United Airlines jumped to the top spot in the latest government rankings for on-time performance, moving up from its fifth-place finish in the month prior.

Delta, the previous No. 1, dropped to second. The carrier was hit by two major schedule disruptions in Atlanta, its busiest hub. An unusual snowstorm and a power outage affected the airport during the month.

Showing significant improvements were Alaska Airlines and merger partner Virgin America, which jumped to No. 3 and No. 4, respectively. They were the two lowest-rated airlines during the previous month.

The ratings are for December 2017, the latest month for which the federal data is available. The federal government counts a flight as on time if it arrives no more than 14 minutes behind schedule. The federal data looks at the 12 biggest U.S. airlines. It also breaks out separately the regional carriers the fly under the brands of their major partners.

Scroll down for the full ratings as by the federal Bureau of Transportation Statistics in its latest Consumer Air Travel Report:

1. United Airlines, 84.6% (Previous month: No. 5)

BY THE NUMBERS: The fleet and hubs of United Airlines, by the numbers

2. Delta Air Lines 83.5% (1)

3. Alaska Airlines, 83.4% (11)

4. Virgin America, 82.5% (12)

5. American Airlines, 82.0% (4)

6. Hawaiian Airlines, 80.8% (3)

7. Spirit Airlines, 80.4% (2)

8. Southwest Airlines, 79.1% (6)

9. SkyWest Airlines, 76.5% (10)

10. ExpressJet, 76.4% (8)

11. Frontier Airlines, 75.8% (9)

12. JetBlue Airways, 74.1% (7)

Total for all covered airlines: 80.3%

Contributing: The Associated Press