A Primer on Gasoline Blending  Refineries produce a more
expensive fuel blend during the summer to cut down on smog
during hot months. Stations nationwide will start selling a
less-expensive winter fuel usually by mid September, which on
average means that winter gasoline is less expensive than summer
gasoline.   There are two very important (although not the only)
specifications that need to be met for each gasoline blend. The
gasoline needs to have the proper octane, and it needs to have
the proper Reid vapor pressure, or RVP. While the octane of a
particular grade is constant throughout the year, the RVP spec
changes as cooler weather sets in.  The RVP is the vapor
pressure of the gasoline blend when the temperature is 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 local atmospheric pressure,
that liquid boils. For example, when y