Fallstreak hole in BOM’s 2016 calendar: how is it formed?

It’s often been attributed to UFOs. The true explanation is no less wondrous — and it does involve a flying object.

TWAM-20151107 EMBARGO FOR TWAM 7 Nov 2015 NO REUSE WITHOUT PERMISSION Cloud formation, Korumburra Pic : David Barton

Korumburra 3950

Your first reaction, on seeing something like this appear over your house, might be to run for the hills. Sure, it’s very pretty — but there’s also something about it that whispers alien invasion, even to sober-minded, rational people. Indeed, this rare meteorological phenomenon has often been attributed to UFOs in the past. The true explanation is no less wondrous — and it does involve a flying object.

One of the weird things about clouds is that the pure water within them can exist in “supercooled” form — that’s to say, as tiny liquid droplets, even though the temperature is way below 0ºC. These droplets “want” to freeze, but none wants to be the first to do so. Think of that Attenborough clip where the great mass of wildebeest are fronting a river they have to cross. When one animal takes the plunge, it sets off a massive cascade.

Heart of our nation

In the right conditions, a trigger can cause some of those supercooled droplets to freeze, and set off a similar chain reaction. The result is a “fallstreak hole”, pictured in a shot from the Bureau of Meteorology’s 2016 calendar (on sale now). And the trigger? It’s an aircraft.

As the plane punches through the cloud, it seeds the first ice crystals and sets the whole thing in motion. As the crystals grow they literally drop out of the cloud (falling crystals are causing the rainbow effect in this shot). Circulating air currents around the edge of the hole feed into the chain reaction. Within an hour a fallstreak hole can grow to 50km across.

If you love clouds, by the way, you’re not alone. The Cloud Appreciation Society (yes, really), which runs a website featuring chat forums, “Cloud News” and a “Cloud of the Month” photo spot, has 39,000 members around the world.