Millions of bogong moths trigger rain warnings on weather maps

For the last month, the weather radars across south-eastern Australia have been lighting up with precipitation every night.

The only problem is that on most nights, skies are not cloudy and it certainly has not been raining for a month straight.

"There has been increased 'echoes' displaying on radars across eastern New South Wales that aren't precipitation," a spokesperson for the Bureau of Meteorology (BOM) said.

But the equipment is not malfunctioning, and this is not a giant UFO.

Instead, the radars are detecting the millions of bogong moths currently migrating south to the Australian Alps for summer, and the swarm appears on radar as widespread rain.

The false echoes are appearing like clockwork just after sunset as the moths take flight after spending daylight hours resting and hiding.

Initially, most of the radar anomalies were concentrated in NSW, but recent nights have also seen the phenomenon across northern and eastern Victoria.

Below is the Monday night radar loop, which shows the odd shower and thunderstorms in the late afternoon. However, the arrival of nightfall coincides with a sudden burst of activity.

Why are weather radars mistaking moths for rain?

A radar works by sending out a short pulse of radio waves, then waiting for the signal to bounce back off airborne particles.

If no echoes are returned, the radar imagery will stay blank. But if the waves reflect off raindrops, hail, or snowflakes, the radar can construct a 3D picture of the precipitation pattern based on the height of the beam, along with both the intensity and the return time of the pulses.

However, there are far more objects floating around in the sky than just precipitation.

Bushfire smoke, aircraft, flocks of birds, and swarms of insects can also result in a return signal, as can mountain ranges and tall buildings.

And the atmosphere itself can lead to deception. Certain conditions can cause the radar beam to bend downwards, a process called ducting, and strike the ground.

Exactly how much of the current interference is from moths is unknown. However, there are a number of clues which suggest a large proportion is currently from the millions of bogongs:

• The false echoes are appearing at around the same time every night which matches the diurnal behaviour of the moths
• The coverage of the radar echoes matches the sightings of moths
• The pattern is widespread light rain, which resembles insects more than ducting

What weather watchers are seeing each night is confirmed by the BOM

"Large numbers of insects can look like very light widespread rain, particularly near sunrise and sunset," the spokesperson said.

Forecasts in a flutter

While meteorologists are not normally fooled by radar interference or clutter, some weather apps that give alerts for imminent precipitation could be sending false alarms, since their algorithms may rely on raw radar data that has not been quality-checked or verified.

However, during the 2000 Olympics, bogong moths did give forecasters a quick flutter.

A crack team of BOM meteorologists predicted a clear and dry night for the closing ceremony, but as the event began, the radar indicated rain was about to hit the stadium.

The meteorologists were puzzled, and their panic only abated when the live broadcast showed a large moth landing on Yvonne Kenny as she performed the Olympic anthem

In this case, a swarm of moths had been attracted to the bright lights of the stadium.

The prolonged spell of interference this year has made radar interpretation less reliable. However, on nights when it is actually wet, it takes a keen eye to differentiate real rain from the clutter.

Take the image below, captured over central-eastern NSW on October 2.

A band of rain is approaching Sydney from the west, while a clump of false echoes is present from northern Sydney to the Lower Hunter.

Thankfully, standard weather forecasts are not affected, since they rely on weather modelling, ground observations and satellites.