(NEXSTAR) — You may think inflation and the war in Ukraine are what’s driving up gas prices this spring – and yes, they are – but they aren’t the only forces at play. One environmental regulation is also causing gas prices to rise, and it happens every year as the seasons change.

The Environmental Protection Agency requires a different blend of gasoline be sold in the summer months than in the winter months. The summer blend has lower volatility, which means it’s less likely to evaporate while sitting in your car’s gas tank in the heat — and therefore less likely to emit harmful fumes into the atmosphere. Winter gas has higher volatility, making it easier to ignite and start your car in frigid temperatures.

Why does that have anything to do with gas prices? Summer blend gas is more expensive to produce, explains Patrick De Haan, lead petroleum analyst at GasBuddy.

The deadline to switch to selling summer gasoline is June 1 for gas stations, but refineries start producing the pricier blend in March, so that’s when you’ll notice prices creeping up.

“It’s like one day refineries go from producing summer to winter. It’s a long time transition because of the nature of pipelines that ship gasoline to the market, the storage tanks where that gasoline sits, and all of it has to happen by June 1,” said De Haan.

At the same time, refineries are typically doing maintenance work during the spring to make sure they’re ready for high season during the summer. Refineries may have to shut down parts of their operation to do the maintenance work, constraining their output and further pushing up prices.

“There’s a lot of logistical challenges that go into this that eventually drive prices up because this transition is so nuanced,” De Haan said.

“We also start to see gasoline demand go up with spring break travel, warm weather, Americans getting outside after winter. So the switch over to summer gas, a higher rise in demand and refinery maintenance is generally what pinches prices. It’s usually fairly noticeable – somewhere between 25 to 75 cents [higher].”

Unlike the other things causing gas prices to skyrocket, like Russia’s invasion of Ukraine and record-high inflation, there’s a firm end date with this: Sept. 15. That’s the last day the EPA requires gas stations to sell 100% summer blend gasoline. After that date, the cheaper winter blend gas can start to be reintroduced.