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
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