THE VERMONT WEATHER BOOK by David Ludlum (Vermont Historical Society, 1996)
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The summer solstice comes on or about June 21 and marks the beginning of astronomical summer. The sun mounts to its highest elevation, its direct overhead rays shining down at the Tropic of Cancer just south of Key West, Florida, and Brownsville, Texas. Along Vermont’s northern border the sun at noon will radiate its warmth at an angle of about 68°30´, or 76 percent of the distance from the southern horizon to the zenith. The longest day of the year occurs at this time (June 20, 21, or 22nd) with about 15 hours and 20 minutes of direct sunshine. With the sun at its most northerly latitude, twilight extends the hours of adequate visibility by almost an hour and 15 minutes at each end of the day. As a result of complexities introduced by the eccentricity of the earth’s orbit and the tilt of its axis, the earliest rising of the sun comes about June 14 and the latest setting about June 27; thereafter, the sun rises later, and sets earlier, and daylight, accordingly, diminishes in duration.
Meteorological summer begins about two weeks before the solstice. The warmest consecutive ninety-three days of the year at Burlington start on June 6 and extend through September 5. This period is called meteorological summer; the mean temperature in Vermont is 63° or above.
By the opening of June the jet stream approaches its most northern location between 45° and 50°N across southern Canada, and the main storm tracks across the continent migrate northward. The most frequented route traverses Ontario and Quebec north of the St. Lawrence Valley, and storms of Alberta origination cross southern Hudson Bay instead of looping southeast to the vicinity of the northern Great Lakes. Some low-pressure disturbances form over the northern Great Plains, head northeast, and pass over Lake Superior and Lake Huron. Their trailing fronts affect Vermont with brief periods of precipitation and a change of air mass. The Atlantic coast route becomes inactive except when an early tropical storm from the Caribbean Sea or Gulf of Mexico retains its energy after passing over the land areas of the Gulf States. In June 1972 Hurricane Agnes drove north with moisture-laden tropical air and gave the Northeast a devastating deluge.
There is less anticyclonic activity in June. The main path from the Pacific Ocean crosses the continent close to the Canadian border. Over the Great Lakes the high centers are joined by a track from the Canadian Northwest, and the joint path leads east over central New York and southern Vermont. Large high-pressure areas occasionally stall over New England at this time of year and produce a series of fine days favoring outdoor activities.
Temperatures continue their upward surge, gaining an average of about 10 degrees during the month. The June mean at Newport is 62.7°, at Burlington 64.9°, and at both Bennington and Vernon 65.2°. The western section along Lake Champlain is the warmest in the state and will continue so during the entire summer. Thermometers in Vermont have dropped as low as 22° and risen as high as 101° in June.
The summer precipitation regime is in full operation in June. Thunderstorms dominate the scene. Some are of the convective type, that is, formed by rising air currents over the mountains. They develop over the central mountain spine of the state and drift eastward over the Connecticut Valley. Other thunderstorms form over the Adirondack Mountains of New York and travel across the northern counties of Vermont to New Hampshire. Many thunderstorms, especially the most violent, are associated with the passage of a cold front that lifts the warm, moist air necessary for the formation of a thunderhead. They may form in lines from north to south and subject the entire state to heavy downpours of rain, the so-called gully-washers, accompanied by loud thunder and vivid lightning.
The northern section in June is slightly wetter than either the western or southeastern. Newport has an average of 4.16 inches, while Vernon’s normal catch is 3.65 inches. Some of the mountain stations report amounts in excess of four inches, led by Peru with 4.34 inches. Dorset in a high valley in the southwest has the top figure for the state of 4.28 inches.
