Listen to part of a lecture in an Earth Science class.(male professor) Uh, getting back to climate patterns, let me point out that weather doesn’t always follow the usual pattern.One of the classic cases of abnormal weather was the summer of 1816 in the northern hemisphere.Actually, in the northeastern United States, 1816 is sometimes referred to as “the year there was no summer.”And the unusual cold was also felt that year in many other parts of the globe.The funny thing is, in January and February, the temperatures in northeastern US weren’t all that much below the normal, no hint of what was to come.That wasn’t seen until May when strong waves of cold are hit the region and covered it with ice and frost.That’s pretty rare for some of those areas in May. And unfortunately, the cold snaps kept on coming.There was widespread snowfall in June, followed by more cold spells in July and August.Historians can point to a number of contemporary diaries from the northeast and beyond, including Jefferson’s, that’s Thomas Jefferson of course, a former president who had retired to his home down in Virginia, diaries that recorded and remarked on the extraordinarily cold temperatures that year.To put it in perspective, the summer of 1816 was the coldest on record for some areas in nearly two hundred years of record keeping.As a result, the corn crop finally harvested in some parts was less than ten percent of the usual yield, an economic disaster for many farmers.So the summer of 1816, the summer that wasn’t, that’s more than just an interesting entry in the weather almanacs.In fact, it led to a dramatic shift in agriculture with many farmers abandoning northeast US for the Midwest.But what was the cause of all this? Just a variation in normal weather patterns, or was there something else going on here?Well, there are a number of theories and several of them involved the Sun.For example, in 1816, it happened that the Sun was in the middle of a period of low magnetic activity.And it turns out that at this point in the cycle, when its magnetic activity is below normal, the Sun is slightly dimmer and the Earth gets less sunlight.So, it’s no surprise that periods of low magnetic activity on the Sun have been associated with low temperatures here on Earth.That’s a pattern observed over and over again and put forward by most authorities of the time to explain the extreme weather of 1816.But the most widely held view among those of us in the field today is that the year with no summer was caused by several large volcanic eruptions just prior to 1816, particularly a volcano in Indonesia that erupted just a year before and by all the dust and gases thrown into the atmosphere as a result.We’re talking huge amounts of dust and gases here, especially in the case of Tambora.That’s the volcano in Indonesia.Tambora is estimated to have thrown two hundred million tons of material into the atmosphere.The idea is that a lot of the gas and dust that shot up from that volcano went clear up into an atmosphere layer called the stratosphere, about ten kilometers up.Then winds in the stratosphere moved this enormous cloud of particles around the Earth.And unlike the so-called greenhouse gases we’re producing so much of today, the particles in this enormous volcanic cloud up in the stratosphere primarily reflected sunlight back into space.And that reduced the amount of sunlight reaching the Earth’s surface down below.And less sunlight means a drop off in temperatures.And even though a volcanic eruption may only last a few days, if it’s big enough, it can apparently impact the weather around the world for up to five years.Incidentally, another way we can study the effects of volcanic eruptions besides reading contemporary reports is to use information from tree rings that grew over the last several centuries.Remember, there’s a growth ring in a tree’s trunk for each year it’s alive.And when the wood in the growth ring is very dense, for example, we’ve discovered this corresponds to a year that was particularly warm.So a recent study was done where the growth rings of exceptionally old trees all over the northern hemisphere were analyzed.Sure enough, the highest concentrations of low-density tree rings, which mean cold temperatures, they almost always follow major volcanic eruptions, including the eruption of Tambora.