U.S. airlines in June turned in their worst on-time performance since December, the Department of Transportation said Tuesday.

The airlines had a combined on-time arrival rate of 76.1 percent compared with 80.5 percent in May. But the carriers had fewer delayed flights this June than in the same month a year ago. The on-time rate in December was 65.3 percent.

The most frequent reasons for flight delays included airport congestion, equipment problems and weather.

June was a bad month for severe weather across the nation. The National Weather Service recorded more than 6,400 incidents involving hail and high winds, almost twice the number reported in May.

Hawaiian Airlines had the best on-time performance in June, while Comair — a subsidiary of Delta Air Lines Inc. — had the worst.