THE VERMONT WEATHER BOOK by David Ludlum (Vermont Historical Society, 1996)
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Having a lower angular slant, the sun’s rays must penetrate a greater slice of the earth’s atmosphere and now impart only 65 percent of the radiant energy to the surface in Vermont compared with the high input at the summer solstice three months before. At the end of the month, the noontime sun stands at 46° or about halfway from the horizon to the zenith, having declined about 11 degrees during the course of the month. The period of direct sunlight shortens noticeably, by about 40 minutes at the end of each day.
The jet stream makes its most northerly entrance into the continent in September, then pursues an easterly course close to the 49°N parallel, crossing Lake Superior and southern Quebec and leaving the continent over the Atlantic provinces of Canada. The major storm tracks, too, continue at their northerly positions. Cyclonic activity is mainly confined to Canada where Alberta-type storms ... move east between 55°N and 60°N across Hudson Bay and northern Quebec. Cold fronts trailing from the storm centers introduce cool polar airstreams into the Green Mountain region, permitting temperatures to drop to the freezing mark and below with increasing frequency as the month progresses.
September brings the peak month of hurricane activity . Large tropical storms originate off the coast of West Africa and make the long journey across the broad Atlantic Ocean to the West Indies. They then either continue west into the Caribbean Sea and Gulf of Mexico or recurve to the northwest and then north along the Atlantic seaboard of the United States or over the offshore waters. The most recent examples of this type that affected Vermont were the Long Island-New England Hurricane in September 1938, Donna in 1960, and David and Frederic in 1979.
The principal September anticyclone tracks resemble those of August. A well-defined primary path extends in a zone across the northern tier of states near parallel 45°N. The northern plains lead all areas in the frequency of high-pressure systems, which blocks storm generation in the middle of the country. An early portent of autumnal conditions appears in the renewal of high-pressure generation in the intermountain region of the West, whence a secondary track leads east to a center of growing importance in West Virginia and the central Appalachian area.
Mean temperatures decline by about 7 to 8 degrees in the course of September, and a chill in the morning and evening atmospheres becomes quite noticeable. Newport has a mean of 56.7° and Vernon of 60.5°. Thermometers in recent Septembers have ranged from a high of 100° at Bellows Falls and Vernon in 1953 to a low of 15° at Dorset in 1947. Burlington has experienced a smaller range: 95° to 25°.
Precipitation in September shows a distinct decline from the high figures of the summer months. The influence of thunderstorms in swelling rainfall totals diminishes; on the average Burlington experiences only three thunderstorms this month compared to eight in July. The southeast becomes the wettest section, mainly the result of tropical storms passing along the coast and extending their rain shield to the near portion of Vermont. Stations in the mountains average slightly over 4 inches: Mays Mills 4.08 inches, Searsburg Station 4.37 inches, and Whitingham 4.23 inches. The driest localities lie in or near the Northeast Kingdom: Montpelier 2.90 inches, Gilman 2.99 inches, and St. Johnsbury 3.06 inches. September can be a droughty month when a renewal of cyclonic storms coming from the west is delayed and tropical storms fail to move northward near enough to the seacoast to influence Vermont skies.
