You may not be laying on the bright beaches of Phuket, but Chiang Mai has its fair share of sun — and heat. The green mountain backdrop is just as beautiful as any beach view, and Northern Thailand — from what I heard and experienced — lacks the the level of humidity that an oceanside city would.
When I was there, it never got below the 70s at night, and was typically in the upper 90s, and even the low 100s during the day.