The Year Without Summer by William K. Klingaman, Nicholas P. Klingaman Page A

west
     to east, the winds drag warm air from the south ahead (i.e., to the east) of the low-pressure
     centers. When this warm air meets colder air, such as was present across New England
     on June 3 and 4, the warm air slowly rises, resulting in steady rain and occasionally
     in thunderstorms. While these warm fronts are usually benign, lows are often followed
     by sharp cold fronts, due to the winds pulling cold air from the north. It is cold
     fronts that most often cause thunderstorms and tornadoes, as the sudden influx of
     cold air causes the existing warm air to rise quickly.
    Highly unseasonable, frigid air lurked behind the cold front of the low that crossed
     the Great Lakes on June 5. In a weather pattern more typical of winter than summer,
     a polar high-pressure system was following the low. In summer, Arctic air is usually
     contained north of Hudson Bay by the subpolar jet stream: strong westerly winds high
     in the troposphere that effectively form a barrier to weather systems. Occasional
     southward excursions of this jet stream in winter can produce frigid, but often clear
     days across the Great Plains and Eastern United States. First in May and then again
     in June 1816, however, the jet stream dipped far to the south, forming a U-shape and
     allowing Arctic air to flow from northern Canada as far south as the Carolinas. The
     collision of this air with the warm, moist air masses that normally prevail in New
     England and eastern Canada produced powerful storms.
    Limited weather observations from the early nineteenth century and the chaotic nature
     of the atmosphere make it difficult to determine with certainty why the jet stream
     moved so far south. One explanation is that a broad area of high pressure, a “blocking
     high,” had developed in late May in the central Atlantic. These systems impede the
     normal west-to-east flow of the jet stream, causing it to shift north and south to
     avoid the block. The effect then cascades backwards and forwards along the jet stream
     in waves, disrupting the jet stream for thousands of miles in each direction and forming
     the type of U-shaped bends that affected eastern North America in 1816. As with water
     moving through a clogged pipe, the block slows the movement of weather systems, stagnating
     the weather and allowing extreme conditions to persist for longer than they might
     otherwise. A slow, meandering jet stream is consistent with the impact of Tambora’s
     aerosol cloud on the North Atlantic Oscillation—a weak polar vortex and frequent incursions
     of Arctic air into the middle latitudes—in the summer of 1816. The aerosol cloud did
     not necessarily cause the early June storm that struck New England, but the stratospheric
     veil almost certainly cooled the air behind the storm and set the atmospheric circulation
     pattern that allowed the air to penetrate so unseasonably far south.
    When the low-pressure center and its trailing cold front passed Lake Erie on June
     5, several Royal Navy ships stationed there reported strong northwesterly gales as
     the polar air rushed in. In New Brunswick, central Ontario, the noontime temperature
     was only 30 degrees. Thunderstorms formed where the air moving behind the cold front
     began to meet the air brought in by the warm front, bringing heavy rain to western
     New York and southern Ontario. The low-pressure center continued to move east, while
     the subpolar jet slipped ever farther south.
    Late on the morning of June 6, the cold front and its powerful northwest wind suddenly
     struck Quebec, turning rain to snow. For more than an hour, snow fell thickly on the
     city streets. When the sky cleared in the afternoon, residents could see the mountaintops
     to the north covered with snow, “the most distant apparently to the depth of a foot.”
     Flocks of birds hitherto found only deep in the forest swarmed into the city in search
     of warmth, “and were to be met with in every street,” reported the