July Was Hottest Month Ever Recorded in U.S. – NYTimes.com http://www.nytimes.com/2012/08/09/science/earth/july-was-hottest-month-ever-recorded-in-us.html

A hot July also contributed to the warmest 12-month period ever recorded in the United States, the statistics showed.

Climatologists at the agency noted that by the end of the month, about 63 percent of the nation was experiencing drought conditions, which contributed to the high temperatures.

“July was a pretty interesting month because there were two different things at play,” Jake Crouch, a climatologist at the agency’s National Climatic Data Center in Asheville, N.C., said in an interview. “We saw very warm daytime temperatures over a large part of the country related to the ongoing drought, just as in 1936. 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.