Masses of moths mistaken for clouds prompt false rain forecasts in New South Wales

There is an odd explanation for all the rainy weather wrongly forecast for the currently sunny southeast of Australia — moths.

Huge swarms of moths, flying below Bureau of Meteorology radars, are being clocked as rain clouds.

The false forecasts are an unexpected side effect of the annual migration of Bogong moths.

Sightings of the curious insect are around six times higher than previous seasons, according to Zoos Victoria senior conservation biologist Dr Marissa Parrott.

It’s just wonderful to see the numbers coming back to where they should be and everyone enjoying a visit from an endangered species,” she said. Parrott also said the Zoos Victoria Moth Tracker website has already recorded 12,000 verified sightings.

A BoM spokesperson told 7NEWS.com.au that “it is possible that radars across eastern New South Wales radars are detecting Bogong moths” but added they were “unable to confirm that these signals are completely attributed to them.”

The BoM radar technology is known to pick up non-meteorological matter.

“The bureau’s radars are not malfunctioning,” a BoM spokesperson told 7NEWS.com.au

“Radar technology can pick up ‘echoes’ from non-meteorological matter such as buildings, birds and insects.

“Large numbers of insects can look like very light widespread rain, particularly near sunrise and sunset.

“We can confirm that there has been increased ‘echoes’ displaying on radars across eastern New South Wales that aren’t precipitation.”

The BoM spokesperson said that aside from the moth migration, there could be other factors such as “temperature inversions” impacting the radar readings.

Horizontal and vertical waves

The BoM radar technology works by sending radio waves out from a rotating dishes that work like antennae. They send out short pulses of electromagnetic waves.

“When these pulses hit water particles, the bounce back to the radar, like echoes in a cave,” according to BoM.

“How long the pulses take to return to the radar determines the distance from the radar, and the strength of the return determines the type of precipitation. This information is converted to data on a map.”

Only some of the BoM radars can correct echoes that are picking up non-meteorological matter.

Some of the more modern radars in NSW, such as those in Terrey Hills and Yeoval, have dual-polarisation capability which means they pick up both horizontal and vertical waves.

The BoM spokesperson said that these radars can tell which echoes are from insects, rather than rain, and filter them out.

Other older radars, such as those in Wollongong and Newcastle, operate with single-polarisation capabilities, and are unable to perform that kind of data filtration.

The moth migration over NSW could continue for up to a few months, so those dressing for light rain forecast in the near future might want to take a quick peek skyward before leaving the house.