Feature Article August 14, 2002
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What was it? A micro-burst, a macro-burst, or a Derecho?by David Brison The storm that raised havoc in the northern portions of Lennox & Addington and Frontenac counties has as yet not been officially classified. One thing is clear, however at least to weather officials; it was not a tornado.
A tornado is characterized by violently rotating columns of air that have contact both with the ground and the cloud base. These types of wind patterns were not noted on radar reports of the storm. In addition, the damage from this storm, as observed both from the air and the ground, appears to result from straight-line winds. For instance, downed trees were all pretty much lined up in the same direction.
Straight-line winds are more likely to result from downdraft or downburst winds. A micro-burst, according to the US National Weather Service, is a small area of rapidly descending air beneath a thunderstorm. When the descending air hits the ground, it quickly spreads out in all directions, causing very strong, straight-line winds. A micro-burst forms, Inside a thunderstorm, [when] water vapour condenses into raindrops, which then fall to the ground. When these raindrops fall through drier air, they start to evaporate. The evaporation cools the air, causing it to become denser that the air around it. This rain-cooled air, along with the falling raindrops, accelerates downwards; it is this down-rushing air that eventually hits the ground causing the damaging straight-line winds.
A micro-burst typically covers an area less than 2.5 miles in diameter. Our windstorm was initially called a micro-burst, but was then referred to as a macro-burst because the area affected was obviously larger that 2.5 miles in diameter. Macro-bursts are the same physically, but cover a much larger area.
After going through the micro-burst and macro-burst explanations, my source at Environment Canada then said that the storm was a Derecho. After sifting through a number of technical explanations of a Derecho, I have concluded that it is best to describe it, as an official with Environment Canada did, as a line of thunderstorms moving very quickly. A Derecho, like a micro-burst and a macro-burst, is a downdraft or down-burst storm.
These storms are difficult to predict, but Environment Canada says that they issued some warnings before our storm hit. Would anyone who heard warnings on radio or television report them to us here at The News. Call 279-3150, fax: 279-3172
