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Weather Almanac for October 2003
ECLIPSED BY STORM
Sometimes, I find it hard to visualize Benjamin Franklin as only one man. During his long, eight-plus decade life, he not only participated in the founding of the United States as a radical thinker, statesman and negotiator, but he found time to be one of the foremost philosophers, scientists and inventors of the day. Perhaps he is best known outside politics for his legendary kite flight into a thunderstorm, and studies of electricity, the invention of the lightning rod, Franklin stove and bifocals, but he also "discovered" the Gulf Stream and proposed a form of daylight savings time long before life was run by clocks.
On the bicentennial of Franklin's birth, patriarch Weather-Bureau forecaster Cleveland Abbe hailed him as "the pioneer of the rational long-range forecasters, and of the physical meteorologists who will, undoubtedly, in the future develop this difficult subject." Franklin was undoubtably America's first meteorological scientist. We know his interest in weather and climate covered at least six decades of his life. While others before him had been weather observers, few set out to explain weather phenomena as Franklin did. He was the first American storm chaser, or at least the first to report on a chase, when in 1755, he took off after a large whirlwind or dust devil on horseback. Franklin also studied evaporation and raindrop formation as well as correlated the Gulf Stream with European climates and an extensive summer "fog" likely the dust from a large volcanic eruption (Asama, Japan) in 1783 on the cold winter that followed. In 1743, Franklin was to have an insight that would be crucial to the development of weather science and weather forecasting. He deduced from concurrent observations, that storms on the American East Coast likely travelled there from the southwest, despite contrary evidence from their winds, and formed from rising air masses.
A Lunar Eclipse Eclipsed
The astronomical almanacs forecast a lunar eclipse for the Autumn of 1743. Franklin, then 37 years old and a Philadelphia newspaper and almanac publisher and editor, had intended to observe that eclipse the evening of 21 October (1 November by our current calendar). When he ventured outdoors at 8:30 pm to view the sight, Franklin was disappointed to see the sky covered with cloud. He much later wrote about the event to his friend Jared Eliot, noting that he was unable to observe the eclipse because "before 8 a Storm blew up at N E. and continued violent all Night and all next Day, the Sky thick clouded, dark and rainy, so that neither Moon nor Stars could be seen."
The disappointed Franklin was later surprised to read accounts of the eclipse in the New England newspapers. He wrote his brother in Boston to confirm the information and was told the stormy weather arrived there about an hour after the eclipse had ended. The prevailing weather theory of the day held that storms moved forward linearly with the general direction of the wind. Since Franklin observed strong northeasterly winds with the onset of the storm, he believed that the storm had travelled from the northeast, the direction of Boston from Philadelphia, to him. Therefore, storm clouds over Boston should have obscured the eclipse unless the storm had already left Boston. But not only were Boston's skies open for the lunar event, Franklin's brother had said that the storm arrived after the eclipse. "This puzzles me, because the storm began with us so soon as to prevent any observation; and being a northeast storm, I imagined it must have begun rather sooner in places farther to the north-eastward than it did in Philadelphia."
The storm that foiled Franklin's viewing of the lunar eclipse was indeed a violent one. By all accounts, it was most likely a tropical storm, perhaps a hurricane, when it reached the northern American coast. Not only would this storm spawn Franklin's storm theory, but it would be the first extensively measured by scientific instrumentation. At Harvard College, John Winthrop, professor of natural history, had begun regular weather observations using scientific instruments the previous year. Thus, he chronicled the weather conditions on October 22 as the storm passed over Cambridge.
"NE by N. worst in years great damage on land as well as at sea. Barometer 29.35". Tide within 4' of 20 years ago. [the highest tide in local memory] Storm abated about 7 P.M. Barometer lowest at 2 P.M."
The Boston Post Boy reported: "The wind being excessive high vast damage was done to the wharves and shipping, some vessels that got loose were drove ashore higher up than was ever known before, and several small vessels were cast upon the wharves and boats floated onto the streets...Tis impossible to enumerate all the particulars the terrible effects of the storm or estimate the damage sustained by it."
The Boston News-Letter commented: "At noon the wind seemed to blow in prodigous [sic] gusts, and with the greatest fierceness and brought in an exceeding high tide,...higher than has been known for these twenty years past."
At Piscataqua, New Hampshire, the storm surge overflowed the wharves. At Newbury, extensive damage was reported to the fields and salt marshes. Philadelphia caught the edge of the storm, experienced violent gusts of wind and strong thunderstorms.
A Theory Proposed
This serendipitous event set off Franklin's thinking on the nature of storms, and he set out to collect data on this storm and others from travellers, his correspondents throughout the Colonies and newspaper weather accounts. From these, he drew a theory on storms. "I find it to be a constant Fact, that N East Storms begin to Leeward; and are often more violent there than farther to Windward," he wrote Eliot.
Franklin deduced that such large, violent storms that persisted along the coast for several days moved from the southwest toward the northeast, even though their winds blew from the northeast quadrant. His conjecture stopped short of ascribing a swirling circulation to the storms that we now call "cyclonic." (Interestingly, the conjecture that tropical storms were big, swirling masses of clouds was forwarded by late-seventeenth-century British privateer and explorer William Dampier.)
Franklin then put forward possible causes for these observations. The most relevant stated the first hint of the convective cyclone model that would find great favour more than a century later. He knew that hot air rises in the atmosphere and that cool air will flow into the region vacated by the rising air, much like hot air rising in a chimney produces currents from the back of a room toward the chimney. He conjectured that for several clear-sky days prior to a storm's formation, the sun heats the surface air over a large tract of land, for example, the Gulf of Mexico and Florida. The heated air is "rarified" (that is, reduced in density/pressure) and is forced by the cooler and moister air from outside the heated region, such as the northern American coastal region, to rise.
The rise of rarified air and flow of denser cold air toward the ascendent region produces a pressure gradient ("The rarified Air being lighter must rise, and the Dense Air next to it will press into its Place; that will be follow'd by the next denser Air, that by the next, and so on.") that establishes the wind field.
Franklin then suggested the terrain of Pennsylvania and New England influenced the prevalence of northeasterly winds in the region: the famed Nor'Easters: "We have on this Continent a long Ridge of Mountains running from N East to S. West; and the Coast runs the same Course. These may, perhaps, contribute towards the Direction of the winds or at least influence them in some Degree."
Franklin had correctly concluded from his analysis that storms do not necessarily travel from the same direction as the wind blows (he did, however, believe that some storms, such as thunderstorms, did move with the general wind field). He was beginning to see the continental migratory nature of storm systems we now take for granted:
"Of these I have had a very singular opinion for some years, viz: that, though the course of the wind is from northeast to southwest, yet the course of the storm is from southwest to northeast; the air is in violent motion in Virginia before it moves in Connecticut, and in Connecticut before it moves at Cape Sable, etc."
Though his theoretical conjectures were incomplete, Franklin laid the foundation for many aspects of synoptic meteorology which will later be exploited to produce weather forecasts.
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