“You may have noticed a lot of fuzziness … on our radar recently,” they wrote. “Our guess? It’s probably the cicadas.”
After deliberating among ourselves and reaching out to entomologists, we mostly concur. Our hunch is that multiple types of insects are contributing to the noisy radar signals — cicadas at low levels and mayflies or termites higher aloft.
Weather radars work by transmitting a sweeping beam of pulsed, microwave energy through a large volume of the atmosphere, listening for “echoes” that result when energy is bounced back to the radar by “targets.” These targets aren’t always meteorological.
The radars are fine-tuned to focus on hydrometeors, or raindrops, snowflakes and hailstones. But a large enough concentration of non-meteorological scatterers lofted in the air — including bugs, tree seeds and even tornadic debris — can light up the radar map, as well.
That’s what weather watchers began noticing over the weekend, when the radar maps went extra fuzzy-looking across the greater Washington area and points north and west.
The signatures appeared like patches of drizzle where the radar beam sweeps within a mile of the surface, but it was sunny and no rain or storm clouds were in the area.
However, the noise has extended higher than cicadas can fly — up to about 6,000 feet in elevation — causing some reason for skepticism.
Daniel Gruner, an entomologist at the University of Maryland, did not initially believe the radar returns were from cicadas. Gruner sent CWG an email stating, “For today’s [radar] images, I am skeptical these signatures are cicadas for multiple reasons. Cicadas are not strong long-distance fliers, they do not migrate, and their biology is tethered to the trees.”
Cicadas are big, heavy and clunky bugs.
They usually prefer buzzing about near tree crowns. In the fuzzy radar images, the fuzziness extends more than 100 miles from the radar. At such long distances, the radar beam is several thousand feet above the surface (the beam follows a relatively straight path through the atmosphere, but Earth’s surface curves away from it, making the beam gain height with range).
That’s where we can turn to more sensitive, high-resolution radars for help. There are several in the region, like the terminal Doppler radar at Joint Base Andrews in Prince George’s County.
It revealed a dense spattering of objects within 300 feet of the ground and then a more diffuse but still present assortment of creatures through about 6,000 feet of elevation.
That 300-foot mark more likely represents the upper cutoff of where the seemingly endless cicadas are swarming. They are probably responsible for the near-radar signals below that, with other insects contributing to what’s being seen upward of a mile aloft.
Upon learning that radar can detect insects as low as the tops of trees, Gruner came around to the idea that radar was indeed showing the cicadas. “If the radar can pick up the returns essentially at canopy level, then that changes the whole equation,” he wrote in a follow-up email.
In the image below, notice how the green and yellow shades are visible only near the radar, where the beam is low and within a few hundred feet of the surface. That’s probably the result of cicadas.
We also noticed that the densest low-level signal roughly corresponded with where cicadas have been sighted. Radar returns abruptly cut off near the Prince William-Fairfax county line, where the Brood X cutoff is.
Beyond that, radar is still detecting insects at elevation up to several thousand feet, raising questions as to what the cause there might be.
How do we know all the targets are insects? Their shapes. By examining the ratio of horizontal- and vertical-polarized components of the radar beam, as well as the amount of backscattered energy bounced back to the radar, the rough shapes and sizes of targets can be determined.
Raindrops, hailstones and snowflakes are shaped differently than the jagged cross-sections of winged creatures like cicadas. Biological targets have “low correlations,” meaning their shapes are unorthodox. All the blue on the below map corresponds to such low-correlation objects:
So what’s going on in the range high above where the cicadas are found?
Our entomologist colleagues point to mayflies and termites, which have been spotted across the area in recent weeks. You may have noticed dead collections of these tiny winged critters on your car hood, as several of us at Capital Weather Gang did this past weekend.
It’s also possible that cicadas, termites and mayflies alike have been carried a bit higher than they’d otherwise fly by convective rolls, or weak overturning circulations that form in the heat of the day. Alternating strips of rising and sinking air a mile or two wide make up these “convective rolls.”
Ordinarily they are invisible, but when tracers like insects are caught up in the converging winds and suspended aloft, they help reveal the structure of such features in the lower atmosphere. That’s what makes the filament-like strands orient parallel to the low-level wind visible on radar.
This is not the first time insects or other wildlife has been spotted on weather radars. Swarms of various insects have been showing up on radar since its inception, as have dispersing flocks of birds, bats and even butterflies. Rocket launches and explosions, aircraft incidents, and wildfires have, too.
The bottom line is that multiple species of insects are probably at play in creating bright, fuzzy, clear-air patterns in the returns of nearby weather radars. It’s likely low-flying cicadas close to the radar and other critters more distant.
What have you, our readers, been seeing in terms of different insect emergences? We look forward to your comments in the blog, as we meteorologists have been “bugged” by this fuzzy radar problem!