butterfly.

(Michael Warwick/Shutterstock.com)

Late last week, meteorologists in St. Louis noticed a cloud acting peculiarly: It was beating a path toward Mexico while changing into a variety of odd shapes. Was it a radar glitch? The debris signature of a south-moving tornado?

The answer was more heartening—and bizarre. After analyzing the reflections, the National Weather Service concluded they showed an immense swarm of Monarch butterflies migrating to their winter home in the Mexican mountains:

(NWS St. Louis)

Here's how it technically arrived at that conclusion, for the weather geeks out there:

Keen observers of our radar data probably noticed some fairly high returns moving south over southern Illinois and central Missouri. High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological targets. High differential reflectivity indicates these are oblate targets, and low correlation coefficient means the targets are changing shape. We think these targets are Monarch butterflies. A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape! NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!

North America's Monarch population has been in decline, reaching record-low numbers in the past couple of years due to habitat loss and perhaps extreme weather. These radar shots provide a spot of good news in that, while struggling, the Monarchs aren't extinct quite yet. Indeed, people on the weather service's Facebook page have reported seeing them fluttering around the region. "They are flying over my Missouri home today," says one. Adds another: "I have been seeing some coming thru OKC in the last week or so. They are beautiful."

Also beautiful—and strange—is that the shape of the swarm itself resembles a giant butterfly. The last time that sort of radar coincidence happened may have been in 2011, when thousands of birds formed into a bird shape above Beebe, Arkansas, right before falling out of the sky, dead.

(NWS St. Louis)

About the Author

Most Popular

  1. Life

    Suburban Jobs Are Growing Fastest, But Urban Jobs Pay More

    New labor data show that the suburbs have the fastest job growth in the U.S. But we shouldn’t assume the future of employment will be suburban.

  2. A sign outside a storefront in Buffalo, New York.
    Environment

    Will Buffalo Become a Climate Change Haven?

    The Western New York city possesses a distinct mix of weather, geography, and infrastructure that could make it a potential climate haven. But for whom?

  3. photo: a commuter looks at a small map of the London Tube in 2009
    Maps

    Help! The London Tube Map Is Out of Control.

    It’s never been easy to design a map of the city’s underground transit network. But soon, critics say, legibility concerns will demand a new look.

  4. Equity

    Why Smaller Cities Are Expanding Their Prisons, and Their Populations

    While many cities are using incarceration alternatives, some smaller cities and rural areas are building—and filling—costly new jails, new research shows.  

  5. photo: a high-speed train in Switzerland
    Transportation

    The Case for Portland-to-Vancouver High-Speed Rail

    At the Cascadia Rail Summit outside Seattle, a fledgling scheme to bring high-speed rail from Portland to Vancouver found an enthusiastic reception.

×