Doppler radar is used to see the location of precipitation, but sometimes it picks up on other things in the sky as well. The National Weather Service in Boulder, Colorado made a unique sighting on Doppler radar Tuesday.

They originally thought that the reflectivity was picking up on migrating birds, but birds fly south at this time of year, not to the northwest. It turns out that it was a migration of monarch butterflies.

Left: Bright purple shows where butterfly wings were flapping Right: NWS originally thought this blue and purple reflectivity was birds Courtesy NWS Boulder Twitter

The left radar image shows the differential reflectivity, a high differential reflectivity like this shows that there was a large mass of wings flapping together. The mass of butterflies turned out to span over 70 miles! The right radar image is the regular reflectivity image. This shows that the radar beam is making contact with something in the sky that is moving together.

The National Weather Service received verification from spotters, who saw the thousands of butterflies flying through the sky. This is a rare occurrence, especially in the Denver area.