[3] Evidence suggests that the anomaly was predominantly a volcanic winter event caused by the massive 1815 eruption of Mount Tambora in April in modern-day Indonesia (commonly referred to as the Dutch East Indies at the time).

In the eastern United States, a persistent "dry fog" dimmed the sunlight, causing unusual cold and frost throughout the summer months.

The Year Without a Summer was an agricultural disaster; historian John D. Post called it "the last great subsistence crisis in the Western world".

[6] The main cause of the Year Without a Summer is generally held to be a volcanic winter created by the April 1815 eruption of Mount Tambora on Sumbawa.

On July 29–30, 1816, a violent storm occurred near Cape Town, South Africa, which brought forceful northerly winds and hail and caused severe damage to shipping.

[17][18] In India, the delayed summer monsoon caused late torrential rains that aggravated the spread of cholera from a region near the Ganges in Bengal to as far as Moscow.

Hungary experienced snowfall colored brown by volcanic ash; in northern Italy, red snow fell throughout the year.

[22] In western Switzerland, the summers of 1816 and 1817 were so cold that an ice dam formed below a tongue of the Giétro Glacier in the Val de Bagnes, creating a lake.

Despite engineer Ignaz Venetz's efforts to drain the growing lake, the ice dam collapsed catastrophically in June 1818, killing forty people in the resulting flood.

[31] Though fruit and vegetable crops survived in New England, corn was reported to have ripened so poorly that no more than a quarter of it was usable for food, and much of it was moldy and not even fit for animal feed.

[34] Massachusetts historian William G. Atkins summed up the disaster: Severe frosts occurred every month; June 7th and 8th snow fell, and it was so cold that crops were cut down, even freezing the roots ...

In the early Autumn when corn was in the milk [the endosperm inside the kernel was still liquid][35] it was so thoroughly frozen that it never ripened and was scarcely worth harvesting.

On September 13, a Virginia newspaper reported that corn crops would be one half to two-thirds short and lamented that "the cold as well as the drought has nipt the buds of hope".

Easterly winds have prevailed for nearly three months past ... the sun during that time has generally been obscured and the sky overcast with clouds; the air has been damp and uncomfortable, and frequently so chilling as to render the fireside a desirable retreat.

Crop failures were aggravated by inadequate transportation infrastructure; with few roads or navigable inland waterways and no railroads, it was prohibitively expensive to import food in most of the country.

Vessels have been sent from Pernambuco to the United States to fetch flour, and what had tended to increase this distress was the interruption of the coasting trade through the dread of war with Buenos Ayres.

[16] High levels of tephra in the atmosphere caused a haze to hang over the sky for several years after the eruption, and created rich red hues in sunsets.

Paintings during the years before and after seem to confirm that these striking reds were not present before Mount Tambora's eruption,[41][42] and depict moodier, darker scenes, even in the light of both the sun and the moon.

[42] The lack of oats to feed horses may have inspired the German inventor Karl Drais to research new ways of horseless transportation, which led to the invention of the draisine and velocipede, a precursor of the bicycle.

[43] The crop failures of the "Year without a Summer" may have shaped the settlement of the Midwestern United States, as many thousands of people left New England for western New York and the Northwest Territory in search of a more hospitable climate, richer soil, and better growing conditions.

British historian Lawrence Goldman has suggested that migration into the burned-over district of upstate New York was responsible for centering the abolitionist movement in that region.

[48] Lord Byron was inspired to write the poem "Darkness" by a single day when "the fowls all went to roost at noon and candles had to be lit as at midnight".

The bright sun was extinguish'd, and the stars Did wander darkling in the eternal space, Rayless, and pathless, and the icy earth Swung blind and blackening in the moonless air; Morn came and went—and came, and brought no day Justus von Liebig, a chemist who had experienced the famine as a child in Darmstadt, later studied plant nutrition and introduced mineral fertilizers.