Volcanoes! Besides being dramatic, volcanic eruptions can have an impact on the weather – not just locally but around the world. Small scale eruptions such as are currently being felt in Hawaii, generally have only local effects from the lava, ash and gases. Larger eruptions, such as Mt. Pinatubo in the Philippines, have impacts that can be felt around the world. During the 1991 eruption of Mt Pinatubo, nearly 20 million tons of sulfur dioxide were injected far up into the stratosphere. Because it was so large and so high, the dispersal of this gas cloud around the world caused global temperatures from 1991 through 1993 to drop by about 1 °Fahrenheit. Similarly, the Tambura eruption in 1815, resulted in such cold summer weather that there were widespread crop failures in Europe and North America. Conditions were so cold that 1816 was known as the year without a summer.
Mary Knapp, Weather Data Library