Why Gas Prices Rise Nearly Every Spring

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Although it wasn’t strongly emphasized in the recent discussions on the rise in gasoline prices, those prices to rise nearly every spring. But why does it happen?

There is no question that it does happen. If you check the history of gasoline prices at the U.S. Energy Information Administration’s (EIA) website you can see that gasoline prices almost always rise between January and May. In fact, this happens about 90% of the time.

Last year was an exception because of the plunge in energy demand due to the Covid-19 pandemic, but prior to 2020 gasoline prices had risen in the first five months of the year for more than 20 years in a row.

Many factors influence gasoline prices, but there are specific reasons behind the seasonal changes.

Understanding Seasonal Gasoline

Two critical specifications that need to be met for each gasoline blend are the octane rating and the Reid vapor pressure (RVP). Octane rating is important for avoiding engine knock. But the octane rating for a gasoline blend is consistent throughout the year and is not the reason for the seasonal price fluctuations.

The RVP spec, however, does change with the seasons and this change can have a major effect on the price of fuel. The RVP is based on a test that measures the vapor pressure of the gasoline blend at 100 degrees F.

Normal atmospheric pressure varies with location but averages about 14.7 lbs per square inch (psi) at sea level. Atmospheric pressure is caused by the weight of the ocean of air pressing down on us. If a liquid has a vapor pressure greater than atmospheric pressure, that liquid boils. For example, when you heat a pan of water the vapor pressure increases until it reaches local atmospheric pressure. At that point, the water begins to boil. (In the mountains where the atmospheric pressure is lower, water boils at a lower temperature.)

In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline be well below 14.7 psi. Otherwise, the fuel may build pressure in fuel tanks and gas cans, and it can boil off lighter components in open containers. Gas that is vaporized ends up in the atmosphere and contributes to air pollution.

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