Why Do Moth Swarms Look Like Rain on Weather Radar?
A weather radar image can show a broad smear of blue over dry ground. The screen says rain. The ground says otherwise. The echo may be coming from moths.
A Beam That Cannot Tell Rain From Wings
Radar sends out short pulses of radio waves. When those pulses strike airborne particles, some energy bounces back. The antenna records the return and plots it as an echo. The strength and timing of that return help forecasters judge whether the target is drizzle, hail or a thunderstorm.
The beam does not discriminate. Smoke, ash, birds, aircraft, mountains and tall buildings can all produce returns. So can insects. The Sydney Morning Herald reported that large numbers of insects can look like very light, widespread rain, especially near sunrise and sunset. ABC News's Behind the News described big patches appearing every evening just after sunset.
That pattern fits nocturnal moth behaviour. The insects take flight in the dark, and a dense swarm presents the radar with a moving mass of bodies rather than a cloud of water droplets. The radar sees the mass, not the individual moth.
The Evening Bloom
A moth echo tends to have a schedule. It appears after sunset, spreads across the screen and fades before morning. Real rain does not keep that timetable. Forecasters use the clock as a clue, along with the strength of the returning signal. Light, diffuse patches are less convincing than the strong returns from heavy rain or hail.
The ABC notes that meteorologists are not usually fooled by this kind of interference, and that similar false echoes have appeared before. Bushfire smoke, aircraft and even tall buildings can leave marks on radar. A single echo is evidence, not proof.
What the Radar Is Really Measuring
Radar is a remote sensor, not a camera. It measures returning energy, and that energy can come from biology as easily as from weather. The species involved in these swarms is not confirmed in the published accounts, and the exact number of insects is unclear. What is clear is that a dense flight of moths can produce a signal that looks like light rain.
The practical effect is confusion. Radar is a primary tool for tracking rain, hail and thunderstorms, and a false rain echo can mislead anyone reading a public map without context. The phenomenon does not mean the radar is broken. The beam is doing its job. The atmosphere simply contains more than water.