Gas prices in New Hampshire have gone up 31 cents over the past month, but they have leveled off in the last week, and experts think they should remain stable for most of the summer.Experts said a major reason for the price increase is that refineries are switching from their winter gas to their summer supply. The average price of gas in New Hampshire is $3.57. “I think we’ll see a little bit of improvement,” said Patrick De Haan, of Gas Buddy. “But again, I think most of the summer will probably see prices in the mid-$3 a gallon range for much of New Hampshire.”>> Download the free WMUR app to get updates on the go: Apple | Google Play <<Winter gas contains a cheaper ingredient, butane, for cold weather starting. Summer gas contains the more expensive alkylate, for hot weather driving.Another factor that leads to higher prices now is when refinery maintenance happens, which reduces the fuel supply.”But now, in the last week, things have really slowed off,” De Haan said. “That’s the good news, is that changeover from winter to summer gasoline is really a one-time hit, and we shouldn’t really see prices going up a whole lot more here in the weeks ahead.”New Hampshire’s average of $3.57 is well below the nation’s high in California, where a gallon of regular gasoline costs an average of $5.40.