There are many factors that contribute to the price of crude oil. Supply and demand is a big one. This summer the demand was viewed as way ahead of the supply, which inherently drives up prices. Now the demand is perceived as slowing as the world economies slow. OPEC announced production cuts, but they have done little to drive the price of crude oil back up. It's not just simple supply and demand that affect the price of crude oil.
Crude oil is bought and sold in American dollars. As a result of this, the price of oil is tied to the strength of the American dollar compared to other world currencies. As the American dollar falls, crude oil prices go up, and vice versa. This summer the American dollar was weaker compared to other currencies, but it has since rallied.
The final big factor in the price of crude oil is commodity trading. There is a lot of agreement among experts that this summer's high oil prices were driven in part by speculators on the commodity markets. The commodity markets have been increasingly used by investors who have less of an interest in the commodity itself than the attempt to make a quick profit. As more people buy into the crude oil commodity, the price goes up. When investors began to panic at the end of the summer, they began to sell their stakes in the oil commodity, driving the price back down.
Those are the three big driving factors in the price of crude oil as far as I understand it.
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