Chiang Rai has a tropical humid and dry climate. The winters are dry and warm although in recent years there have been some waves of cold in December and January (temperatures between 14 and 18 °C in the plain but for Thailand it is cold). Temperatures rise until April, the hottest month here. The monsoon season extends from late April to October, with heavy rains and cooler temperatures during the day.
It is only in July-August that coming to Chiang Rai is not really advised because of the risk of rains. Be careful in winter (December to March), morning temperatures can be quite cool, bring a sweater in your luggage and even a warm jacket if you plan to go in the mountains.
See weather forecast in Chiang Rai