From Climate Central:
The summer of 2012 has been one for the record books in the lower 48 states. On the heels of the country’s warmest-ever spring, several record-breaking June and July heat waves kept the Southwest, Midwest and Atlantic Coast sweltering. July went on to become the all-time warmest month on record for the country. In fact, 2012 to-date has been the hottest year for the U.S. since instrument records began in 1895, and the summer was the third warmest summer on record.
The record-breaking heat has affected nearly every part of the country at some point this year, and so far there have been more than 28,000 daily high-temperature records broken or tied. This begs the question: which state was the biggest record-breaker in 2012? Or put another way: Which state had the most extreme heat?
Answering this question is not as simple as counting the number of records broken or tied in each state. If that were the case, the states with the most thermometers would almost always be the top record-breakers. When quantifying record-breaking heat by state, there are more factors to consider: How often do these stations set records? How long have records been kept for at each station? (It’s harder to break a record at a station with 100 years of data compared to one with 40.) How does this year compare to average? How many record-high temperatures compared to record lows were set in the state this year? To put it simply, finding the biggest record-breakers depends on how you look at the data.