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atmosphere. While the editors of the North American Review dismissed such speculation, they did admit that “the observation … that the light
     of the sun is less brilliant and dazzling than usual, is unquestionably well founded.
     We have remarked at different times during the present season, on days when the sky
     was perfectly clear, that there was a degree of feebleness and dimness in the Sun’s
     rays, not unlike the effect produced by a partial eclipse.”
    Yet the first four months of 1816 were not noticeably colder than normal in the Eastern
     United States. In New England, the winter had been one of the mildest in a decade,
     with significantly less snow than usual. “The winter was open,” noted Noah Webster
     in his diary at Amherst, Massachusetts. “A snow in January, which was sufficient for
     sledding, was swept away in a few days. The ground was uncovered most of the winter.”
     Judging by the measurements of several amateur meteorologists at Northeastern colleges,
     January’s temperatures appeared to have been slightly above normal, with a warming
     trend at the end of the month. In Maine, the days were so pleasant “that most persons
     allowed their fires to go out and did not burn wood except for cooking.” Similarly,
     the Connecticut Courant reported that “January was mild—so much so as to render fires almost needless in
     sitting rooms.” (Thomas Jefferson, on the other hand, wrote to a friend from his retreat
     at Monticello, just west of Charlottesville, Virginia, shortly after New Year’s Day
     that he was “shivering and shrinking in body from the cold we now experience.”)
    February brought generally mild temperatures with only a few snowstorms. “The first
     of March was very warm,” noted Adino Brackett, a farmer and schoolteacher in Lancaster,
     New Hampshire, “and almost all the snow went off.” The weather then turned clear and
     cold for several weeks, but the month ended with another warm spell and a rare appearance
     of early spring thunderstorms in the Northeast. There had been sharp cold snaps along
     the East Coast in mid-March, however, including a bout of sleet in Richmond, Virginia,
     that left fruit trees covered in icicles. As winter departed, the first week of April
     was slightly warmer than usual in New England, with very little precipitation.
    Although it appears counterintuitive, the stratospheric aerosol cloud from Tambora
     was partly responsible for both the mild winter of 1815–16 in North America and the
     stormy conditions across central Europe. The aerosol cloud not only scattered sunlight,
     preventing it from reaching Earth’s surface, it also absorbed some of the incoming
     energy, reradiating it as heat. This warmed the stratosphere immediately above the
     cloud. If the aerosol cloud had warmed the stratosphere evenly around the globe, its
     effect would have been minimal. In the depths of winter, however, the high northern
     latitudes are plunged into continual darkness for several months. Without sunlight
     to absorb, the aerosol cloud could not heat the Arctic stratosphere; yet it continued
     to heat the stratosphere in the sunlit middle and lower latitudes.
    A strong, cyclonic vortex forms near the North and South poles each winter. Strong
     west-to-east winds surround the vortex and expand to cover much of the high latitudes.
     These winds are created by the difference in winter temperatures between the sunlit
     middle and perpetually dark high latitudes: Air always flows from warmer temperatures
     toward colder ones, but Earth’s rotation turns the air off its path, towards the right
     in the Northern Hemisphere and the left in the Southern, to produce westerly winds.
     These westerly winds prevent cold, polar air from moving into the middle latitudes.
     When the vortex is particularly strong, lower atmospheric pressures exist near the
     pole; higher pressures are found in the middle latitudes; and the westerly winds provide
     an