Weather radar doesn’t just pick up rain and snow, it can also detect other objects such as birds, insects and even flying debris during tornadoes.
That’s what happened earlier this week in Colorado, where a weather radar there “saw” what turned out to be a 70-mile-wide swarm of butterflies.
The butterflies in question were painted ladies, which are sometimes mistaken for monarch butterflies. The bugs have descended on Colorado’s Front Range in recent weeks, feeding on flowers and sometimes flying together in what seem like clouds.
At first, meteorologists thought they were birds, since “insects rarely produce such a coherent radar signature,” according to the National Weather Service in Boulder. “Migrating birds do all the time.”
— NWS Boulder (@NWSBoulder) October 3, 2017
Weather service meteorologist Paul Schlatter asked birdwatchers on social media what it might be, and soon had his answer: People reported seeing a loosely spaced net of painted lady butterflies drifting with the wind across the area.
He said the colors on the radar image are a result of the butterflies’ shape and direction, not their actual colors.
Radar works by sending out a beam of energy then measuring how much of that beam is reflected back and the time needed for the beam to return. If more of the beam is sent back, the object is said to have a high reflectivity and is indicated by a bright color.
Objects that return a small part of the beam have a low reflectivity and are indicated by darker colors.
In recent years, birds, ants, bats, termites, mayflies, grasshoppers, and beetles have all been spotted on radar. Birds last year were even seen taking refuge in the eye of Hurricane Matthew.
While capturing animals on radar is just a fun byproduct of radar technology, more seriously, debris blown around by tornadoes can also be spotted. This gives forecasters high confidence that a tornado exists, ramping up the danger level of the warning to more people in its potential path, according to the Storm Prediction Center.
Known as a tornado debris signature, it’s a very important tool, especially at night, in remote areas without spotters, and for rain-wrapped tornadoes that spotters can’t see safely.