Upvote:22
The answer to this is more geological than historical or meteorological; the reason for 1816's incredible cold was a massive volcanic eruption in Indonesia.
As long before as 1783, Ben Franklin had suggested that volcanic eruptions may cause dips in temperature due to blocking out light from the Sun, and his ideas were brilliantly confirmed by Mother Nature in 1816 when the Indonesian Mount Tambora exploded.
Mount Tambora's explosion ejected over 100 cubic kilometers of ash into the atmosphere, making the sky so hazy that all over the world, stars dipped by a full magnitude. This also meant the earth received (comparatively) much less light from the Sun than before, and due to that, temperatures dipped so low than 1816 became known as the "year without a summer".
The 1883 eruption of Krakatau, the 1991 explosion of Pinatubo, and even Mount Saint Helens' 1980 eruption further proved this by noticeably lowering global temperatures, albeit or short periods of time and less severely than in 1816.