On last Friday morning, observers at the Saint Louis National Weather Service (NWS) noticed some fairly high radar returns moving south over southern Illinois and central Missouri. Even though the reflectivity levels of these clouds were on the order of a moderate rainstorm, weather was quickly discounted, because there wasn’t a single cloud in the sky. For technical reasons, Birds were also ruled out. The above radar map accompanied the following rather technical NWS Facebook post on the phenomenon:
High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological in origin. High differential reflectivity indicates these are oblate [flattened] targets, and low correlation coefficient means the targets are changing shape. 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. NWS Saint Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!
The cloud even resembles the shape of a butterfly. I recall seeing Monarchs in the yard on both Saturday and Sunday. The picture of one is from the previous weekend and was taken at the gardens. Organizations dedicated to the preservation of the Monarch have disagreed with the NWS interpretation of its radar data. These NGOs point out that the Monarch population has fallen from a billion individuals to only 33 million in recent years and that the Monarch is now considered endangered. Still, the timing is right for their annual migration south to Mexico, as was the altitude of the returns, about 6,000’ and for lack of any better theory; I’m inclined to believe that those clouds were orange.