Thailand has three "seasons." The Thais refer to them as the "cool," "hot" and "rainy" seasons. A more accurate description would be "hot," "really HOT" and "really hot AND wet." The "cool" season from October/November through to March is the prime tourist season. Current weather conditions for the major cities covered (Chiang Mai, Phuket, Samui and Bangkok) are displayed for your information. If you're viewing these pages in the Americas during the day, keep in mind that the conditions you see reflect night-time conditions in Thailand. For more detailed information, and forecasts, click through the weather banners to get to Weather Underground, where you can also find information on additional cities.
Tradition also has it that the worst time to visit is during the rainy season which starts in earnest in May or June and lasts until September or so. This particular piece of traditional wisdom is not entirely accurate. During the rainy season, it may rain for only an hour or less each day, usually in the late afternoon. Yes, the downpours are drenching, but they are short lived and you get up to an hour's notice that they are coming -- if it starts to get dark at 4:00 in the afternoon, run for cover! We're not talking cold showers here either. The rainy season is still quite hot, so the rains can sometimes be warmer than you might be used to in your bath at home.
The rainy season is actually an annual monsoon that moves east and northward in the first half of the year before shifting south and west in the second half. This means that when the rains are their heaviest on Phuket in May they are still rather light in Samui and haven't even started in Chiang Mai.