The cool, dry season from November to February is by far the best time to visit, with warm days, cool evenings and clear mountain air. March to April is the burning season, when agricultural fires blanket the valley in smoke and air quality can turn genuinely unhealthy, so it is best avoided if you can. The June-to-October rainy season is green and quiet with short, heavy downpours, and the Yi Peng and Loy Krathong lantern festivals light up the city in November.
From roughly late February through April, crop-burning fills the Chiang Mai valley with haze and air quality can reach unhealthy levels. If you visit then, check a live air-quality index, keep an N95 mask handy, limit strenuous outdoor activity on bad days, and take special care if you have asthma or a respiratory condition.
Rented scooters are a leading cause of tourist injuries in northern Thailand. Only ride if you are licensed and experienced, always wear a helmet, avoid riding at night or after drinking, and watch for gravel and sharp bends on the mountain roads to Doi Suthep and beyond.
Days are hot for much of the year and temple-hopping means time in the sun. Carry water, wear sunscreen and a hat, and pace yourself, especially on the long stairway and grounds at Doi Suthep.