Needless to say, you can't expect very cool temperatures in Chiang Rai. Even if you visit this destination during the cool season (which is of course the ideal time to visit its waterfalls), you have to expect temperatures in the low 30s. So, visit Chiang Rai from October to February and explore the cascading waterfalls. The skies will be all clear, but you may expect some evening showers here and there. From August to September is the wet season where there can be unpredictable heavy rainfalls all the time. If you plan your Chiang Rai tour during this period, make sure you are aware of the flood situations and landslides before you head to areas with waterfalls.