The Carlisle Mosquito Online

Friday, December 3, 2010

 

Microburst hits Carlisle on October 1, narrowly missing a residence

The destructive power of a microburst is shown clearly in these 50-year-old downed trees. (Photo by Bob Zielinski)

As Tropical Storm Nicole approached New England on October 1, a few trees fell across West and Acton Streets. The Police Log marks the events between 8:30 and 10 in the morning. And, in the middle of a porkchop lot between West and Acton Streets approximately 45 mature pine trees fell over or were fractured at a height of ten to 20 feet. I was made aware of this event when, several days later, Sarah Sampson, a neighbor, called to tell us that some heavy equipment was going to be working in back of our property. She said that when she arrived home just before noon on October 1, she noticed something different: “There was more sunlight in the house.” Indeed, within 70 feet of the back of the house there was a new seven-tenths acre clearing. A UFO landing site? No, she identified it as a “microburst” site.

Although photographs show the damage, it was only after the toppled trees were harvested for lumber and the fractured trees were removed or chipped that the true scope is evident. Many of the trees on the porkchop lot were 50 to 55 years old.

It was through the work of severe storms researcher Tetsuya “Ted” Fujita at the University of Chicago that this type of phenomenon began to be understood. He coined the term “microburst” after he studied two wind-shear-related airplane crashes in 1975. The accidents were caused by the aircrafts’ encountering concentrated thunderstorm downdrafts rather than gust fronts.

Wikipedia offers the following description: “A microburst is a very localized column of sinking air, producing damaging divergent and straight-line winds at the surface . . . A microburst often has high winds that can knock over fully grown trees. They usually last for a couple of seconds.”

Almost an acre is revealed after the cleanup. A house is clearly visible at the far edge of the new clearing. (Photo by Bob Zielinski)

The formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate – this demands a lot of energy so the air is cooled. Cooler air has a higher density than the warmer air around it, so it falls as a “cold air balloon.” As the cold air balloon hits the ground, it spreads out as a gust front. Areas under and immediately adjacent to the downburst are the areas which receive the highest winds. ∆


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