Was it a bird? A plane? A tornado cell? Nope. The huge, slow-moving cloud picked up on radar in Illinois and Missouri last week was nothing anyone expected. The US National Weather Service reports it was actually a swarm of millions of monarch butterflies—which have been disappearing at alarming speeds over the past decade.


As Citylab's John Metcalfe says, the swarm was actually on its way south to Mexico in search of warmer climes in which to spend the winter. On Facebook, the US National Weather Service explained that "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!"


Image: NCG

Back in June, a similar radar glitch was observed when millions of locusts took to the skies in New Mexico, and The Vane was kind enough to tell us how biological matter is picked up by radar with this excellent explanation:

Weather radar works by sending out pulses of microwave radiation tilted at different angles above the horizon; the beam bounces back and the speed/intensity/shape of the return tells the radar what it's looking at. As the radar beam goes out in a straight line, the curvature of the earth causes the beam to go higher into the atmosphere as it moves further away from the radar site.

The journeys of the Monarch is long—stretching over 2,000 miles—and very mysterious, since no one quite knows how the newly-hatched butterflies already know the route either south or north (depending on when and where they're born). On Facebook, the Weather Service added: "NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!"

Despite the sighting, these distinctive, lovely insects are critically endangered. Some estimates say their numbers have declined by 90 percent over the last two decades, with conservation groups calling for Monarchs to be designated as "threatened," according to Newsweek. Keep your eyes peeled over the next few weeks—the remaining Monarchs should be making their ways south en masse through October. [CityLab]