When bushfires create their own 'perfect storm'
IN A mind boggling revelation, The Bureau of Meteorology explains how bushfires can create their own weather, generating 'pyrocumulonimbus' clouds and storms.
But conditions have to be just right to do so.
According to The BoM, Pyrocumulonimbus clouds are a thunderstorm that forms in the smoke plume of a fire (or nuclear bomb blast, or volcanic ash cloud).
These pyrocumulonimbus clouds can cause dangerous and unpredictable changes in fire behaviour, making the fire more difficult and hazardous to fight.
But in Australia they most commonly form in large and intense bushfire smoke plumes.
Clouds forming above fires are a phenomena commonly called pyrocumulus, but they are officially known as flammagenitus.
These clouds are said to form when intense heat from the fire causes air to rise rapidly in the smoke plume.
The rising hot air is turbulent and draws in cooler air from outside the plume, which helps cool the plume as it rises.
As the plume rises to higher and higher elevations the atmospheric pressure reduces, causing the plume air to expand and cool even further. If it cools enough, the moisture in the plume air will condense and forms cumulus cloud, which, because it comes from the fire plume, we call 'pyrocumulus'.
The condensation process causes latent heat to be released, which makes the cloud warmer and more buoyant and causes the cloud air to accelerate upwards. Further expansion and cooling causes more moisture to condense and the cloud air to accelerate upwards even more.
In the right conditions the cloud can accelerate into the lower stratosphere before losing buoyancy. Collisions of ice particles in the very cold upper parts of these clouds cause a build-up of electrical charge, which is released by giant sparks-lightning, which can potentially light more fires.
Having produced a thunderstorm, the cloud is now known as 'pyrocumulonimbus'.
Why are they dangerous?
Change in fire direction and intensity
These clouds can produce intense updrafts that suck in so much air that strong winds develop, drawn in from all directions towards the plume. These can cause the fire to burn hotter and spread faster. The inflowing winds can cause nearby fires (perhaps caused by spot fires or lightning strikes, see below) to change direction unexpectedly as they are drawn in to the parent fire.
The updrafts become very tall and because they are so strong they can carry large burning embers and lift them to great heights before they're carried far downwind, where they can ignite new fires (called spot fires). During Victoria's Black Saturday bushfires in 2009, one spot fire was recorded more than 30km from the 'parent' fire.
Lightning can form in these storms, which can cause new fires. Again in Victoria's Black Saturday fires, lightning from pyrocumulonimbus clouds started a new fire 100km from the fire front.
Rain can also form in pyrocumulonimbus clouds, producing intense downbursts (caused by cooling from evaporation of the rain) that hit the ground and 'burst' outwards, producing very strong and gusty winds that can last 20 minutes or more. These winds can be strong enough to blow the fire in any direction, and have been responsible for deaths of fire fighters on a number of occasions.