(NewsNation) — Gas prices increased the last week of February, and a AAA report suggests they’ll keep rising as drivers hit the road for spring break.

On Friday, the national average cost for a gallon of gas increased to $3.31 per gallon — up 21 cents higher than a month ago and four cents less than a year ago, when the average was $3.35, AAA reports.

The national average currently stands at $3.36 per gallon — up from 9 cents one week ago, and 22 cents one month ago.

“We’re entering Spring Break season, and historically, March and April bring higher gas prices as demand rises,” said Aixa Diaz, a AAA spokesperson. “With milder temperatures come more road trips, and this time of year tends to be a precursor to the summer driving season.”

Aside from spring break, prices have increased ahead of the annual switch to a higher grade of gasoline known as the summer blend, which is more expensive to make, according to NACS, the trade organization for fuel and convenience stores.

Gas prices have increased seasonally by about 50 cents from a low at the beginning of February to the seasonal high in May since 2000, NACS reports.

Axios reports retailers must sell summer-blend gas from June 1 to Sept. 15, which is required under the Clean Air Act’s 1990 amendments. Most retailers switch over early.

Gas prices typically begin to drop around Memorial Day.