On Friday, September 19, the National Weather Service radar spotted a huge, mysterious, butterfly-shaped cloud over St. Louis in Missouri. The cloud was changing into all sorts of odd shapes as it moved towards Mexico. Turns out, it wasn’t a cloud at all. Upon closer analysis, the National Weather Service found that it was a large swarm of Monarch butterflies. They were migrating to their winter home in Mexican mountains.

Huge Cloud Over St. Louis Was A Swamp Of Butterflies: NWS

Butterflies migrating to Mexico for the winter

ON NWS’s radar, the swarm of butterflies resembled a butterfly for a short while as it was changing shapes. The National Weather Service said on Facebook that the cluster of monarchs was flying about 5,000-6,000 feet above the ground. Their wings were radar targets. Nobody saw the butterflies, but radar signal analysis showed that the “targets” were flat, flapping and biological, reports LiveScience.

The butterfly swarm’s timing is in line with a recent exodus from the Great Lakes region. According to Monarch Watch, eastern population of monarch butterflies spends summer across the northeastern U.S., the Great Lakes and Canada. And then they migrate to western central Mexico to spend winters. The 3,000-mile journey takes more than two months. It’s the longest butterfly migration on Earth.

Population of monarch butterflies falling dramatically

Western population of monarchs summers in California, but winters in Mexico. Butterflies may have gathered together when passing over St. Louis due to favorable weather conditions. They take advantage of air currents to conserve their energy during two-month long travel. Monarchs are the most recognizable butterfly species in North America because of their distinct black, orange and white markings.

During their migration, the monarchs help pollinate about 35% of fruits and vegetables we consume. The sheer size of the recently spotted swarm is a positive sign. Monarch population in North America has been declining at an alarming rate over the past few years due to exposure to pesticides, drought, cold temperatures along the migration route.

According to Monarch Watch, their population fell to record low of 33 million last year.