Marine Glossary

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downburst

A (rare) strong and potentially destructive downdraft of wind in a squall or thunderstorm, usually of short duration (5 to 30 minutes) and sudden onset. The downburst winds fan out when they hit the water leading to dangerous horizontal winds at the surface emanating from the point of impact. See note at the end on terminology downdraft vs.downburst.

Depending on size (ranging from less than 1 kilometer in horizontal extent to tens of kilometers), they are classified as either a microburst (less than 2.5 kilometers) or macroburst (greater than 2.5 kilometers in extent). The strength of the winds, however, does not depend on the size of the downburst. Both can have winds well over 100 knots in extreme cases.

On August 1, 1983 a microburst peaking at 120 knots hit the runway at Andrews Air Force Base just 7 minutes after Air Force One landed with President Reagan on board. The runway was dry at the time of the aircraft landing with an average wind of about 17 knots. Just 10 minutes after the gust the wind was back down to 10 knots or less.

On July 7, 1984 a microburst embedded in a macroburst capsized a 92-foot sternwheeler "Scitanic" on the Tennessee River with the subsequent loss of 11 lives. Rain began at 1120 CDT with 30-kt winds from 270. In the next couple minutes the wind built to 60 knots from 290, and the captain turned the vessel into the wind. At 1125 a strong microburst gust (strong compared to 60 knots!) struck the vessel from about 90° to the left (from 200°) which capsized the vessel. By 1130 the wind was calm.

The schooner "Pride of Baltimore" (replica of the historic Baltimore clipper) enroute from the Virgin Islands to Chesapeake Bay was struck by a severe downburst at midday on May 14, 1986 at about 23° N, 67° W with the loss of the vessel and 4 lives. From reports of the 8 survivors, the wind increased suddenly and briefly to 70 to 80 knots from an average wind of 25 to 30 knots which capsized the vessel. They took on water quickly through an open companionway hatch and sank.

G131 shows the development of downburst winds in a squall.

Note that all squalls have some region or phase of "downdraft" winds, meaning wind from above pointed down towards the surface. Not all of these incidents, however, are rightfully called "downbursts," which by definition means some level of "strong" or potentially destuctive winds. Much of the terminology originates with Professor Fujita of University of Chicago who did so much research on the topic (see book list in the Source Book). This research was focused in part on aviation and other commercial concerns.

Small craft under sail are particularly sensitive to sudden onsets of new winds, and in these cases a "downdraft" might more often be considered a "downburst." In any event, I mention this because in the various discussions of squalls throughout this course this distinction in nomenclature is not made. In this course, the word downburst is meant to impy any downdraft that might influence the stability of a sailing vessel. There are certainly cases where it might have been better to use the the word drowndraft.