Tambora

Tambora was a volcano on Earth which erupted in 1815.

This eruption caused an accumulation of volcanic ash in the upper atmosphere. The effect was so significant that even in June of next year the half delirious Eighth Doctor could still guess it to be the "Year Without a Summer" by observing the atmosphere on the shores of Lake Geneva. (AUDIO: Mary's Story) However, the Thirteenth Doctor pondered whether something other than volcanic ash might be the cause of the Year Without a Summer. (TV: The Haunting of Villa Diodati)

Behind the scenes
The on 10 April 1815 in Indonesia was one of the most powerful recorded volcanic eruptions. The resulting bad weather during the "Year without a Summer" was largely the reason why Mary Shelley, Lord Byron and their friends spent so much time indoors in Villa Diodati around the time of the Doctor's arrival. Mary Shelley wrote about the horrible weather in the introduction to the 1831 edition of Frankenstein.