Nationwide, there have been more than 4, 500 daily record highs in the last 30 days, according to the National Climatic Data Center.

CNN: Cold front breaks heat wave, brings storms

The cool-down follows a heat wave that roasted much of the country for more than a week and comes as the National Climatic Data Center reported the mainland United States has experienced the warmest 12 months since record-keeping began in 1895.

CNN: Northwest heat endangers crops

Nationwide, more than 4, 500 record-high daily temperatures have been set in the past 30 days, according to the National Climatic Data Center.

CNN: July 7, 2012 -- Updated 0222 GMT (1022 HKT)

At the same time the National Oceanic and Atmospheric Administration's National Climatic Data Center has released a report noting that this spring in the United States has been the warmest since record-keeping began in 1895, a group of scientists has published a paper in the journal Nature warning that the planet is approaching a critical tipping point because of climate and other factors.

CNN: STORY HIGHLIGHTS

"This past year, unlike the US they were not a record globally but they certainly were warm, " said Thomas Karl, director of Noaa's National Climatic Data Centre.

BBC: 2012 was in top 10 warmest on record