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Heat picture - a construction worker cools off with a garden hose in Carpentersville, Illinois.

Construction-company owner Joe Weston cools off in Lake Geneva, Wisconsin, last month.

Photograph by Laura Stoecker, Daily Herald/AP

Christine Dell'Amore

National Geographic News

Published August 8, 2012

July was the hottest month on record in the United States, perhaps due to a combination of global warming and a widespread drought, experts say.

The lower 48 U.S. states experienced an average July temperature of 77.6 degrees Fahrenheit (25.3 degrees Celsius).

That's about 3.3 degrees (1.8 degrees Celsius) above the 20th-century average and the highest July average since record-keeping began in 1895, according to a report released August 8 by the U.S. National Oceanic and Atmospheric Administration (NOAA).

For 2012, July wasn't an anomaly, either. Taken together, the first seven months of the year have been, on average, the warmest January-to-July period on record in the contiguous U.S. states.

Drought conditions in more than 60 percent of the country helped keep temperatures high, explained Kevin Trenberth, a senior scientist at the Boulder, Colorado-based National Center for Atmospheric Research.

"If it is wet, it tends to be cool, while if it is dry, it tends to be hot," Trenberth said. With no water on the ground, "all the heat goes into raising temperature and not evaporating moisture."

(Related pictures: "Surprising Effects of the U.S. Drought.")

That creates a "powerful feedback loop," since "drought begets drought. And drought causes heat waves," Trenberth said by email.

On its own, the July record "is not such a major feat," Trenberth said. "But the fact that the first seven months of the year is the hottest on record is much more impressive, from a climate standpoint."

The consistent heat "highlights the fact that there is more than just natural variability playing a role: Global warming from human activities has reared its head in a way that can only be a major warning for the future."

Heat Ridge Kept July Hot and Dry

The current heat wave throughout the continental U.S. is due to what's called a giant heat ridge: when a high-pressure system in all layers of Earth's atmosphere creates a clockwise wind flow that forms a sort of barrier at its edges.

AccuWeather.com meteorologist Mark Paquette likens a heat ridge to a rock in a stream. Just as water is deflected around a rock, other weather systems are forced to go around a heat ridge.

As a result, during a heat ridge event, "nothing goes on but sunshine," Paquette said.

A heat ridge commonly forms over the continental U.S. in summertime, but "generally it's not as far east and north and not as persistent as this past summer," Paquette said.

The 1930s Dust Bowl was a "classic heat ridge" that led to record temperatures, he said—all of which have been eclipsed in recent years.

Paquette agreed with NCAR's Trenberth that the persistence of high temperatures suggests that global warming may also be at play. (Test your global warming knowledge.)

"Eventually it gets to the point where you've got to say that something strange may be going on here—if we haven't gotten to that point, we will get to [it] soon."

Heat Wave May Continue

There's some debate about whether the heat ridge will continue into the fall, Paquette noted.

AccuWeather's fall forecast calls for above-average warmth to remain in the southern and central plains. States in the Ohio Valley, central Appalachia, and the Northeast are expected to exceed their averages by greater margins than any other states.

"Unless you're going to get some sort of weather feature that ends the drought, strong summer sun is going to continue that pattern."

For instance, a tropical storm, common in late summer, can bring rain and clouds. And, Paquette noted, there's a potent storm now moving across the Great Lakes now, which is driving some cooler air into the plains and rain into Texas and Oklahoma.

NCAR's Trenberth said the heat may not be as bad next year in the U.S., "but these conditions will likely occur somewhere else" on the planet, he said.

"Human-induced climate change has a cost, and one that is ultimately much larger than slowing down or stopping the basic problem: burning of fossil fuels," Trenberth said.

"As a nation we are not doing that," he said—"nor are we prepared for the consequences."

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