The Power of Microbursts

A view from the Mount Tom ridgeline shortly after an October 2014 microburst shows clear evidence of the damaging straight-line winds. Photo by John Burk.

In the early morning of October 8, 2014, an autumn thunderstorm unleashed winds of more than 100 miles per hour as it crossed the western slopes of Mount Tom, a familiar landmark in the Connecticut River Valley of western Massachusetts. Within a matter of minutes, thousands of trees in a mile-long corridor were uprooted or snapped. Fortunately, there were no human fatalities. The damage was a testament to the power of microbursts, which as their name suggests, are small columns of sinking air that produce strong straight-line winds on the ground. They were first identified by noted meteorologist Theodore Fujita in the 1970s, after an analysis of storms that had caused fatal plane crashes. As explained by Mount Washington Observatory staff meteorologist and observer Ryan Knapp, “Thunderstorms have an upward and downward component that most often stays in balance, but occasionally, a stronger upward movement causes an opposing reaction to evolve and ultimately send a small shaft of air to the ground.”

The strongest winds, which are at the point where the downdraft reaches the ground, can exceed 100 miles per hour, comparable to a small tornado. The outflow then spreads away from the initial contact point, like water being poured onto a floor, until friction causes the winds to dissipate within a matter of seconds or minutes. Downbursts where the swath of damaging wind is less than 2.5 miles in diameter, as was the case at Mount Tom, are classified as microbursts, while those that impact larger areas are called macrobursts. When a group of storms combine to produce consistent straight-line winds along a front that’s hundreds of miles wide, it’s called a derecho. Because of their rapid formation and short lifespan, all of these phenomena are notoriously difficult to forecast.

Microbursts occur with greater frequency than tornadoes, which require more complex weather conditions to form. The orientation of fallen trees is one of the primary clues meteorologists look at when trying to determine the type of storm that struck. Tornado damage produces a swirling pattern indicative of rotating winds, while downburst windfall is linear or radial. It is possible for a storm to produce both straight-line and tornadic winds, which happened during the July 2006 storm that damaged the Wendell State Forest in central Massachusetts.

Though microbursts in the Northeast are most likely to occur in interior regions, where mountains and hills pinch the wind into a thinner slice of atmosphere, causing it to accelerate, they can strike anywhere. Microburst storms in 2014 caused damage in areas ranging from Mount Mansfield, Vermont, to the Maine and Massachusetts coasts. In July 1995, straight-line winds in New York’s Adirondack Mountains damaged 125,000 acres of forest and brought back memories of the 1950 derecho that blew down 800,000 acres. The strongest wind gust ever recorded in the U.S. was at the summit of Mount Washington and was of the straight-line variety.

Because of its cool climate, the Northeast generally experiences fewer thunderstorms than hotter regions of the country, and therefore has fewer microbursts. The storms in the eastern U.S. are considered wet microbursts, meaning they are associated with rainstorms. In arid regions, especially west of the Rocky Mountains, dry microbursts often occur without accompanying precipitation.

Though forest damage often appears catastrophic following microbursts, such disturbances are part of the natural cycle. Some trees that are blown down can still be suitable for lumber, while the remainder can often be salvaged for firewood or chips. And when left undisturbed, the fallen wood, snags, and early successional regrowth will benefit a variety of wildlife that have suffered from the loss of such habitats in recent decades.

 
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