Much of the United States has felt winter’s bite this week, with fresh but frigid cold to the east and a weeks-long spell of stagnant, polluted chill to the west.

Two leading climate-change researchers cast new light on one of the most prominent findings of the last major IPCC report, issued in 2007.

New research points to gravity waves, which ripple unseen through the atmosphere, as the culprit in many cases of clear-air turbulence. If those waves can be forecast, the research suggests that planes in many cases could be rerouted around them.

Even as the rest of the nation baked in 2012, Alaska froze. The contiguous 48 U.S. states saw their warmest year on record by far, but it was one of Alaska's chilliest.

More than two days ahead of landfall, it was clear that Hurricane Sandy could bring higher water than New York and New Jersey had seen in decades. But for thousands of people in the area, the threat simply didn’t register. (Part 1 of 2)