A 70-Mile Butterfly Swarm Was Picked Up By Colorado's Weather Radar


Rachel Baxter

Copy Editor & Staff Writer


The left-hand image shows birds picked up by weather radar, whilst the right-hand one shows the pattern created by butterflies. National Weather Service

Is it a bird? Is it a plane? Nope – it’s a 112-kilometer (70-mile) swarm of butterflies gliding over Colorado.

That’s what puzzled weather officials discovered when a mysterious multicolored mass heading towards Denver popped up on weather radar readings in Boulder, Colorado, last Tuesday, October 3.


The National Weather Service (NWS) of Boulder and Denver took to Twitter to see if any of their followers could help work out what the strange patterns were. It’s quite common for migrating birds to be picked up on weather radar, so initially, weather officials assumed that they were to blame.


“We detect migrating birds all the time but they were flying north to south,” NWS meteorologist Paul Schlatter told CBS News. This would be an unusual journey for birds to be making around this time of year.

A number of Twitter followers solved the conundrum with answers like “Could it be butterflies? Butterflies EVERYWHERE in #Denver!”

The NWS Boulder later tweeted: “We believe migrating butterflies are the cause of yesterday’s radar signature.Thanks for all the reports and sightings!” 


Many local people had seen clouds of butterflies floating over the area. Sarah Garrett, a lepidopterist at the Butterfly Pavilion in Westminster, Colorado, told ABC News that people as far as the Dakotas had witnessed the butterflies.

The butterflies in question were Painted Lady butterflies, often mistaken for Monarch butterflies. The insects were traveling with the wind, on their way to northern Mexico and the US southwest, where they spend the cold winter months. Miraculously, the tiny creatures can float hundreds of miles a day, just by hitching a ride on the wind.

The Painted Lady (Vanessa cardui) can have a wingspan of just over 7 centimeters (2.8 inches) and has orangey-brown wings with black markings. The species can be found all over the world, on every continent except Antarctica.

A pair of Painted Ladies (Vanessa cardui). Alina Wegher/Shutterstock. 

The array colors picked up by the weather radar sadly aren’t actually representative of the butterflies’ markings. Instead, they represent the shape of the insects and the direction in which they were traveling. Apparently, it’s quite rare for insects to show up on weather radars, and officials believe this is the first time that butterflies have been detected by the equipment in Denver.


"We haven't seen this type of signature of butterflies before," said Schlatter. "We're excited about it."


  • tag
  • butterflies swarm,

  • weather radar