Excerpt: 'Weather's Greatest Mysteries Solved!' by Randy Cerveny

Previous research had shown that the Great Plains–type of storm undergoes a specific life cycle. When a thunderstorm cell -- a small individual storm -- begins its existence, it consists of air being violently uplifted into the upper atmosphere. As the air rises, it cools and eventually reaches its dewpoint, the temperature at which condensation occurs -- and a cloud forms. As the thunderstorm cell matures, some parts of the thunderstorm cell containing raindrops begin to fall and begin to pull the air down with them. At this stage, the storm has both updrafts and downdrafts. Finally, as the thunderstorm cell begins to dissipate, the overall motion in the storm is downward.

What if, thought Fujita, occasionally the downward winds were incredibly concentrated from a single thunderstorm cell -- perhaps when a sudden blast of cold rain forced the air down? We have long known that large-scale downdrafts -- sinking air associated with a line of thunderstorms -- can occur. For example, in the desert, these massive downdrafts, which crash into the ground and spread outward, have been observed to create massive lens-shaped walls of dust. Such dust walls are commonly called haboobs in the Middle East. These gust fronts are formed along the leading edges of large domes of rain-cooled air that result from the merger of cold downdrafts from adjacent thunderstorm cells. At the leading edge of this gust front, there is the dynamic clash between the cool, out-flowing air and the warm thunderstorm inflow that produces the characteristic wind shift, temperature drop, and gusty winds that precede a thunderstorm. These gust fronts were long thought to be the main wind shear threat presented by thunderstorms to aircraft during takeoff or landing.

But, Fujita theorized, what if much smaller individual thunderstorm cells could produce concentrated downdrafts of greater intensity than the more massive gust fronts? The University of Chicago professor conjectured that the crash of Eastern Flight 66 might be due to the impact of a small-scale, jetlike downdraft that Fujita labeled a microburst. Specifically, he defined a microburst as "a small downburst with its outburst, damaging winds extending only 4 kilometers (2.5 miles) or less from a central origin point. In spite of its small horizontal scale, an intense microburst could induce damaging winds as high as 75 m/sec (168 mph)." The key point here is the size of the event. While a downburst involves any sudden downdraft of air, a microburst is a concentrated, small-scale air blast.

In essence, one can picture a microburst as a massive air bomb -- a bomb that explodes air down to the ground and then sends it abruptly outward with wind speeds comparable to those of tornadoes. That downward blast of air produces massive wind shear, a rapid change in wind direction or speed. A microburst can produce the severe wind shear with horizontal wind speed changes greater than fifteen knots (roughly seventeen mph) or vertical wind speed changes greater than five hundred feet per minute (around five to six mph in the vertical direction).

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