flickr / futureatlas.com

Credit: flickr / futureatlas.com

Lots of drivers out there probably didn't know (or care) that gasoline is blended differently for summer or winter. Recently, though, that issue has been capturing headlines, thanks to a showdown in California, where Gov. Jerry Brown wants to allow the early use of the so-called winter-blend gas in a play to bring down spiking gas prices in the Golden State.

The price of gas is a confusing, convoluted issue. But before we get into that, what exactly is the difference between summer and winter gas, anyway? Basically, winter gas is cheaper but not as pure, and worse for the environment.

The nation has some 20 different blends of gasoline to meet overlapping state and federal guidelines. The reason for the different grades of gas comes down to trying to control VOCs (volatile organic compounds) that are more likely to evaporate the hotter it gets. More VOCs equal more smog, especially in summer, when the heat in the atmosphere increases the propensity for atmospheric ozone and adding in the VOCs increases the intensity of the smog.

The different grades of gas are measured on a system of RVP, or Reid Vapor Pressure, which is measured in pounds per square inch (PSI). The higher the RVP number of a particular gas blend, the easier it is to vaporize and the worse it is for the environment. All gasoline blends have to be below 14.7 PSI, which is normal average atmospheric pressure. Any number higher than that and gasoline would become a gas.

During the summer heat, the RVP of gas has to be especially low to keep it from boiling off. The EPA mandates an RVP maximum of anywhere between 9.0 PSI and 7.8 PSI for summer-grade fuel, depending on region (though you get a fudge factor of 1 psi for using gas blended with 10 percent ethanol). There are even lower RVP-rated fuels for cities like Houston, New York, and L.A. Different states and cities have their own rules based upon their seasonal temperatures—Washington state needs different summer gas than, say, Florida. That's why there are so many blends. To make it more complicated, the time for switching from summer- to winter-blend gasoline varies by state too.

Generally, the lower the RVP of a gas blend, the more it costs. For example, in winter you can blend butane, which is relatively plentiful and cheap, with gasoline. But butane, which has an RVP of 52 on its own, can't be used in summer, when it would immediately boil off as a gas. So "purer" summer gasoline is by default costlier. (And there are other factors at play too. More people travel in summer during peak driving season, for instance, putting more stress on demand.)

Back to California's woes: First, an August fire at a Chevron refinery diminished the ability of the oil industry to meet demand, driving up delivery times and transportation costs. That especially hurts in California, which has stricter standards than anywhere else in the nation as far as what blends it will sell, making the state especially vulnerable to supply issues.

So Brown's loosening of the rules on gas blends is an attempt to fight back against that vulnerability and bring prices back down. But there's no guarantee. Gasoline is a commodity, and yet the price-setting system has only a loose relationship to supply and demand. Rather than having an independent governing body oversee the supply side of the equation, suppliers voluntarily report on the supply of oil. And any time the numbers could be fudged for profit, we ought to raise our eyebrows.

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