science:

Volcanic Sunsets and Years without Summer

On April 10, 1815, Mount Tambora exploded. Tambora is a stratovolcano in Indonesia, and has been overshadowed in the popular imagination by Krakatoa, another Indonesian volcano that erupted in 1883. But Tambora was larger. It was, in fact, the largest eruption in recorded history, topping 7 on the Volcanic Explosivity Index. The casualties are estimated to be around 75,000-90,000, more than any other known eruption. Most interesting from a scientific perspective is the profound effect such eruptions have on the global climate.

1816 was called “the year without a summer” in the northern hemisphere, especially in Europe and North America. Now, the years of 1790-1830 saw a minimum of solar activity, which no doubt contributed to the unusually cold summer. But the reason this year was outstanding even for a cold period was almost certainly Tambora. Put simply, dust and volcanic ashes blocked the sun. Some of the effects, from an old article at The Smithsonian:

In China and Tibet, unseasonably cold weather killed trees, rice, and even water buffalo. Floods ruined surviving crops. In the northeastern United States, the weather in mid-May of 1816 turned “backward,” as locals put it, with summer frost striking New England and as far south as Virginia. “In June … another snowfall came and folk went sleighing,” Pharaoh Chesney, of Virginia, would later recall. “On July 4, water froze in cisterns and snow fell again, with Independence Day celebrants moving inside churches where hearth fires warmed things a mite.” Thomas Jefferson, having retired to Monticello after completing his second term as President, had such a poor corn crop that year that he applied for a $1,000 loan. (…)

In Europe and Great Britain, far more than the usual amount of rain fell in the summer of 1816. It rained nonstop in Ireland for eight weeks. The potato crop failed. Famine ensued. The widespread failure of corn and wheat crops in Europe and Great Britain led to what historian John D. Post has called “the last great subsistence crisis in the western world.” After hunger came disease. Typhus broke out in Ireland late in 1816, killing thousands, and over the next couple of years spread through the British Isles.

In Portugal and Spain, droughts and unseasonally low temperatures caused problems.

The effects could be seen not only in crops and thermometers, but also in art. A survey of paintings by notable artists done before or in the years immediately following major volcanic eruptions shows that sunsets, as seen by artists, were significantly redder immediately following eruptions. The findings correlate well with historic estimates of the Dust Veil Index, a measure of how much dust and aerosols a particular volcano released, compared to background conditions.

Why would volcanic dust in the atmosphere make sunsets redder? The reason the sky is blue at midday and red at dusk and dawn is Rayleigh scattering. Molecules and tiny particles in the atmosphere scatter incoming sunlight. Shorter wavelengths like blue light are scattered more strongly, resulting in a blue sky. However, when the sun is low in the sky, the angle means sunlight must pass through a much larger volume of atmosphere before it reaches us. This causes the bluer wavelengths to be scattered away, leaving reddish light, giving us red sunsets. A denser atmosphere due to a volcanic eruption would exacerbate this effect.

Above are The Lake, Petworth (circa 1827-28) and Sunset (circa 1833), both painted by J. M. W. Turner. The first painting was done before the 1831 eruption of Babuyan Claro, a volcano located in the Phillipines, while the second, redder one was most likely done less than two years after said eruption. One of the most famous red sunsets in art, Edvard Munch’s The Scream, is also speculated to have been inspired by atmospheric conditions after a major eruption, Krakatoa in 1883.