The tendency of gasoline to increase in price during the summer is well known. Partly, this reflects increased demand (which leads to an increased quantity sold at an increased price, given a particular supply curve). Partly, this is the consequence of how summer gasoline is a different blend of hydrocarbons. The reason for this is the need to prevent too much pressure from building up inside gas tanks as more of the liquid turns to vapour in the summer heat. This is standardized in terms of Reid vapour pressure (RVP): the pressure of any particular gasoline blend at 100Â°F (37.8Â°C) expressed in kilopascals, calibrated to a standard atmospheric pressure of 101.3 kPa.
RVP is used to specify which blends of gasoline are acceptable for sale at different ambiant temperatures. Gasoline with an RVP of over 14.7 will fairly easily pressurize gas tanks and gas cans in summer heat. It will also boil if left in open containers. As such, regulations require summer gasoline to contain less butane than the winter sort. This is on account of how butane is relatively inexpensive (making companies want to include more of it), but is also the most active contributor to vapour pressure. As such, the butane content of summer gasoline must be very low – one factor behind the higher price.
I learned all this from R-Squared, an energy blog that seems to be commonly cited. The blog makes one other important point: anyone considering storing cheap winter gasoline for use in the summer should consider the dangers of having the butane therein turn to vapour and start pressurizing the container in which it has been stored.