Fire tornado on Port Hills a first
An extremely rare fire tornado seen whipping through the Port Hills in Christchurch may be the first of its kind in the South Island.
The towering tornado, described as a column of flames tens of metres high and the same across, is likely the result of the change in the air temperature as the two initial fires that had burnt through the hills early last week merged.
The Port Hill fires, which started on February 13, ripped through about 2075 hectares of land, destroying several homes and forcing the evacuation of hundreds residents. Efforts to extinguish the blaze are continuing.
The tornado was spotted by aerial fire crews in the early days of the fire.
Fire response co-ordinator Richard McNamara has fought fires all over the world and said he had never seen anything like it. He said the blazing tornado was at least 50 metres tall, hidden ‘‘under that big mushroom cloud’’.
Fire behaviour scientist Grant Pearce said fire tornados, like the one seen in the Port Hills fire, were incredibly rare. He could think of few other instances of the tornados in New Zealand’s history.
The flaming red whirls were present at the devastating fires in Chicago in 1877 and San Francisco in 1905 and were reported at Raetihi in 1919 and again at Taupo in 1946.
Pearce said the tornados behaved in much the same way as a wind tornado and picked up everything in their path, except at their core was a flaming inferno.
They scooped up vegetation, whole logs and branches. The wind equivalents had such power that grass blades became bullets.
He said while it was relatively common for a small whirlwind to occur, usually on the perimeter of the fire, it was very unusual for it to escalate further than a tiny twister.
As the Port Hills blaze was an example of an ‘‘extreme fire’’, the conditions made for the perfect storm.
‘‘You need an unstable atmosphere and something to get the circulation going. As hot air rises, it naturally rotates due to wind or variation in the amount of burning because different levels of activity within the fire cause updrafts and downdrafts,’’ Pearce said.
‘‘They usually occur in gullys or in the middle of a fire as it meets another.
‘‘As the hot air rises, cool air is sucked into a central hot spot and begins to spin, creating a vortex of flame.’’