THE VERMONT WEATHER BOOK by David Ludlum (Vermont Historical Society, 1996)
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During the days of January, the solar angle at noon increases its altitude above the horizon by 5°30´, and the duration of daylight lengthens by about fifty minutes, resulting in an increase of 17 percent in the amount of insolation received over its annual nadir at the solstice in December, yet the increased thermal input does not make a sensible impression on the coldness of the Vermont atmosphere until the very last week of January. The “turn of winter” comes about January 26, more than a month after the solstice, with a gradual increase in the normal mean temperature following day by day. January 26 also marks the midway point of the winter heating season. Half of the winter’s total of heating degree days should have been accumulated by then, and one should have at least half one’s woodpile left.
In concert with the jet stream, the cross-continental storm tracks also attain their southernmost locations. The St. Lawrence Valley stormway continues to serve as the main path for cyclonic disturbances moving eastward from the Great Lakes. During their passage to the north, Vermont lies in a southerly airflow for a brief period. If the precipitation begins as snow, it may turn to rain for a while, then return to snow showers with the arrival of the cold front and northwest winds as the storm system to the north passes eastward.
Occasionally, storm centers coming from the Lower Lakes region or the Ohio Valley cross New England or pass just to the south in the vicinity of Long Island. Then a northeast flow prevails over the Green Mountains, and the precipitation falls as snow. At this time of year coastal storms (nor’easters or sou’easters) carry an abundant supply of oceanic moisture as they travel along the Atlantic seaboard and bring a threat of heavy snowfall to portions of Vermont. The exact trajectory of the storm center determines the distribution of snow over the Green Mountain region. In general, if the center passes inside Cape Cod over southeast Massachusetts, all of Vermont may get a heavy snowfall; if outside the Cape, only the southeast corner usually gets substantial amounts.
The principal high-pressure areas that direct atmospheric traffic across the continent have also retreated to southerly latitudes. The core of the Azores-Bermuda high-pressure area lies well offshore, allowing coastal storms to form along the South Atlantic coast and track northeast toward New England. The most frequented path of anticyclones over the interior of the continent from northwest Canada passes west of the Great Lakes. Vermont receives a strong northwest flow of very cold polar or of arctic air during each high pressure intrusion into the United States. Once the gusty winds subside and the atmosphere becomes relatively stable, sparkling days, clear and calm, ensue and supply a delightful interlude to the normally stormy midwinter scene. Sunshine during January increases to about 39 percent of the possible, a rise of 9 percent from the low point during the gloomy days of November.
Mean temperatures in January range from 13.8° at Newport in the northeast to 21.1° at Bennington in the southwest. The mean daily minimum at Newport averages the low figure of 3° and is probably close to zero or slightly below in the cold hollows that abound in surrounding valleys. All stations except Vernon in the southeast have a mean daily maximum below 32°. The highest January temperature reported during this century in Vermont was 70° at Dorset and Rutland in 1950, and the lowest, -44°, was reached at Enosburgh Falls in 1904 and 1914, and at Bloomfield in 1934. In the record coldest January 1970, on only two days, the 29th and 30th, did the thermometer rise above freezing in the northeast corner of the state at West Burke.
January receives much less precipitation than December, by 21 percent in the northeast and by 17 percent in the west and southeast sections. The southeast has more precipitation by about one third than the rest of the state, mainly because of its proximity to coastal storm tracks. Mountain stations, as usual, report the greatest amounts; locations above 2,000 feet in the south, exposed to an easterly flow of Atlantic Ocean moisture, average as high as 4.5 inches, and valley exposures receive about 3 inches. Much drier conditions prevail across the north, where a continental airflow lacking precipitable moisture predominates; some station totals fall below 2 inches for the month.
The January thaw is an interesting feature in the midwinter calendar. Weathermen refer to its occurrence as a singularity comprising a reversal in the seasonal temperature trend. A thaw does not come every January, but statistical studies for the Northeast have demonstrated that warm spells may be expected toward the end of the month in a sufficient number of years to affect the normal trend of mean temperatures temporarily. A slight increase appears at the beginning of the last ten days in January and is most noticeable from January 23 to 26; then a slight decrease follows until a definite upward trend comes with the advent of February. Afternoon temperatures often rise into the forties and fifties during the January thaw, and nighttime readings may remain above freezing for a night or two. If the thaw is sustained, ice may break up in the rivers and lakes, creating ice jams and local flooding.
A typical January thaw occurred in 1972. On the morning of January 22, temperatures were generally below zero, though several locations registered single figures. The next day above-freezing conditions occurred at all stations, mainly in the high 30s with some western stations in the 40s. On the 24th, readings soared into the 40s, with a maximum of 50° at Chelsea. Thermometers again mounted to similar readings on the 25th throughout the state, but on the 26th only in the south were the 40s reached again. A cold front entering the northwest early that morning, pushed southeast during the day and overran the entire state by evening. During this period, temperatures over the state ranged from a low of -15° on the 22nd to 50° on the 24th. Readings were generally below zero again late on the 26th as the January thaw vanished.
