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Massive 70-Mile-Wide Butterfly Swarm Shows Up On Denver Radar System (bbc.com)
dryriver shares a report from BBC: A colorful, shimmering spectacle detected by weather radar over the U.S. state of Colorado has been identified as swarms of migrating butterflies. Scientists at the National Weather Service (NWS) first mistook the orange radar blob for birds and had asked the public to help identifying the species. They later established that the 70-mile wide (110km) mass was a kaleidoscope of Painted Lady butterflies. Forecasters say it is uncommon for flying insects to be detected by radar. "We hadn't seen a signature like that in a while," said NWS meteorologist Paul Schlatter, who first spotted the radar blip. "We detect migrating birds all the time, but they were flying north to south," he told CBS News, explaining that this direction of travel would be unusual for migratory birds for the time of year. So he put the question to Twitter, asking for help determining the bird species. Almost every response he received was the same: "Butterflies." Namely the three-inch long Painted Lady butterfly, which has descended in clouds on the Denver area in recent weeks. The species, commonly mistaken for monarch butterflies, are found across the continental United States, and travel to northern Mexico and the U.S. southwest during colder months. They are known to follow wind patterns, and can glide hundreds of miles each day.
Why would it be unusual to see migratory birds going from north to south this time of year? That seems like what we'd expect from any migratory creature.
Also, Paul Schlatter knows his stuff, so I don't think it's his error, but rather the reporter. It is very common for flying insects to be detected by radar, despite what the article says. More likely, it's rare to detect swarms of migratory insects. However, we see insects all the time on weather radar, usually seen as an area of relatively low reflectivity compared to storms (say, 10-20 dBZ, compared with 50 dBZ for storms) and it's seen close to the radar where the beam isn't too far above the ground. Sometimes the winds converge and cause the concentration of insects to increase in an area. When the winds converge, the air generally rises, but the insects resist ascending to a higher altitude. We see this a lot of times along fronts, drylines, outflow boundaries, and horizontal convective rolls. They usually appear as a line, usually a few kilometers wide, of stronger reflectivity (perhaps 20-25 dBZ), and it allows us to see where things like fronts and outflow boundaries are. We also use the motion of the insects with increasing height (as the beam gets farther from the radar) to estimate the wind speed and direction in the lowest kilometer or two of the atmosphere.
When it's warm enough, it's usually very common to detect insects with weather radar. Sometimes this is actually very useful to meteorologists, too.