When Illinois and Missouri weather services detected something strange on their radar last week, they weren't sure exactly what it was. However, the National Weather Service has cleared up the mystery: the strange blue cloud seen on radar screens was actually swarms of monarch butterflies, making their annual journey south for the winter.

Radar picked up the migration on Fri., Sept. 19 when hundreds of monarch butterflies flew about 5,000 feet above the ground as part of their annual trek south to Latin America. Because their wings flutter, the radar picks them up.

Interestingly enough, there are no official reports about the butterflies, but some Facebook users in Missouri and Illinois posted about sightings. The National Weather Service is using what they know about radar and butterflies for a scientific answer to the mystery. By determining that the radar was picking up something flat, biological and moving, the answer was simple: butterflies.

"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," says The Vane. "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."

Nonprofit group Monarch Watch confirmed the migration of the butterflies from the Great Lakes area, where the insects spend their summers. Once cold weather begins settling in, they migrate to Mexico. The cold weather also brings air currents that help them conserve energy during their journey.

The monarch butterfly is easily the most recognized butterfly in nature with its most distinguishing feature being the orange and black patterns on its wings. However, this beautiful creature is also very close to endangerment. Its yearly journey is a dangerous one, made more so by climate change and human impact on the environment. Their trip south includes exposure to extreme weather, drought and pesticides. This has decimated their numbers. In 2013, there were only 33 million monarch butterflies in Mexico, a dramatic decline from previous years. Organizations, such as Monarch Watch, are urging officials to put them on the endangered list.

As the butterflies made their journey through the Midwest, the St. Louis Weather Service sent a message.

"Good luck and a safe journey to these amazing little creatures on their long journey south!" says the agency.