Weather is a prime consideration for most people visiting Thailand. There are three seasons in Thailand
Summer runs essentially from March through May
Temperatures can reach over 100 degrees F (38 C) with the heat index going as high as 120 F (48 C). It may not get below 80 F (27 C) at night. This is also the Dry Season, with very little rain. It is oppressively and dangerously hot for some people. Rainy Season runs from June through October
Frequent heavy rains, most often at night. Temps from the 60s F (16 C) at night to the 90s (32 C) during the day.
Cool Season runs from November through February.
Temps from the 60s F (16 C) at night (can be much cooler – indeed cold – in the North) to the 90s (32 C) during the day. Not much rain.
Most years in Bangkok, the three seasons are, in effect, The Hot Season, The Hot and Rainy Season and The Hottest Season you’ve ever experienced in your life.
In terms of weather, the choice for most people is the Cool Season – November through February.