Technology: Why Victoria's bushfires generated their own lightning
There are few sights more terrifying for a fire fighter: a vast, dark storm cloud brewing above a bushfire, shooting out lightning. On Sunday, 3 March 2019, the Licola bushfire east of Melbourne burnt with such intensity it generated a huge thundercloud that blasted more than 1 200 lightning strikes at nearby forests, igniting several small fires. The Bunyip fire, which is believed to have wiped at least one town off the map, also generated its own weather system which fed the fire with extreme winds. “It’s absolutely terrifying. And it’s dangerous as well, because that lightning can start new fires before the main fire,” says Dean Narramore, a meteorologist at the Bureau of Meteorology’s extreme weather desk.
As a bushfire burns, it generates hot, smoke-filled air. This air is hotter than surrounding air and rapidly rises, forming a smoke plume. As the plume rises, atmospheric pressure falls. This causes the plume to spread out, generating a "mushroom" on top of the plume. The smoke plume is filled with moisture which is released by burning trees. The higher you go in the atmosphere, the cooler it gets, so the top parts of the plume get chilled.
This causes the moisture in the plume to condense (turn from water vapour into tiny water droplets) and form a cloud. As the plume rises rapidly into the sky, cool air is sucked in to replace it. This causes extreme winds near the firefront.
To fight a fire, you need to know which way it is burning. But when a fire-cloud forms and starts generating strong and unpredictable winds, the fire can become chaotic. “All this air rushes into the fire. You can imagine air coming from all different angles, feeding in and oxygen is a very important part of fire. It causes fires to race up and down hills,” Narramore says.
These winds can be extremely strong; during Black Saturday, trees were ripped in half or torn from the ground. Other buildings, which survived the fire, ended up getting destroyed by the powerful gusts.
If the fire is very hot, it makes so much hot air the cloud it generates can rise high into the atmosphere. As the cloud races up into the sky, it enters the freezing upper reaches of the atmosphere. This leads the condensed moisture to turn to ice. You end up with a cloud full of ash, ice and powerful winds. The winds bash the ice chunks together in the cloud, building up static electricity, just like rubbing a balloon does.
That electricity wants to get to ground. And it does, in huge spikes of lightning.
The Licola fire burned through forest, meaning it was much hotter, which is why it produced fire lightning and the Bunyip fire did not.
Source: The Age