Gasoline evaporates. You’ve probably noticed this when you accidentally spill a bit on the pavement during a fill up. Gasoline evaporates at faster or slower depending on the temperature. It’s called volatility, and unbelievably, volatility directly influences the volatile price of gasoline.
Summer Heat. In the hot months of summer, gasoline evaporates faster than when its cool. Since that’s not good for the environment, regulatory bodies require that gasoline sold during the summer have a lower Reid vapor pressure. In other words, summer gas must be less volatile, and evaporate more slowly at higher temperatures.
Winter Starts. When the weather is cooler, gasoline is produced that will ignite more easily in the lower temps. This gasoline has more butane, and as a result has a higher RVP. This gasoline is cheaper.
Federal law says that refiners must switch to summer blends by June 1, and can switch back to winter blends on September 16. You can see these switches reflected at the pump!