For the last month, the weather radars across south-eastern Australia have been lighting up with precipitation every night.
asserted
radars → light → precipitation
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.
asserted
spokesperson → rain → Meteorology
But the equipment is not malfunctioning, and this is not a giant UFO.
asserted
this → malfunction → ?
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.
asserted
swarm → detect → rain
The false echoes are appearing like clockwork just after sunset as the moths take flight after spending daylight hours resting and hiding.
asserted
moths → appear → hours
Initially, most of the radar anomalies were concentrated in NSW, but recent nights have also seen the phenomenon across northern and eastern Victoria.
asserted
nights → concentrate → Victoria
Below is the Monday night radar loop, which shows the odd shower and thunderstorms in the late afternoon.
asserted
which → show → afternoon
However, the arrival of nightfall coincides with a sudden burst of activity.
asserted
arrival → coincide → activity
Why are weather radars mistaking moths for rain?
asserted
radars → mistake → rain
A radar works by sending out a short pulse of radio waves, then waiting for the signal to bounce back off airborne particles.
asserted
signal → work → particles
If no echoes are returned, the radar imagery will stay blank.
asserted
imagery → return → ?
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.
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radar → reflect → pulses
Bushfire smoke, aircraft, flocks of birds, and swarms of insects can also result in a return signal, as can mountain ranges and tall buildings.
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ranges → result → signal
And the atmosphere itself can lead to deception.
asserted
atmosphere → lead → deception
Certain conditions can cause the radar beam to bend downwards, a process called ducting, and strike the ground.
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beam → cause → ground
However, there are a number of clues which suggest a large proportion is currently from the millions of bogongs:
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proportion → be → bogongs
- The false echoes are appearing at around the same time every night which matches the diurnal behaviour of the moths
-
asserted
which → appear → moths
The coverage of the radar echoes matches the sightings of moths
-
asserted
coverage → match → 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.
asserted
spokesperson → resemble → sunrise
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.
uncertain
that → fool → data
However, during the 2000 Olympics, bogong moths did give forecasters a quick flutter.
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moths → give → 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.
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rain → predict → 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.
asserted
interpretation → puzzle → interference
However, on nights when it is actually wet, it takes a keen eye to differentiate real rain from the clutter.
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it → take → clutter
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.
asserted
clump → approach → Hunter
Thankfully, standard weather forecasts are not affected, since they rely on weather modelling, ground observations and satellites.
asserted
they → affect → modelling
The Aporia
