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As Eastern Airlines flight 66 approached JFK Airport in New York City on June 24, 1975, the pilot encountered a thunderstorm. At 175 metres in altitude, 3.5 km from the runway, he turned on the windshield wipers. Ten seconds later, at 130 metres, he put the wipers on high speed. A headwind temporarily interrupted the plane’s descent, but was followed by a strong downdraft that caused an abrupt loss in altitude and drop in airspeed. The pilot powered his engines but crashed 1 km short of the runway, killing all but 12 of the 124 persons aboard.
Other aircraft landing and taking off near the accident time also experienced severe weather. Seven minutes earlier, Eastern flight 902 rapidly lost altitude approaching the runway. Its pilot barely avoided crashing after applying full power at 80 metres and descending to 20 metres.
Meteorologist T. Theodore Fujita was asked to investigate. During the next four years he photographed patterns of wind damage in corn fields and forests from a low-flying Cessna aircraft. He found many cases where corn or trees were blown down in a starburst pattern, as if a jet of descending air had hit the ground and then burst out violently in all directions.
Fujita coined the term “downburst” for this phenomenon. He defined a macroburst as a large downburst with damaging winds extending outward over 4 km, and a microburst as having winds extending less than 4 km. He found that the more common microbursts could have wind speeds equal to or greater than macrobursts — up to 150 mph — and proposed that microbursts are the main cause of aircraft crashes.
Fujita’s observations led to intensive research in the late 1970s and early 1980s. Radar stations were placed in triangular arrays, first near Chicago (at 60-km spacing) and later near Denver, Colorado and Huntsville, Alabama (at 20-km spacing). While colleagues focused on computer analysis of radar data, Fujita drove around photographing storm clouds and microbursts visible as blowing dust. He took photos at the radar stations synchronized with radar data. His Colorado photographs showed that seemingly innocuous high clouds could generate powerful microbursts that were not always accompanied by precipitation. In the moister climate of Alabama, microbursts were accompanied by hail and intense pools of cold air near the surface.
More airplane crashes added urgency to this research. On July 9, 1982, Pan American Airways flight 759 took off from the New Orleans airport into a moderate headwind. Ten seconds later, at an altitude of 50 metres, the plane passed through the centre of a microburst and into strong tailwinds in excess of 60 mph. It rapidly lost altitude and crashed 1.5 km from the airport, killing all 148 passengers and eight people on the ground.
On August 2, 1985, Delta Airlines flight 191 approached Dallas-Fort Worth. Its digital flight data recorder measured both vertical and horizontal wind speeds. It showed that the plane encountered a 30-mph headwind, then a 30-mph downdraft, and then a 60-mph tailwind combined with lateral winds before crashing. Of 163 crew and passengers aboard, 137 died.
Nearly nine years elapsed before another microburst-related accident occurred in the United States. Better radar instrumentation, pilot training in microburst avoidance, and use of flight simulators with realistic wind information from actual microbursts contributed to this turnaround. Fujita’s research likely saved many lives by preventing deadly airplane crashes.
Downbursts and tornadoes are both associated with thunderstorms. A tornado originates near the ground as a fast-rotating column of rising air. A downburst originates at high altitudes as a column of rapidly descending air. On May 20, 1983 the Houston area was hit by a combination of tornadoes and downbursts, with five people killed by eight tornadoes in a 6-square-mile area, and six people killed by nine downbursts in a 1,000-square-mile area.
A thunderstorm initially consists only of rising air currents, as a warm, unstable air mass is forced upwards (for example, by a mountain range or a cold front). As the thunderstorm matures, rising and sinking currents coexist; as it dissipates, sinking currents dominate. Just as hot air rises, cold air falls. T.T. Fujita’s colleagues were surprised by his discovery that Colorado downbursts occur in the absence of precipitation. It turns out that ice and snow in the upper atmosphere, in contact with a rising warm air mass, change from solid to gaseous form (sublimation). This causes rapid cooling and generates a column of cold, heavy air that can descend very rapidly.
For several years after Fujita coined the term “downburst” in 1976, scientists questioned his findings. Because downward wind velocity goes to zero at the earth’s surface, they believed that downdrafts could not damage objects on the ground. Even after his theories won wider acceptance, meteorologists failed to appreciate that the greatest potential for damage occurs near the centre of the diverging outflow, not at the outer edge where wind speeds are lower. Strong winds generated by microbursts probably caused damage that had been attributed to tornadoes in earlier years.
What will climate change mean in terms of future wind damage? It is widely accepted that a warmer atmosphere holds more water, and can produce more of the energy needed for stronger thunderstorms. Partly countering this trend is a likely decrease in high altitude winds that mix cold and warm air masses and initiate thunderstorms, owing to smaller equator-to-pole temperature gradients as the planet warms.
On balance, conditions favourable for severe thunderstorms should increase in the future. But scientists are very reluctant to make predictions. Despite the social and economic significance of wind damage, the summary for policy-makers of the recent report of the Intergovernmental Panel on Climate Change contains no mention of thunderstorms, tornadoes or downbursts.
Scientists also debate whether data on the frequency and intensity of extreme wind are already showing significant upward (or downward) trends. Recent work suggests that tornado incidence appears to be growing more variable: fewer days with tornadoes, but more tornadoes occurring on those days. Perhaps someone following in the footsteps of T.T. Fujita can make new discoveries that will shed additional light on these important phenomena.
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Ole Hendrickson is a forest ecologist and current president of the Ottawa River Institute, a non-profit charitable organization based in the Ottawa Valley.
Photo: West Texan/flickr