The rainy season in Chiang Mai is fascinating and lasts from around June to about October. As opposed to most guidebooks I actually recommend people visit the city during this time of the year. There are several reasons for this: a) the rainy season doesn’t mean that it rains day and night, far from the truth. It may rain for one hour in the late afternoon and then not rain for the next 5 days or rain all night (when good children are asleep 🙂 ), b) Everything becomes green and the air is fresh after the little rain has gone. This time is beautiful, with many wild flowers around, and it is nice for trekking or visiting the mountains which is one of the main reasons people come to Chiang Mai, c) There is a very low level of pollution as opposed to the end of the cold season (from February to April) in which the levels of pollution becomes a hazard to health. During the rainy season the air is fresh in the morning, and the daytime is not too hot, d) The accommodation in the city is much cheaper than in the high season (from November until January).
All of this being said, I will have to warm you about the amounts of rain that comes down when it does rain. Within a matter of minutes it can change (and usually does) from a few drops to a torrential downpour which often leaves the streets flooded. The rain is usually heaviest in September, with an average precipitation of 250mm for that month. Another downside to the rainy season is the amount of mosquitoes in the beginning (May – June) – do put lots of mosquito repellent on.
The "rainy season" video below was taken on September 16th and shows you how much rain comes down at one time – enjoy 🙂