SeaTac-based Alaska Airlines today reported an 11.8 percent increase in traffic in January compared with January 2012.

The passenger increase didn’t quite keep up with the airline’s increase in capacity, which rose by 12.2 percent last month.

That meant Alaska planes were slightly less full last month compared with a year ago. The airline filled 83.4 percent of its seats with paying passengers last month compare with 83.6 percent a year earlier.

Alaska’s on-time performance improved greatly last month  to 86.7 percent.  It’s January 2012 performance, 79 percent on-time, was affected by a severe snow and ice storm that hit the Puget Sound area in January of last year.  Sea-Tac is the airline’s main hub.