Temperatures in the contiguous United States last year were the hottest in more than a century of record-keeping, shattering the mark set in 1998 by a wide margin, the federal government announced Tuesday.

The average temperature in 2012 was 55.3 degrees, one degree above the previous record and 3.2 degrees higher than the 20th-century average, scientists at the National Oceanic and Atmospheric Administration said. They described the data as part of a longer-term trend of hotter, drier and potentially more extreme weather.