Its not that often you observe these but they are truly awesome to behold. I have never been able to take a picture of one of these myself but I have observed quite a few. Here is a picture I found on the web of a particularly large one photographed in Australia.
Growing up in rural areas on the highveld in South Africa you learn early about fire.
The Highveld biome is a grassland and the "veld" or wild grass grows form waist high to taller than a person in good years. This time of year is the fire season... really all of winter is fire season because the grass is dry... but late winter early spring is the worst.
The countryside is at its driest... the days are just beginning to heat up and we have the late August winds. These winds signify the change of the season as the Inter Tropical Convergent Zone begins to reverse and major weather systems realign themselves for the onset of the summer rains.
Little boy's that play with matches learn very early, that fire gets out of control fast in these conditions...
and every year as soon as you are able to swing a wet hessian sack you are out there with all the other farmers in the area fighting fire, saving pasture or preventing animals, houses and property from destruction.
Its in this often yearly event that I have managed to occasionally observe fire tornadoes.
The most magnificent ones I have seen have generally formed on days when there has been a strong cold wind blowing cross-ways to the advancing fire front. They are also more prevalent when there aren't trees around forming a wind-break.
They form when a very hot fire front with large flames create a rapidly rising column of hot air. This hot expanding air rises even more rapidly when the surrounding air is cold and dense.
Friction in a rising or descending column of air or water is most easily overcome when the air or water adopts a spiral motion. This spiral motion introduces centrifugal forces into the mix. The centrifugal forces, force the air or water outwards from the center of the spiral resulting in a very low pressure in the center of the vortex.
In the case of a fire tornado the flames of the fire and sometimes flammable material is sucked up into this area of very low pressure in the center of the vortex leaving us with a very spectacular visual display whether its night or day.