Thailand experiences tropical monsoons, and there are three clearly-divided seasons.
The hottest season is from March to June when the high temperatures may reach 40 degrees Celsius or more than 100 degrees Fahrenheit. This is the best time to visit the beaches and go swimming and canoeing. The weather is dry.
From July to October, the temperatures are not as high usually, but it rains a lot. The main monsoon usually starts in September.
The most comfortable season is from November to February. In the mountainous north the temperatures might reach single digits Celsius or the forties Fahrenheit. But in the south, the temperatures remain quite hot. Thailand is a long country north to south, and each region has its own climate.