As the temperatures rise in California, so do the gas prices.

The average pump price in California has been creeping upwards for weeks.

Today’s average price in the Golden State is $0.14 cents higher than a month ago, according to the AAA Northern California monthly gas survey.

Northern California’s average price for a gallon of regular unleaded gasoline is $3.98, that’s $0.14 cents more than last month’s AAA report on March 11.

For perspective, California’s average gas price, $4.05, is $0.03 higher on this date than it was a year ago.

“There is a reason why we typically see gas prices rise as the weather becomes warmer. Each spring, refiners must start producing their summer-blend gasoline by May 1, and that process is well underway,” said Bob Brown, AAA Northern California spokesperson. “Additional additives are put into to the gasoline to make it burn properly, so that it will meet clean-air standards. The cost of those additives is passed on to the consumer.”

Today’s national average price at the pump is $3.59 per gallon, which is the highest price in nearly six months. This price is $0.02 higher than one week ago and $0.10 more than one month ago. The national average price is the same as a year ago.

The national average continues to steadily climb and has now increased for 18 straight days.

In addition to warm-weather blends of gasoline coming into play this time of year, seasonal refinery maintenance has also helped to push up the pump price.

However, there is good news. Many refineries in the United States have come out of their seasonal maintenance cycles, lessening the impact of restricted output due to maintenance.

That said, demand for gasoline was at a three-month high in the United States just two weeks ago, according to the U.S. Energy Information Administration.

This higher demand has continued to breathe life into the upward trend in pump prices.

News from Libya might soon bring some relief. Libyan rebels have agreed to reopen two oil ports they previously closed. That announcement has now started to put downward pressure on oil prices.

Following an eight-month closure, the two ports will return a combined 180,000 barrels per day of crude oil supply to the global market. Also yesterday, for the first time in three days, the costs of both Brent and West Texas Intermediate crude oils dropped.

The least expensive average price in Northern California can be found in Marysville where regular unleaded gasoline is $3.82 per gallon. Of all the metro areas tracked by AAA in Northern California, Eureka has the highest price at $4.14 per gallon of unleaded regular.

To get the best mileage possible, AAA recommends keeping tires at the proper pressure suggested by the vehicle manufacturer, performing routine maintenance and making sure fluids are clean and belts and hoses are in good repair. The way you drive can also impact fuel economy. Smooth driving to avoid sudden stops and starts, combining trips and lightening your load also helps conserve gasoline.

AAA’s Fuel Gauge Report is the most comprehensive retail gasoline survey available, with over 100,000 self-serve stations surveyed every day, nationwide. Data is provided in cooperation with OPIS Energy Group and Wright Express, LLC.