The beginning of rainy season is a great time to visit Chiang Mai.  Everything looks green and the air is fresh after a little rain has gone.  It has not a big rain, just a short rain. This time is beautiful with wild flowers, nice for trekking or visiting the mountain which is the main tour program in Chiang Mai. No pollution from wild fire or anything else.  There is fresh air in the morning, and the daytime is not too hot.