They can come in different sizes and intensities. Most commonly, fire whirls occur when hot, strong winds, often whipping through trees, come into contact with already raging bushfires. Updraughts of hot air catch the fire and surrounding winds send it whirling into the air, sucking up debris and flammable gases. They also acquire a vertical vorticity. CSIRO fire researcher Dr Andrew Sullivan says that in order for a vortex to form, a ‘shear’ must be present, which is a flow (for example a prevailing fire or wind) coming perpendicular to another.
The gif above is from one of the the most recent firenado video filmed last year in Australia, by filmmaker Chris Tangey. For 40 minutes he watched in amazement as several 30-metre (98 ft) tall fire tornadoes danced just a short distance away from him.