What Causes the Jet Stream to Change its Course?

Usually changes in “normal” weather patterns reflected in the movement of the jet stream are short lived. However, sometimes the jet stream moves north or south for more extended periods of time. So what can make the path of the jet streams change?

Remember that it is the sun's heating of the earth, of its land, and its water that drives the weather. To change the weather then, it is necessary to change the absorption and distribution of the sun's energy.

There are two basic ways to do this:

1) Change the amount of solar radiation that reaches the earth or

2) Change the amount of sun's energy that radiates back into space.

Clouds, for example, block the sun’s rays from reaching the ground by reflecting the sunlight back into space. We all have experienced the cooling effects of clouds on a warm summer day. Clouds also prevent the heat radiating back from the earth from reaching outer space. As any farmer knows, cloud cover can prevent a very cold winters night from reaching freezing temperatures. Because cloud cover has both a cooling and a warming effect the net effect on the overall climate is nil.

Dust in the upper atmosphere also reflects sunlight back into space but unlike clouds its net effect is anything but nil. Dust tends to cool the high latitudes more that those at the equator. Remember that the path of the sunlight that reaches the poles is longer than that at the equator. If the atmosphere is dusty, the sunlight has a longer path through the dust and more of the sunlight is reflected back. This of course increases the temperature differentials at the boundary between Arctic and equatorial air and therefore increases the force of the winds and the strength of the storms. One of the most important sources of atmospheric dust is violent volcanic explosions. These explosions can throw tremendous amounts of debris high into the atmosphere which then circulate around the globe for years and are believed to affect the climate all over the globe.

The correlation of volcanic eruptions and climate has been made relatively recently although the consequences of such eruptions have been recorded, albeit unwittingly, throughout history. For example, Roman poets tell us of the eruption of Mt. Etna in Sicily in 42 BC and at the same time an historian in China writes of a "veiled and indistinct" sun and crop failures. More recently, Benjamin Franklin wrote in May 1784 of a constant fog over all Europe during the summer months of 1783 and the most severe winter in Europe for many years. He knew about volcanic eruptions in Iceland about that time and speculated that there might be a connection but he did not know of the eruption in Japan in the 1783. There were crop failures and poor harvests in France during this time that are believed to have contributed to the social unrest preceding the French Revolution in 1789. In June 1991 the dust from the eruption of Mt. Pinatubo in the Philippines more than doubled the normal amount of dust in the air. The dust cooled the climate, brought spectacular sunsets to Hawaii, caused ash induced damage to jet plane engines, and some believe, disturbed weather patterns in southern Africa. In 1992 and 1993 Mozambique, Zimbabwe, and South Africa suffered a drought which withered crops and caused a food shortage throughout the region.

In his book Catastrophe, D. Keys believes there is strong evidence that in 535 AD there was a major volcanic eruption that devastated the climate all over the world. Its location was mostly likely in Indonesia between the islands of Sumatra and Java where the notorious volcano of Krakatoa is located. The climate disruption from this massive eruption lasted for 20-30 years and affected every know civilization of the time. If this is true, and it probably is, this was the final blow to the Roman Empire.

For many places in the world, recovery did not come until 800 AD or later. But this recovery was short lived as the global temperature fell again with devastating effects throughout the world.

Ice cap studies together with historical records and tree ring growth data showed that there was a long time of increased volcanic activity from 1250-1500. This was exactly at the time of the Bubonic Plaque in Europe, the end of the Mississippi culture (the Mound Builders) in North America, and demise of the Viking colonies in Greenland and Iceland. It was the beginning of another period of global cooling that became what scientists call the Little Ice Age.

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