Story highlights July's average U.S. temperature topped 20th-century average by more than 3 degrees

The first seven months of 2012 are the warmest since record-keeping started in 1895, NOAA says

The high temperatures have contributed to a drought that has battered farmers

The July heat wave that wilted crops, shriveled rivers and fueled wildfires officially went into the books Wednesday as the hottest single month on record for the continental United States.

The average temperature across the Lower 48 was 77.6 degrees Fahrenheit, 3.3 degrees above the 20th-century average, the National Oceanic and Atmospheric Administration reported. That edged out the previous high mark, set in 1936, by two-tenths of a degree, NOAA said.

In addition, the seven months of 2012 to date are the warmest of any year on record and were drier than average as well, NOAA said. U.S. forecasters started keeping records in 1895.

And the past 12 months have been the warmest of any such period on record, topping a mark set between July 2011 and this past June. Every U.S. state except Washington experienced warmer-than-average temperatures, NOAA reported.

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The high temperatures have contributed to a "rapid expansion" of drought across the central United States, NOAA found. Dozens of cities and towns already have seen the mercury hit record levels this summer, and three states -- Nebraska, Kansas and Arkansas -- saw record dry conditions between May and July.

That's battered American farmers' corn and soybean crops, driven farmers to sell or slaughter cattle they can't feed and spurred the U.S. Department of Agriculture to designate more than half of all U.S. counties as disaster zones.