SEATTLE - Alaska Air Group Inc. reports that 58 percent of its Alaska Airlines flights arrived on time in December. The 13 percent decline from a year earlier was due to winter storms that affected operations at Seattle and Portland the last two weeks of the month. Horizon Air, Alaska's subsidiary,  reports 54 percent of its flights arrived on time last month, a 13-point decrease.

The Group also said Tuesday its December passenger traffic fell 5.2 percent on its main carrier, Alaska Airlines.

Alaska Airlines flew 1.48 billion revenue passenger miles, down from 1.56 billion in the same month of 2007. A revenue passenger mile is a widely watched industry measurement accounting for one paying passenger flown one mile.

Capacity fell 10.9 percent to 1.84 billion available seat miles from 2.06 billion in December 2007. Load factor, or occupancy, rose 4.9 percentage points to 80.7 percent.

For the full year 2008, traffic rose 1.4 percent to 18.71 billion revenue passenger miles, while capacity was flat at 24.21 billion available seat miles. Occupancy rose 1.1 percentage points to 77.3 percent.

Alaska Air said Horizon Air flew 183 million revenue passenger miles, down 21.5 percent from 233 million in December 2007.

Capacity fell 21.8 percent to 251 million available seat miles from 321 million in the year-ago period, while load factor rose 0.4 percentage point to 72.8 percent.

For the full year, Horizon's traffic fell 9.7 percent to 2.64 billion revenue passenger miles, capacity fell 9.1 percent to 3.62 billion available seat miles and occupancy dropped 0.5 percentage points to 72.9 percent.