The numbers are in: 2012, the year of a surreal March heat wave, a severe drought in the corn belt and a massive storm that caused broad devastation in the mid-Atlantic states, turns out to have been the hottest year recorded in the contiguous United States.
How hot was it? The temperature differences between years are usually measured in fractions of a degree but last year blew away the previous record, set in 1998, by a full degree Fahrenheit.
If that does not sound sufficiently impressive, consider that 34,008 daily high records were set at U.S. weather stations, compared with only 6,664 record lows, according to a count maintained by Weather Channel meteorologist Guy Walton, using federal temperature records. That ratio, which was roughly in balance as recently as the 1970s, has been out of whack for decades as the country has warmed, but never by as much as it was last year.
"The heat was remarkable," said Jake Crouch, a scientist with the National Climatic Data Center in Asheville, N.C., which released the official climate compilation Tuesday. "It was prolonged. That we beat the record by one degree is quite a big deal."
But as so much of the country baked, the Bay Area avoided the off-the-charts heat. In fact, local meteorologist Jan Null calculates that San Francisco was 0.6 of a degree below average for 2012 while San Jose's average temperature was 0.2 under the normal.
Nationally, scientists said, natural variations almost certainly played a role in last year's extreme heat and drought. But many of them expressed doubt that such a striking record would have been set without the backdrop of global warming caused by the human release of greenhouse gases. And they warned that 2012 was likely a taste of things to come, as continuing warming makes heat extremes more likely.
Even so, the last year's U.S. record is not expected to translate into a global temperature record when figures are released in coming weeks. The year featured a La Niña weather pattern, which tends to cool the global climate overall, and scientists expect it to be the world's eighth- or ninth-warmest year on record.
Assuming that prediction holds up, it will mean that the 10 warmest years on record all fell within the past 15 years, a measure of how much the planet has warmed.
Last year's weather in the United States began with an unusually warm winter, with relatively little snow across much of the country, followed by a March that was so hot that trees burst into bloom and swimming pools opened early. The soil dried out in the March heat, helping to set the stage for a drought that peaked during the warmest July on record.
The drought engulfed 61 percent of the nation, killed corn and soybean crops and sent prices spiraling. It was comparable to a severe drought in the 1950s, Crouch said, but not quite as severe as the Dust Bowl drought of the 1930s.
Meanwhile, California experienced its fourth-warmest year on record. But some coastal regions of the state were affected by "the trough of the West," as Null called it. He notes that the jet stream looks much like ocean waves. As America's heartland cooked beneath a persistent high-pressure ridge, we were experiencing a cooling low-pressure trough.
"Our weather was a by-product of the high-pressure ridge throughout the entire middle of the country," said Null, of Golden Gate Weather Services. "That kept us relatively cool. It's like I tell my college students: Weather is complicated."
In fact, San Francisco had 219 days of below-average temperatures and only 133 days of above-average temps, according to the National Oceanic and Atmospheric Administration.
Bob Benjamin, observation program leader at the National Weather Service, added that it makes sense that the Bay Area would be slightly cooler than normal even as the state as a whole state suffered through such a hot year.
"It's a repercussion of the warm weather in the Central Valley because it makes the cooling effects of the sea breezes much more prevalent," Benjamin said. "It acts almost exactly like an air conditioning system."
Crouch pointed out that until last year, the coldest year in the historical record for the lower 48 states, 1917, was separated from the warmest year, 1998, by only 4.2 degrees Fahrenheit. That is why the 2012 record, and its one degree increase over 1998, strikes climatologists as so unusual.
"We're taking quite a large step above what the period of record has shown for the contiguous United States," he said.
Among the cities that set temperature records in 2012 were Nashville; Athens, Ga.; and Cairo, Ill., all of which hit 109 degrees June 29.
Our region also set records. San Jose's high of 84 degrees on Nov. 5 broke a record set in 1901 by two degrees. Records also were broken that day in Gilroy (90), San Francisco (79), Oakland (84) and Redwood City (84).
Crouch pointed out that at the beginning of January, 61 percent of the country still was in moderate to severe drought conditions.
"I foresee that it's going to be a big story moving forward in 2013," he said.
Staff writer Mark Emmons contributed to this report.