A WALL OF WIND (by Glenn Field, NWS - Taunton, MA)

The thunderstorm is one of the most common, most awesome, and most deadly inventions of Mother Nature. All thunderstorms produce lightning (that's what causes the thunder!). But they also have the capability of producing downburst winds, gigantic hailstones, flooding rains, and in some cases tornadoes.

On a typical summer afternoon, with sufficient heating and moisture in the atmosphere, puffy cumulus clouds form and begin to billow up. If the air is unstable enough (cooling rapidly with height, for example), they continue to develop into cumulonimbus clouds (thunderstorms).

A characteristic "anvil" shape forms at the top of the storm, where it flattens out upon encountering the stable stratosphere. In most cases, the upward motion caused by the afternoon heating is cancelled out by the downward motion from the falling rain and the storm will dissipate within about 30 minutes.

Certain mechanisms in the atmosphere (such as very strong upper level winds, a clockwise turning of the wind direction with height, an approaching cold front, etc.), can help sustain a storm for hours at a time, because the updraft is not cancelled out by the downdraft. These storms can become severe.

The National Weather Service defines a severe thunderstorm as one capable of producing hail 3/4" in diameter (dime/penny size) and/or wind gusts 58 mph. Keep in mind that tornadoes can be spawned by severe thunderstorms, especially those that have produced hail approximately golf ball size or larger. Although heavy rain and lightning can be deadly, they are not considered to be distinguishing criteria for a "severe" thunderstorm, since even the smallest thunderstorm can produce them.

A downburst is a strong downrush of wind from a thunderstorm. The damage generally is in a straight line and spreads out with distance. Small downbursts, known as "microbursts," can be deadly to aviation. Downbursts with damage swaths more than 2.5 miles wide are called "macrobursts." The damage can be just as extensive as that of a small to medium tornado. Downbursts can cause a roaring sound like a train and can slice off large trees. Thus, they often are mistaken for tornadoes. The strongest downburst ever recorded was 158 mph at Andrews Air Force Base, MD in 1986. On May 21, 1996, a downburst caused extensive damage in Plymouth County, MA where winds were clocked at 104 mph. Brockton, Whitman, and Abington were the hardest hit towns. Nine elementary school children were killed in East Coldenham, NY when a microburst blew down a cafeteria wall in November, 1989.

Small hail can damage crops. However, when hail reaches nickel to quarter size, it begins to dent cars. You definitely do not want to be caught in a hail storm with baseball size hail falling from the sky! There were accounts of baseball size hail (2.75" diameter) during the Worcester, MA tornado in 1953 and golf ball size hail (1.75" diameter) in Berkshire and Hampden Counties in MA during the May 29, 1995 killer tornado in Great Barrington, MA. Three weeks later, on June 20, 1995, baseball size hail fell in Ellington, CT from a severe thunderstorm. The largest hail recorded in MA since 1960 was in Essex County, MA on August 28, 1965, when 3.5" diameter hail (between tea cup and grapefruit size) was observed!

Stationary thunderstorms, or storms that repeatedly traverse the same area, can result in flash flooding. On June 13, 1996, 5.5" of rain occurred in just three hours in Montague, MA and Leverett, MA, sending the Spaulding Brook and Sawmill Creek roaring out of their banks. All but one access road into the town of Leverett was washed out in what some said was the worst flooding since the Great 1938 New England Hurricane. Damage was estimated near $2 million.

Lightning bolts are hot...50,000C...yet amazingly, people struck by lightning can be revived by prompt mouth-to-mouth resuscitation (if not breathing) or CPR (if no pulse). Victims do not carry an electrical charge. Lightning is attracted to metal, so you should be aware that hair pins, belt buckles, money clips, dog leashes, umbrellas, and golf clubs all attract lightning. Lightning also is attracted to the tallest objects, so you should not seek safety underneath a large tree, especially an isolated tree. If outdoors, do not stand in an open field. Stay away from fences, poles, etc. Water conducts electricity so you should get out of swimming pools. Ideally, you should move indoors to a place of safety, keep off the phone, and stay away from doors and windows. If no building is available, get inside a car (hard top, not a convertible) but do not touch any metal. It is not the rubber tires of the car that protects you; rather the metal "Faraday cage" of the car's hard top. Lightning can strike up to 15 miles outside of the main portion of the thunderstorm. This has implications for outdoor sporting activities. You should wait at least 10 or 15 minutes after the rain has ended and the last rumble of thunder is heard before resuming play.

The National Weather Service issues a Severe Thunderstorm WATCH when conditions are favorable for the development of severe thunderstorms over a large area. You should go about your normal business, but pay close attention to the weather. The National Weather Service issues a Severe Thunderstorm WARNING when its state-of-the-art doppler radar indicates that a severe storm is imminent or occurring, or if a timely, reliable report of large hail or damaging winds has been received. You should seek immediate shelter until the storm has passed.