Chiang Mai's climate is different from Bangkok and southern Thailand thanks to its mountainous setting in the north.
The cool season runs from late October to the end of February, with average daytime temperatures around 21°C (70°F) and noticeably cooler nights. December and January are the coolest months and the most pleasant time to visit — warm days, cool evenings, and little rain.
The hot season runs from early March to the end of May, with average daytime temperatures around 30°C (85°F). March and April can be hazy from agricultural burning in the surrounding hills.
The rainy season begins in early June and lasts through October, with September typically the wettest month. Showers tend to be heavy but short, and the countryside is at its greenest.
The major festivals are worth timing a visit around. Songkran (Thai New Year) in April is celebrated enthusiastically in Chiang Mai — expect city-wide water fights for three days. The Yi Peng Lantern Festival in November, when thousands of paper lanterns are released into the night sky, is one of the most visually spectacular events in Southeast Asia. Book accommodation months ahead for both.