Why Bogong Moths Can Look Like Rain on Weather Radar

How can moths confuse weather radar?

Weather radar sends out brief pulses of radio waves. When those waves meet something in the air, some of the signal bounces back. The returning signal, called an echo, can appear on a radar display. ²

Radar detects targets, but an echo is not automatically proof of rain. The Bureau of Meteorology says large numbers of insects can look like very light, widespread rain. ¹ So a broad patch on a screen can be caused by moths as well as water droplets.

Why are Bogong moths appearing on Sydney-area radar?

Bogong moths are migrating south towards cool caves in the Australian Alps, where they shelter over summer. Reports say the swarms have been large enough to appear on radars in parts of eastern New South Wales. ¹

The echoes have reportedly appeared in the evenings, often just after sunset. ABC meteorologist Tom Saunders described radar signals appearing on repeated evenings despite clear skies and no rain. ¹² That timing is consistent with the reported moth activity, but an individual radar image by itself does not identify the target.

How can forecasters tell moths from rain?

The reports point to comparing radar signals with what is happening outside the radar display. If a radar shows a rain-like patch but skies are clear and there is no rain, insects may be a possible explanation. The repeated evening timing during migration is another clue described in coverage of the event. ¹²

These sources do not give a full technical checklist or describe a specific test that conclusively separates moth echoes from rain. They also note that other things, including smoke, aircraft, birds, mountains and tall buildings, can appear on radar. ² A radar image therefore needs interpretation rather than a simple assumption that every echo is precipitation.

Does a moth echo mean the radar is broken?

No. The Bureau of Meteorology said its radars are not malfunctioning and explained that large numbers of insects can resemble light, widespread rain on radar. ¹

This is a limitation of what the signal can show, not necessarily a fault in the equipment. Radar detects reflected radio waves; deciding what reflected them takes context. Moth echoes are not described in the sources as evidence that rain is falling.

Why does the moth migration matter?

The radar story may also point to a change in Bogong moth numbers. The Sydney Morning Herald reported that conservationists welcomed the large swarms as a sign that populations may be recovering after a severe crash in 2017. ¹

That is a reported interpretation, not a population count or a guarantee of recovery. The sources do not provide figures for the current moth population or establish how long the apparent increase will last. The radar echoes offer a striking sign of many insects in the air, but they cannot by themselves measure the health of the species.