Spring is approaching, and gasoline prices are once again climbing. But you may not know that this ritual of climbing prices happens almost every year about this time. If you check the history of gasoline prices at the Energy Information Administration’s (EIA) website you can see that gasoline prices almost always rise between January and May.
The primary reason this happens is due to a seasonal switch in gasoline blends. There are two key (although not the only) specifications that refiners must meet for gasoline. The gasoline must have the proper octane, and it must have the proper Reid vapor pressure (RVP). While the octane specification of a particular grade is constant throughout the year, the RVP specification changes with the seasons. (See Refining 101: Winter Gasoline for a more detailed explanation of gasoline blends).
The RVP is based on a test that measures vapor pressure of the gasoline blend at 100 degrees F. Normal atmospheric pressure varies, but is usually around 14.7 lbs per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than atmospheric pressure, that liquid boils. For example, when you heat a pan of water the vapor pressure increases until it reaches atmospheric pressure. At that point the water begins to boil.
In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline is well below 14.7 psi. Otherwise, it can pressure up your gas tanks and gas cans, and it can boil in open containers. Gas that is vaporized ends up in the atmosphere, and contributes to air pollution. Therefore, the Environmental Protection Agency (EPA) has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others. The particulars vary, but key considerations are the altitude and motor vehicle density of a specific location.
The EPA publishes a schedule for the RVP transition:
The schedule varies somewhat from region to region, but in general is as follows. After allowing vapor pressures as high as 15 psi in the winter, the limit drops starting on May 1st:
May: 9.0 psi
June – Sept. 15: 7/7.8 psi
More congested areas and hotter areas will tend to have a limit of 7.0 psi, while cooler climates generally opt for 7.8 psi. Some cooler climates maintain a 9.0 psi limit throughout the summer. One of the disadvantages of having different requirements for different areas is that summer gasoline is less fungible. This can cause price imbalances in different areas, and sometimes prevents product from flowing from one area into another to ease the shortage.
Refiners will start to pull down their inventory of winter gasoline well in advance of the May 1st deadline. On that date, all gasoline in the system has to meet the stricter requirements, and this “summer blend” is costlier to produce because it contains less butane.
Butane, which has an RVP of 52 psi, can be blended into gasoline in higher proportions in the winter because the vapor pressure allowance is higher. There are two advantages in doing this. First, butane is a cheaper blending component than most of the other ingredients. That makes fall and winter gasoline cheaper to produce.
But butane also adds to the total gasoline pool, so that means that gasoline supplies increase in the winter as more butane is added to the mix. Not only that, but this takes place after summer driving season, when demand typically falls off. These factors normally combine each year to reduce gasoline prices in the fall (even in non-election years). The RVP is stepped back down to summer levels starting in the spring, and this usually causes prices to increase.
One misconception some have is that they can save money by buying cheap gasoline in the winter and storing it for the summer. Remember that winter gasoline will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix. You will end up with less gasoline than you paid for, and that would also contribute to the air pollution problem that summer gasoline was designed to avoid.
If, on the other hand, you were to buy summer gasoline and try to store it until winter, you might find yourself having problems getting the fuel to ignite, due to the lower vapor pressure. This would be like putting a little bit of diesel in your gasoline – not very good for your car.
So how high might gasoline prices climb this spring? The EIA’s gasoline inventory database can provide some guidance. In the spring of 2007 gasoline prices spiked above $3.00 a gallon for the first time. But that year gasoline inventories also dropped sharply. Rapidly falling gasoline inventories are a good predictor of sharply higher gasoline prices. In the fall of 2005, Hurricane Katrica also caused a sharp drop in gasoline stocks, leading to an atypical fall price increase.
So far in 2010, gasoline inventories have been at very healthy levels. While some inventory draw down can be expected during the transition to summer gasoline, it is a pretty safe bet that the current high level of gasoline stocks will prevent a rapid escalation of prices this spring. I would expect no more than a mild price increase between now and summer, and at the current inventory levels it would not be surprising to see prices start to decline from present levels.
However if oil prices escalate, that could trump high gasoline inventory levels. This is why gasoline is presently about $1 more than it was last year at this time; oil prices were $30-$40 lower than they are now. But that’s a topic for a future essay.