While not as notorious and certainly not as spectacular to witness as a tornado, downbursts, or straight-line thunderstorm winds, are responsible for most thunderstorm related wind damage. This is especially true across southern New England.
A downburst is a strong and relatively small area of rapidly descending air found beneath a thunderstorm. It can result from stronger jet stream winds being transported downward to the surface or it can also often be a result of air within the downburst being cooled significantly as rain evaporates. A downburst differs from a common thunderstorm downdraft because the wind it produces has more potential for causing significant damage. These downburst winds tend to spread out or diverge considerably as they reach the surface. Conversely, the damage patterns resulting from a tornado generally converge toward a narrow central track.
Intense downbursts can be phenomenal. Speeds have been clocked as high as 175 mph a few years ago near Morehead City, North Carolina, and at 158 mph at Andrews Air Force Base in Maryland in 1986. Closer to home, 104 mph downburst winds were measured at both Worcester Massachusetts on May 31, 1998, and Whitman Massachusetts on May 21, 1996. Strong downbursts will definitely cause roaring sounds and people may often refer to a sound like a freight train, which are terms typically associated with tornadoes. Although downbursts are not tornadoes, they can cause damage equivalent to that caused by a weak to moderate tornado.
Downbursts are classified as either macrobursts or microbursts. The difference depends on the areal extent of the damaging wind swath. A macroburst's damage extends horizontally for more than 2.5 miles. A microburst is a small downburst with its damaging winds extending 2.5 miles or less. The small horizontal scale and short time span of a microburst makes it particularly hazardous to aviation as well.
Source: National Weather Service, Taunton, Massachusetts