“I kind of thought it was a tornado,” the woman said, her disappointment apparent, “but they said it was straight-line winds or something.” This was from a newspaper article earlier this summer, after a crazy afternoon of thunderstorms broke up the first big heat wave. The poor woman’s barn had been blown over (fortunately not injuring any cows), and now the weather service was adding to the indignity by refusing to call it a tornado.
In the informal moral calculus of these things, it’s somehow OK if your barn gets blown down by a tornado. Or destroyed during a hurricane. But if a building is leveled during a random wind storm on a Thursday afternoon, there’s a lingering implication that maybe it was somehow your own fault. Perhaps you’d let those sills get too soft or neglected to keep up with the roofing tin?
We need to meet this misperception head on, because it’s working against us here in New Hampshire and Vermont. We’re not tornado country. We do have tornadoes hereabouts – more often than people think – but they tend to be small and short lived. Our terrain is too rough and complicated to allow a Wizard-of-Oz storm to develop. But this same terrain leads to ferocious wind gusts, which can be just as formidable.
Mountain ranges and steep terrain accelerate winds in two ways. The first is the straightforward Bernoulli effect, where mountains and hills pinch the wind into a thinner slice of atmosphere, causing it to accelerate. That’s why winds are, as a rule, higher on the summits and why the Green Mountains and White Mountains have the region’s highest winds. Our mountainous terrain cuts tornados to ribbons but can enhance straight-line winds.
Topography adds a random factor, too, which can lead to isolated yet severe gusts that can take down a building or two. Winds in a thunderhead are moving up and down inside the cloud as well as laterally across the ground. You’ve undoubtedly felt one of these downdrafts during a thunderstorm – they’re the sudden, cold blasts of air on your neck, perhaps 20 or 30 degrees colder than the sweltering summer air of a few moments before.
If one of these downdrafts happens to drop from a cloud just as a thunderhead is, say, crossing the head of the valley just above your house, the downdraft can be funneled by the terrain and focused into much higher wind speeds than would otherwise occur. This is a local effect, and hence not likely to make the evening news. But the resulting wind can be just as powerful and destructive as what you’d experience in a tornado. Entry-level tornadoes have wind speeds as low as 70 miles per hour.
Although tornados and hurricanes dominate the popular imagination when it comes to extreme winds, straight-line winds can be just as destructive and can affect just as large of an area. Meteorologists call large-scale, straight-line storms ‘derechos’, which is from the Spanish for straight ahead (and meant to be in contrast to ‘tornado’, which derives from the words for “thunder” and “twist”.) Derechos are long-lived storms, sometimes lasting for more than a day, in which straight-line winds average more than 58 miles per hour along a front that’s at least 280 miles wide.
The most recent derecho to pummel Vermont and New Hampshire was on the morning of July 15, 1995, when high winds in the southern counties blew down trees, overturned cars, and destroyed buildings. The twin states were on the tail end of that particular derecho, which formed over southern Ontario and covered 800 miles in 12 hours before crossing Nantucket and heading out to sea. The storm’s average speed over the ground was 67 miles per hour; at peak, gusts measured close to 100 mph. Total damage across the region was more than a billion dollars.
Large-scale derechos such as that one aren’t any more common than tornados but do illustrate the point that winds don’t need to be twisting in order to be serious. At our farm a few summers ago, a straight-line gust touched down in our pasture, blew down three acres of white pine, crossed a road, knocked over a trailer, and blew a perfect stripe down the middle of a corn field before dissipating over the Connecticut River.
It was just a garden-variety, straight-line wind event, New England style. Small, local, and not important enough to make the evening news. But serious just the same.
Lest you’re still clinging to the belief that winds have to be twisting to be legitimate, consider the highest surface wind ever recorded by a ground-mounted gage. The speed was 231 miles per hour, the year was 1934, and the location was the Mount Washington Observatory in New Hampshire. And the type of wind? Straight line.
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