...GOOGLE "Winter blend gasoline"
...here's EPA document:
http://www.epa.gov/otaq/regs/fuels/rfg/f01033.pdf
...quote from the EPA website:
"Volatility/Reid Vapor Pressure (RVP)
Volatility is the property of a liquid fuel that defines its evaporation characteristics. RVP is an abbreviation for "Reid vapor pressure," a common measure of and generic term for gasoline volatility. EPA regulates the vapor pressure of gasoline sold at retail stations during the summer ozone season (June 1 to September 15) to reduce evaporative emissions from gasoline that contribute to ground-level ozone and diminish the effects of ozone-related health problems."