May 19, 1780 - New England's Darkest Day

On this day in engineering history, a strange darkness fell over New England. Along the western border of Vermont, the sun rose in near-darkness. By mid-morning, students at Harvard College in Cambridge, Massachusetts were forced to study by candlelight, prompting Professor Samuel Williams to remark that "the darkness was so great that persons could not see to read common print in the open air". As rains began to fall, Williams noted that the drops "have an uncommon appearance, being thick and dark and sooty." Later, when the gloom departed for the East, student Nathan Read rejoiced that "cocks have continued to crow as at day breaking". By evening, residents of Cape Cod shuttered their cottages for a night that came all too early.

What happened across New England on May 19, 1780? From the many diaries available from that dark day, meteorologist David McWilliams Ludlum reconstructed the events. In a 1972 study called "New England's Dark Day", the former weather forecaster for the U.S. Army Air Corps claimed that a dark "cloud" traveled a distance of 180 miles over 7.5 hours. Advanced at a speed of 25 mph, Ludlum's "cloud" combined sooty smoke from distant forest fires with a low pressure-trough frontal system. For most of the month of May, New England had enjoyed cool temperatures and clear skies, indications of an anti-cyclonic influence from Canada. On the morning of May 19, however, a southwest wind began, bringing falling barometric pressures and warmer temperatures to the region.

Although many diarists described an eerie haze which could have been caused by forest fires, Harvard College's Samuel Williams provided the closest scientific explanation. In positing that the "remarkable darkness" was due to an atmosphere laden with "vapours", Professor Williams reasoned that some highly-charged substance refracted, reflected, absorbed, and ultimately weakened much of the incoming sunlight. Williams' "vapours" probably weren't so mysterious, however, especially in light of subsequent events. In September of 1881, smoke from forest fires in Ontario and Michigan covered the northeastern U.S. in an eerie, sunlight-dampening haze. Over 75 years later, "The Great Smoke Pall" of 1950 sent smoke from forest fires in western Canada to the eastern U.S. Most recently, smoke from forest fires in Quebec sent a smoky haze as far south as Washington, D.C during the summer of 2002.

Resources:

http://www.islandnet.com/~see/weather/almanac/arc2004/alm04may.htm

http://www.vermonthistory.org/arccat/findaid/ludlum.htm

http://en.wikipedia.org/wiki/New_England's_Dark_Day