The next consideration to keep in mind is the time of year you’ll be visiting. While it’s warm year round, there are three distinct seasons in Thailand: hot, cool, and wet.
Hot season runs from March through June and you can expect to be sweating excessively all day every day. You’ll get the occasional rain shower, but most of the time you’ll be walking around under the beating sun with little reprieve.
During this time of year, you’ll want lots of clothes made from quick-dry fabric to wick sweat instead of absorb it, and lots of lightweight flowy fabrics as well.
Cool season is from November to February. Cool is totally relative because it’s still just as hot as summer in Europe or the U.S. During cool season, Thailand is drier and less humid than hot season, making it more comfortable, but it’s still super warm.
That being said, you may want to bring a pair of leggings, sweatpants and a flannel, if you’re traveling during this time because temperatures at night can drop.
Finally, monsoon (wet) season varies from place to place but is generally from May to October, with the most rain coming from August to October.
Rainy season in Thailand will completely redefine what you think you know about rain the second you get caught in a downpour. The roads flood with water, you’re pelted constantly and then it will stop as suddenly as it started.
It’s rare that you’ll have an entire day of rain, but intermittent rain is almost a guarantee during this time of year.
Because monsoon season is so unpredictable, you should carry a raincoat or small umbrella with you all the time. No, seriously, get a lightweight, compressible raincoat and keep it with you always. You will be so happy when you’re the only dry one when the heavens open up.
Note: You can check more detail about Thailand weather and best time to visit