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Why gas prices drop in autumn

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Toby Talbot/AP/File

(Read caption) A gas pump is shown in Montpelier, Vt. Is it worth it to fill up with premium gas?

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Fall Means Falling Gasoline Prices

Fall is always a welcome change of pace for most people after a long, hot summer. Not only from the temperatures, but fall almost always brings relief at the gasoline pump. Pundits frequently notice this phenomenon during election years, and assume that vested interests are trying to manipulate prices to win elections. But there is a more straightforward explanation to what’s going on, and it isn’t limited to election years.

Regulating Smog

Everyone knows that gasoline evaporates. What you may not know is that there are numerous recipes for gasoline, and depending on the ingredients, the gasoline can evaporate at very different rates. And because gasoline vapors contribute to smog, the Environmental Protection Agency (EPA) seasonally regulates gasoline blends to minimize emissions of gasoline vapors.

The way the EPA regulates these vapors is by putting seasonal limits on the Reid vapor pressure (RVP). The RVP specification is based on a test that measures vapor pressure of the gasoline blend at 100 degrees F. Vapor pressure is a measure of the tendency to evaporate; the higher the vapor pressure the faster the evaporation rate. Normal atmospheric pressure is around 14.7 lbs per square inch (psi) at sea level. Substances with a vapor pressure higher than normal atmospheric pressure are gases, and those with a vapor pressure lower than normal atmospheric pressure are liquids (assuming they are exposed to normal atmospheric pressure). 

But vapor pressure is also a function of temperature. Under normal atmospheric temperatures water is a liquid because its vapor pressure is below 14.7 psi. It still evaporates (i.e. it still has a vapor pressure), but very slowly. As water is heated, its vapor pressure increases, and as the boiling point of water is reached the vapor pressure of water reaches that of atmospheric pressure and the water becomes a gas (steam).

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