The term "summer blend" is shorthand for a menu of federally and locally mandated summertime fuel recipes that are designed to cut down on smog. The gasoline we use is always refined from an intricate blend of ingredients; the process combines coffee-dark crude oil with various additives that increase performance or make fuel burn more cleanly. Because overlapping federal and local requirements call for different recipes in different locales and seasons, there are approximately 20 distinct "boutique blends" of gasoline sold in the United States. Some Americans end up pumping a blend called Carb (named for the California Air Resources Board) while others burn Atlanta (named for Georgia's capital, where it's sold).

Refineries brew their summer blends by removing hydrocarbons that are more prone to evaporate in hot weather. These chemicals, called volatile organic compounds, react with airborne pollutants in the summer sun to form ozone, one of the main components of smog. From June 1 to Sept. 15, the EPA mandates that pumps in 12 high-ozone urban areas—such as Los Angeles, New York City, and Baton Rouge—deliver gasoline that meets special low-evaporation standards. Several states have voluntarily adopted the rules, and 15 have enacted their own seasonal-blend regulations on top of the EPA's. For example, pollution-conscious California has mandated that service stations must start selling its summer blend in May.

Summer-blend gas isn't new. It was first sold in 1995, as required by the Clean Air Act's 1990 amendments, and the current, even cleaner, concoction was phased in for the summer of 2000. Since then, there have been sharp spikes in fuel prices every spring as summer blends get rolled out. This is not so much because it's expensive to make the gas—the added cost per gallon is only 1 or 2 cents—but because refineries generally try to sell every last bit of winter fuel before mixing in the slightly more expensive summer batch. Sometimes they draw down the stock too far, creating shortages before the first deliveries of summer blend enter the supply chain. The return to normal blends in the fall causes a far less pronounced spike because the industry, free from summer standards, doesn't bother selling off the summer gas before mixing in the less pricey stuff.

So why not use the summer blend year-round? The main reason—apart from the fact that the 1990 law isn't written that way—is that summer-blend gas doesn't work as well in the winter. Summer blend's low-evaporation rate makes engines less likely to stall in hot weather but can make them difficult to start in the cold.