The graphic below shows the year-to-date departure from average temperature in the US for the past 118 years. Each month includes the average temperature of the previous months in that year: March 2012, for example, shows the average temperature for the January-February-March period.
The five warmest and five coldest years are also highlighted, showing that three of the five warmest years in the US have occurred since 1998. The coldest five years were all before 1924. 2012 has had the warmest first seven months of all the years on record.
"It is often hard to put an event into proper historical perspective while it is still unfolding," says Crouch. "It is almost the same as describing a car accident while the vehicles are still moving."* “We saw very warm daytime temperatures over a large part of the country related to the ongoing drought, just as in 1936, Crouch said in an interview with the New York Times. "When soils are dry, especially during the summer, it drives the daytime temperatures up. But this is a very local effect.
“On the other side, at the national level, we have also seen very warm nighttime temperatures, and that is part of a long-term trend we’ve seen across the contiguous U.S. over the past several decades,” he said. “The hotter days increase the amount of moisture the lower atmosphere can hold, and this means it doesn’t cool off as much at night anymore.
“This clearly shows a longer-term warming trend in the U.S., not just one really hot month,” Mr. Crouch said.
The Daily Galaxyvia NOAA's National Climatic Data Center and NY Times