Vermont wind storm
© Green Mountain Power
An unusual "gravity wave" induced wind storm produced wind gusts of hurricane force in parts of western Vermont and eastern New York Friday afternoon. The unusual wind storm occurred suddenly in the late afternoon and appears to be driven by what is known as a gravity wave.

A gravity wave is a wave (think just like the ocean) that occurs in a stable atmosphere. This vertical wave pushes the air up and down just like a ripple in a pond after you toss a rock into it. We can see this perturbation in the atmosphere by looking at the surface pressure on a barometer.

These two pressure traces from Williamstown, MA and Stockbridge, MA in the Berkshires show a sharp drop and rise in the atmospheric pressure just after 4:00 p.m. This happened as the gravity wave moved over Massachusetts - the ripple overhead in the atmosphere produced a quick drop and rise in the pressure.

gravity wave air pressure
Pressure trace from a home weather station in Stockbridge, MA (above) Williamstown, MA (below) showing a sharp drop in pressure as the gravity wave passed.

North of the Berkshires - on the west side of the Green Mountains - the gravity wave was more than just a curiosity. A ferocious period of winds developed as the gravity wave moved overhead with gusts up to 74 mph in the town of Wells, VT and substantial tree damage in the cities of Rutland and Bennington, VT.

East of Albany in the town of Brunswick, NY you can see a big pressure drop (~7mb in an hour) along with a sudden surge of winds up to 65 mph.

Brunswick gravity wave
© Weather UndergroundWind speed/gust graph at the top and pressure at the bottom from Brunswick, NY yesterday afternoon. Notice the sudden jump in wind speed that corresponds with the sudden pressure drop.
The gravity wave appears to have been of a substantial enough amplitude to bring down very strong winds from aloft. The winds from 5,000 feet in the atmosphere were about 75 mph and appear to have mixed right down to the ground as this vertical wave resulted in one hell of an atmospheric ripple. The terrain absolutely played a part in this - as all of the powerful winds occurred just west of the Green Mountains (Killington, near Rutland, is ~4200 feet) and another band of strong winds occurred just west of the Taconic Mountains in eastern New York.

What triggered the gravity wave isn't clear either. Gravity waves are not uncommon - we see small ones all the time during storms but gravity waves that produce a 9mb pressure drop in an hour are very unusual and can produce really nasty winds.