FIRE WHIRLS

Imagine a twelve-foot wide, hundred story tall fire spinning at speeds more than 100 miles per hour.  Sounds like a scene from a science fiction movie, right?  But these fire whirls, also called “fire tornados” or “fire devils” actually occur all around the globe.  They range in height and width, and there are certain conditions that must be in place for a fire whirl to form.  There are four main types of fire whirls [1]:
Thermally driven fire whirls: Occur when high levels of wind come into contact with fire.  They    are the most common type of fire whirl.
Lee side fire whirls: Thermally driven fire whirls that are formed on the downward side of     ridges.  Fire whirls that develop on ridges pose as a higher threat because they are able to spread down the ridge at much faster rates.
Wake type fire whirls: Caused by airflow around an obstacle, such as a tree or rock.  They are comparable to an F2 tornado on the Fujita Scale (roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees uprooted).  They are sometimes called leading edge fire whirls because they develop at the front, or “leading edge” of a fire.
Convection column vortex: Start out high and touch down to the ground.  They can spread well outside the range original fire and are the tallest kinds of fire whirls.

Like tornados, fire whirls vary in size and speed.  They can range from less than one foot in diameter to several dozen feet wide. And from only a few stories to over 100 stories tall.  The speed at which they travel varies as well, from 20 mph to more than 100 mph [2].  The duration of fire whirls usually last for less than one minute, but there are accounts on record that lasted for almost an hour.  Fire whirls used to be thought of as an extremely rare occurrence, but with new technology and communication, fire whirls are being reported much more frequently.
There are several factors that contribute to fire whirls.  While it seems that fire whirls would form in high wind conditions, they actually cannot form if the winds are too high.  Fire whirls typically occur in areas with atmospheric instability.  Fire whirls, unsurprisingly, develop in drought conditions and in high heat.  Cloudless skies, low humidity, bare soil, and direct sunlight all increase the chance of a fire whirl.

Fire whirls can be extremely dangerous.  The most devastating one on record occurred in 1923 during Japan’s Great Kanto Earthquake.  It is estimated that it killed 38,000 people in less that 15 minutes [2].  Fire whirls are especially dangerous because not only can they destroy objects in their path, but also because  they shoot debris which can set additional fires [3].  Fire whirls are not very well known, but they are a powerful, and sometimes deadly, natural hazard.