Average gas prices in Fresno have hit the $4 mark.
AAA of Northern California released its monthly gas price report Tuesday. The price of a gallon of unleaded gasoline in Fresno rose 14 cents over the last month.
The average price in the Visalia-Tulare-Porterville area rose 18 cents to $3.99.
Valley prices are running slightly below the state average, which is at $4.05.
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Nationwide, prices rose 10 cents to an average $3.59.
Prices in California rise each spring as refineries switch to a cleaner-burning but more expensive blend of gasoline. Refinery maintenance that accompanies the switch also contributes to higher prices, according to AAA.
Most refineries are finished with that work, but demand for gas recently reached a three-month high nationwide, which pushes prices up, a AAA spokesman said.
Other factors are putting downward pressure on prices. The cost of crude oil dropped Monday. And Libyan rebels agreement to reopen two ports to oil shipments is expected to return 180,000 barrels of oil a day to the global market.