A fire whirl is a vertical vortex of flame. They're sometimes called fire tornadoes, but that's not really accurate. A tornado always forms top-down, while the most common types of fire whirl rise up and out of the fire the same way a dust devil rises out of sand.
Basically, a fire whirl can happen whenever air vortices meet fire. You need the vortices to give the air its spin, and you need the fire for the updraft and the flames, if any. Just like a tornado doesn't need a visible funnel to be a tornado, a fire whirl doesn't need flame inside the spinning updraft to be a fire whirl. However, if it moves outside the burning area, it's lost its heat source and is going to die fast.
A lot of fire whirls are being caught on camera lately. That's mostly because there's a lot more cameras everywhere than there used to be, although if wildfires are getting more common, there's going to be more fire whirls as well. With all the new information, scientists are finally able to study them properly. However, wildfire firefighters put together the first classification of fire whirls. In their years of fighting brush fires, they've seen a lot of them! They've come up with four types of fire whirls.
1. Thermally driven fire whirl
This is your classic fire whirl. It happens whenever fire meets wind shear. They're also called energy release fire whirls.
2. Lee side fire whirls
These form where the air eddies on the downwind side of hills and can move a forest fire forwards fast! Firefighters expect to see them any time a downhill fire run coincides with prevailing downslope winds. They're sometimes lumped in with energy release fire whirls, but they're a lot more dangerous.
3. Wake type fire whirls
These can be the equivalent of an F2 tornado! They're caused by the flow of air around an obstruction, like a rock, tree, or even a convection column. That's why you'll often find them at canyon bends. They're also called leading edge fire whirls, because that's where they form.
4. Convection column vortex
These are the tallest kinds of fire whirl. They're also the only kind to start high up in the air and reach down to the ground, sometimes well outside the burning area. The entire region around a convection column vortex plays havoc with water bombers and spells extreme danger for everyone on the ground.
Surprisingly, you don't need much of a wind for a fire whirl to develop. With most fire whirls, ground wind speeds are less than 30 klicks. The winds usually come from the local weather conditions, especially when the Haines Index is 6. Fire whirls can't form at all if the ground wind is too high!
The winds inside the fire tornado are another thing altogether. They've been known to uproot trees. Some scientists think the winds in the strongest fire tornadoes could be more than 480 kilometres per hour! Of course, that's a little difficult to measure, even in normal tornadoes where you don't have fire to contend with as well.
Low level atmospheric instability is a must for fire whirls to form. Obviously they're a lot more likely in drought conditions, just like forest fires are more likely, but the temperature gradient close to the earth is much more important than the actual temperature. That's why more fire whirls form on the east side of slopes in the morning, and on the south and west side of slopes in the afternoon.
That temperature differential is what makes a fire tornado vortex completely backwards from a standard tornado vortex, or even from a standard fire. The hottest air is spinning and rising in the outside wall of the vortex, like smoke rings from a cigarette. However, the colder air is drawn down in the middle at the same time. It's the complete opposite of a standard tornado's low pressure. It's even backwards from the surface in-draft that usually happens with fires.
All that fresh air downdraft is also full of oxygen, which goes right to the base of the fire and spreads in all directions, even upwind. it's just like a gigantic bellows!
Clear sky, low humidity, and perpendicular sunlight increase the chances of a fire whirl forming. Bare soil creates the strongest temperature gradients, and so does desert scrub. However, even a little bit of cloud cover cuts the chances of a fire whirl down drastically.
Thermally driven fire whirls can grow to more than a kilometre high. That's close to your typical tornado. At the other end of the scale, under tightly controlled conditions, you can even make your own tiny fire whirl at home. Small fire whirls aren't much of a problem to firefighters, but they indicate that the conditions are right for larger fire whirls and unpredictable fire behaviour.
The typical height of a standard tornado is the same as the Lifting Condensation Level (LCL), which is more or less the same as the cloud base. It ranges from 400 metres or so for the strongest tornadoes all the way up to an occasional odd funnel at 1500 metres. Most supercell tornadoes average around 780 metres high. Most fire whirls are a lot smaller, at just ten to fifty metres high.
Convection column vortexes are in another league entirely. Those kinds of fire whirls are pretty much making their own weather, but they're not usually visible the same way a pyrocumulus cloud is. Under perfect atmospheric conditions, these vortexes can grow as high as eighteen kilometres. That's high enough to poke right through the troposphere and into the stratosphere!
A strong fire whirl can last up to an hour. During that time, it can pick up flaming pieces and fling them far away from the main burn area, starting new satellite fires. It can even travel outside its burn area. It'll soon overreach itself and collapse, but it's going to spread the main fire in the process.
If a fire whirl shows up at all, the underlying fire's a lot more extreme and unpredictable than you thought it was. These kinds of fires can break all the normal rules of surface fires. They can spread any direction, even upwind, with little warning. They can even start new satellite fires a long distance away. That's extremely dangerous for firefighters.