Evidence suggests that the anomaly was predominantly a volcanic winter event caused by the massive 1815 eruption of Mount Tambora in the Dutch East Indies (the largest eruption in at least 1,300 years after the extreme weather events of 535–536), perhaps exacerbated by the 1814 eruption of Mayon in the Philippines.
The Year Without a Summer was an agricultural disaster. Historian John D. Post has called this "the last great subsistence crisis in the Western world". The climatic aberrations of 1816 had the greatest effect on most of New England, Atlantic Canada, and parts of western Europe.
In the spring and summer of 1816, a persistent "dry fog" was observed in parts of the eastern United States. The fog reddened and dimmed the sunlight, such that sunspots were visible to the naked eye. Neither wind nor rainfall dispersed the "fog". It has been characterized as a "stratospheric sulfate aerosol veil".
The weather was not in itself a hardship for those accustomed to long winters. The real problem lay in the weather's effect on crops and thus on the supply of food and firewood. At higher elevations, where farming was problematic in good years, the cooler climate did not quite support agriculture. In May 1816, frost killed off most crops in the higher elevations of Massachusetts, New Hampshire, and Vermont, as well as upstate New York. On June 6, snow fell in Albany, New York, and Dennysville, Maine. In Cape May, New Jersey, frost was reported five nights in a row in late June, causing extensive crop damage.
At the Church Family of Shakers near New Lebanon, New York, Nicholas Bennet wrote in May 1816, "all was froze" and the hills were "barren like winter". Temperatures went below freezing almost every day in May. The ground froze on June 9. On June 12, the Shakers had to replant crops destroyed by the cold. On July 7, it was so cold, everything had stopped growing. The Berkshire Hills had frost again on August 23, as did much of the upper northeast.
A Massachusetts historian summed up the disaster: