In 1816 it snowed in June – because of a volcano in Indonesia.
In April 1815 Mount Tambora on Sumbawa erupted, the most powerful documented eruption in history. Its sulphur cloud cooled the planet by roughly 0.4 to 0.7 °C the following year. Snow fell in New England in June, grain rotted across Europe, and famine followed. Meanwhile, by a rain-soaked Lake Geneva, Mary Shelley wrote 'Frankenstein'.
Useful: Half a degree globally sounds harmless; 1816 shows what it means for harvests.
Source: en.wikipedia.org