Coal's share of electricity generation in Dec. fell below 40 percent, for the first time since 1978

In December, only 39 percent of the electricity produced in the U.S. came from burning coal, according to preliminary data from the Energy Information Administration. It marked the first time in more than 34 years that coal's share of monthly generation fell below 40 percent. Three years ago, it was just under 50 percent.

"A combination of mild weather (leading to a drop in total generation) and the increasing price competitiveness of natural gas relative to coal contributed to the drop in coal's share of total generation," EIA reports. Gas accounted for 26 percent of December generation, up from 22 percent in the previous December. Nuclear (to 22% from 20%) and hydro (to 7% from 6%) made up the rest of the shuffle. Other sources, including wind and solar, accounted for 6 percent.

"Natural gas prices have dropped significantly this winter, leading the generators in some states (such as Ohio and Pennsylvania) to significantly increase the share of natural gas-fired generation," EIA reports. Natural gas combined-cycle units operate at higher efficiency than do older, coal-fired units, which increases the competitiveness of natural gas relative to coal."