Whilst nearby Chiang Mai enjoys year-round temperatures in the 20-30C range, Doi Inthanon is decidly cooler, with temperatures reaching zero during winter (December-January). The mountain is often shrouded in mist and heavy rain is not uncommon. The entire falls within the Doi Inthanon National Park, and while you may think this would open up all sorts of hiking trails, there are infact no hikes/trails to the mountain summit. The only way up is via road.
Covered in forest, the mountain is temperate and you'd certainly need at least a light sweater during the winter months. During our November climb, the temperature ranged from 2-12C.