Turns out that the Year Without a Summer was caused by the great amount of ash stationed in the atmosphere after the eruption of the Indonesian volcano Tambora in april 10th 1815. The sun rays couldn't get past that heavy air and, as a result, there were great snowstorms, frosts and rains in the following summer. That made survival in the Northern Hemisphere really hard. Many countries in Europe went through famine, a lot of people died because of this and also from diseases and the livestock almost didn't make it. Of course, a great part also suffered and died from the eruption itself.
In Portugal and Spain, however, this "winter" was not so harsh - although many died there too -, and they were able to export cereals for the other countries, where the crops were basically destroyed by the weather.
At times like these, I'm glad to be born in Brazil!