THE VERMONT WEATHER BOOK by David Ludlum (Vermont Historical Society, 1996)
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July is the warmest month of the year in Vermont, showing an increase of four to five degrees over June at all stations. Mean temperatures reach their highest point about July 20, a full month after the solstice. Thermometer readings over the different sections of the state are more uniform than in any other month: in the northeast Newport’s mean is 67.1° and in the southwest Bennington has 69.1°. Several localities average slightly over 70°, such as Cornwall in the flat Champlain Valley 70.3°. The highest July temperature reading ever registered in Vermont was 105° at Vernon in 1911 and the lowest was 30° at Somerset in 1926.
From June to July a continued northward shift of most features of the general wind circulation normally takes place, and a corresponding northward displacement in the latitude of storm movement results. No main storm tracks prevail within the United States at this time, though occasional cyclonic developments may take place over the northern Plains. These move directly into Canada and pass over Quebec and Labrador well north of Vermont. The main continental storm path crosses central Hudson Bay near latitude 60°N. Trailing fronts from these disturbances may affect northern New England by bringing fresh Canadian air southward to temper a summer heat wave. Tropical storm activity increases in the vicinity of the West Indies, but few disturbances move north along the Atlantic seaboard this early in the season.
A center of anticyclonic activity prevails over the cool waters of the Great Lakes and Hudson Bay; ridges of high pressure from this activity often break away and drift eastward over Quebec and northern New England, attended by cooling northwest winds and sparkling skies, providing some of our best summer days. In more southerly latitudes, the Bermuda-Azores high-pressure area moves north with its axis extending west over the coastal plain as far as the Appalachian Mountains. It is to blame for causing some of the extended heat waves that plague the Northeast each summer. The wide-ranging circulation around the western periphery of the Bermuda High stimulates a vast flow of southwest winds that carry heated air from the interior of the continent to the Green Mountain region.
The jet stream flows across the continent along a northerly latitude trending east-northeast from Oregon and Washington and passing north of the Great Lakes to make an exit over the Atlantic provinces of Canada.
The distribution of precipitation over Vermont in July is quite uniform, as in June. The western section receives about the same amount as the southeast, with the northern section leading both by a slight margin. Burlington averages 3.43 inches and Newport 3.95 inches. Thunderstorms are responsible for the bulk of the precipitation; Burlington reaches the season’s peak with an expectancy of eight thunderstorms during the month. The mountain stations continue the wettest with about four inches of rainfall.
St. Swithin is the patron saint of weathermen, in honor of the hoary tradition that, if it rains on July 15, rain will fall for forty days thereafter. This is supposed to have occurred when the remains of the worthy Bishop of Winchester, who died in 862 A.D., were to be transferred from the churchyard to a more honored place within the church, but a long series of rainy days delayed completion of the removal. Many European countries have similar saint’s days with the same tradition, but on different summer dates.
The Vermont Chapter of the St. Swithin Society does not have much positive evidence to support its reason for existence. A study of the past fifty years of records for Burlington reveals that the greatest number of consecutive rainy days following a wet July 15 is only five. In 1961, it rained on every day from the 15th through the 20th, but on three of the days only traces occurred. In 1955, four days with measurable rain followed, but all other cases were three or less. In fact, it has rained on July 15 on twenty-three of the fifty years since 1931. So it is fun to talk about St. Swithin once a year, but he really does not exert much remote control on Vermont weather.
