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The landscape of the Portland Street area in St. Johnsbury in 1884. Notice the large areas cleared of trees. Open land heats up and cools off more rapidly than forested land.

If April teases us with warmth, and May offers us some encouragement as the landscape greens and the gardens get underway, then June finally delivers, with the longest days of the year, and potentially, some of the hottest weather of the summer season.

As a rule, June is not the hottest month of the year, in spite of receiving the greatest amount of sunshine, in terms of total minutes. June averages slightly more sun than July and August, plus the days are longer, so the supply of solar energy exists. The difference comes from what is known as “seasonal lag.” Although the length of the day climaxes on June 20, with 15 hours and 32 minutes of possible sunshine, the warmest average temperatures during the year occur about 4 weeks later, thanks to the more gradual heating of water and land. This isn’t a recent discovery from our growing knowledge of atmospheric science. You can extend the credit back to at least the Romans, and quite possibly the Greeks before that. The best example comes from something still talked about over 2,000 years later – Dog Days. The Dog Days of July and August actually refer to the presence of the star Sirius, the Dog star, during the steamy afternoons of July and early August. Sirius blazes forth as the brightest star in the heavens, primarily seen in the winter. Naturally, you can’t see the star in the summer daylight, but the ancients thought its light, combined with the heat of the sun, made those days the hottest of the year. Their observation of the timing of summer’s heat was right on, but their reasoning was not.

A look at the number of heat waves by month illustrates the effects of this seasonal lag. Over the 125 years of records at the Fairbanks Museum, June has registered 14 heat waves – that is, three or more consecutive days when the temperature reaches or exceeds 90 degrees. July finds more than twice that number, at 31, while August sees the number decrease to 17. That is not to say June does not measure up in terms of hot weather. At the Fairbanks Museum, our hottest temperature ever recorded, on July 3, 1911, pushed the mercury to 101 degrees Fahrenheit. Later that decade, in June of 1919, one century ago, we equaled that figure during one of the most intense heat waves on the record books.

The spring of 1919 offered no clues to the sweltering heat that would bake northern New England in early June. Following a mild, less-than-average snowy winter, March warmed as expected, until a late-season snowstorm from the 27th to the 29th brought eight inches of snow, and sharply colder weather, lasting into the first week of April, dropping to eight degrees on the 3rd. The remainder of April, and the bulk of May settled back into a normal pattern, with both temperatures and rain showers fitting the long-term averages.

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Figure 1

By late May, the weather pattern favored high pressure through Ontario and western Quebec, extending into the southeast U.S. Initially, cool air over Canada controlled our weather, with nights in the 30s and 40s, while daytimes warmed to the 70s. At the same time, the stagnant high pressure over the southeastern U.S. allowed heat and humidity to build. On June 2, this high moved off the East Coast, while cool air to our northeast retreated, as seen on this map (figure 1) allowing the temperature to reach 90 degrees that afternoon. By June 3, the high-pressure area crested over Bermuda (Figure 2). This location created a stifling southwesterly airflow, pushing any cool air here into the Canadian Maritimes. That’s when the real sauna arrived, as the temperature reached 100 degrees in the back yard of the Museum on the afternoon of the 3rd. For the next two days, the heat reached an all-time high, with 101 degrees on the afternoon of the 4th, tying the record for the hottest temperature ever recorded at the Museum, set on July 3rd, 1911. The thermometer on the next day, June 5th, also registered as 101, marking the only time that the temperature reached 100 degrees or more for three consecutive days. At 4 p.m. on the 5th, the temperature “cooled” to 98 degrees, and appears to be the temperature recorded as the maximum for the next day, the 6th. The official 24 hours at the Fairbanks Museum runs from 4 p.m. to the next day at 4 p.m. Although 98 degrees was the maximum for the 24 hours ending at 4 p.m. on the 6th, the actual temperature at 4 p.m. was only 73, suggesting that the day itself was much cooler.

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Figure 2

The relief from the heat on the 6th came from a cold front that swept east through the area, though only squeezing out 0.07 inches of rain. The cooler weather was brief, as the heat returned, in modified fashion, about every five days, lasting for two to three days at a time, near the 15th, the 20th, and the 25th. The final cold front that pushed through on the 26th rattled the windows with some thunderstorms and soaked much of the Northeast Kingdom with an inch of rain. Behind this front, the region was in for a shocking change, as temperatures on the morning of the 29th, slipped to the upper 20s and low 30s, with scattered frost. Gardeners may have been caught off-guard by this plunge in temperatures, as the reading of 33 degrees at the Museum set a record for the coldest so late in the season.

Rarely do we see such temperature extremes today. While there are climate factors at work, the explanation can be seen with an image (figure 3) of the landscape of St. Johnsbury, dated to 1884. Notice the large areas cleared of trees. Open land heats up and cools off more rapidly than forested land. This change accounts for at least part of the changes in temperatures we have seen over the past several decades. I’ll discuss that in more detail at some point in the future.