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
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.