The National Climatic Data Center has been measuring weather extremes since 1910. A value of 0% on its U.S. Climate Extremes Index means that there were no extremes of temperature or precipitation in that timeframe. A value of 100% means extreme conditions throughout the year — a virtually impossible scenario. The ranking for 2013 was 22.10%. The most extreme year in recent history was 2012, with a ranking of 49%.