The ash particles that are thrown up into the atmosphere from a volcanic eruption cause a number of weather changes. For example, ash particles attract water droplets, causing the weather immediately close to the eruption to be stormy, as in rain, thunder, and lightning (from all the positive and negative particles rubbing each other). There is also the formation of vog, or volcanic fog. The ongoing eruption is very quiet, with lava flowing through lava tubes and then into the ocean. Up at the vent is an almost constant plume of volcanic fume that contains a lot of sulfur dioxide. This SO2 combines with water in the atmosphere to form sulfuric acid droplets that get carried in winds around to the leeward side of the mountain. The air quality becomes very poor and can decrease the amount of rain on that side of the mountain.
The world-wide affects of volcanic eruptions happens when there are large explosive eruptions that throw material into the stratosphere. If it only gets into the troposphere it gets flushed out by rain. The effects on the climate haven't been completely figured out. It seems to depend on the size of the particles (again mostly droplets of sulfuric acid). If they are big then they let sunlight in but don't let heat radiated from the Earth's surface out, and the net result is a warmer Earth (the famous Greenhouse effect). If the particles are smaller than about 2 microns then they block some of the incoming energy from the Sun and the Earth cools off a little. That seems to have been the effect of the Pinatubo eruption where about a 1/2 degree of cooling was noticed around the world.That doesn't just mean that things are cooler, but there are all kinds of effects on the wind circulation and where storms occur.
A new study from the University of East Anglia's Climatic Research Unit in England reports that not only do large volcanic eruptions cause a global cooling effect but they also cause warmer winters over the continents of the Northern Hemisphere. The study, published in the International Journal of Climatology (Vol. 16, p.537), looked at the past 120 years of global temperature records for evidence of effects of major volcanic eruptions in the tropics from Krakatoa in 1883 to Pinatubo in 1991. The authors conclude that in every case but one, the world cooled by about 0.2 degrees C for up to two years after a major eruption. The exception was the eruption of El Chichon in Mexico in 1982, which coincided with a countertrend of warming caused by one of the strongest El Nino events in the Pacific Ocean this century. An even more controversial connection involves whether or not volcanic activity on the East Pacific Rise (a mid-ocean spreading center) can cause warmer water at the surface of the East Pacific, and in that way generate an El Nino. In other words, there is belief that large eruptions can cause the weather phenomena called "El Nino" to start. This is a huge disruption of the Earth's atmospheric circulation. The connection hasn't been accepted by everybody though. Dr. Dan Walker from the University of Hawaii has noticed a strong correlation between seismic activity on the East Pacific Rise (which he presumes indicates an eruption) and El Nino cycles over the past ~25 years. Lots of people seem not to like this idea, but it hasn't been disproved either.
The study also reports that over continental areas of the higher latitudes of the Northern Hemisphere, each eruption was followed by an unusually warm winter. Lead author Mick Kelly explains, "The net effect is to suppress the seasonal cycle with colder summers and warmer winters." Several researchers had predicted the winter warming with computer simulations of global climate, based on changes in the heat gradient of the atmosphere that strengthen the winter jet stream in polar regions. This in turn "strengthens surface westerly winds in the middle latitudes, particularly in the North Atlantic, resulting in a stronger maritime influence over land," says Kelly. In winter, winds blowing from the sea tend to keep the land warm.
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