Although it may have been camouflaged in recent years by fluctuating fuel prices, the average price of gasoline at the pump typically rises by a few cents every summer before easing back down in mid-September. The increase in demand that the surge in summer driving creates certainly contributes to the higher prices, but so does "summer-blend" gas.

According to statistics provided by AAA Carolinas, the average price for a gallon of regular gasoline increased 31 cents, on average, from the first quarter to the second quarter for each of the years 2000 through 2008. This spike has been as much as 65 cents in years when fuel costs were increasing wildly anyway. However, even during those years in which pump prices decreased, such as 2001, when the average price per gallon in the fourth quarter was 25 cents less than in the first quarter, there was a 16 cent jump in the second quarter. Most experts credit the extra cost of refining summer-blend gasoline with about 10 cents of any first to second quarter increase.

What's "summer gas"? Making gasoline is somewhat like brewing beer. A number of different ingredients can be added or subtracted to change the nature of the final product. Summer-blend gasoline is the catch-all term applied to a hodgepodge of recipes used to meet federal environmental standards as well as an assortment of local standards during the warmer summer months. Different areas require different recipes.

During much of the year a number of inexpensive additives, such as butane, are blended into gasoline, usually to reduce costs. But in warmer temperatures these chemicals can evaporate from the gas and seep into the air as pollution. Particularly in more traffic-congested areas, these pollutants are a key component of smog. Removing such additives creates a purer gasoline mixture; but oil is more expensive than most additives thus translating into higher prices at the pump. What's more, as refineries try to time the changeover from winter to summer gas, supplies can run short, which exacerbates pricing problems.

Some localities require the sale of summer-blend gas for longer than the federally mandated timeline, but generally, summer-blend gas is in the pumps from June 1 until Sept. 15 and has a minimal effect on mileage. The slightly higher cost will be offset to some degree as many consumers should notice a slight increase in fuel economy during that time period because the gasoline to additive mix is greater.

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