The weather in Thailand is classified as hot and humid, with a long monsoon season.  Thailand has three official seasons - hot, cool,  and wet.

The hot season runs from March through to June, with April and May the hottest months of the year.

The monsoon season usually is accompanied by heavy rain.. This season may start earlier than years before-- as early as April or May, although it officially begins in June and runs as late as October, when the temperatures begin to cool slightly... The rain isn't constant in the early months of the period as It will rain for one or two hours, and then dry out. But it can rain torrents when it does rain. Be aware that late in the monsoon season is when the rains are at their heaviest and most persistant, and flash flooding can occur. Carry an umbrella or raincoat, and you can still enjoy a great vacation in the Thai rainy season. In the south, (including the Amdaman islands), the rains don't end until late November. 

The cool season normally runs from November thru February and will still feel hot to most visitors, but much of the country does experience a short cold snap for a week or so in December, especially in the north and notheast where it can get quite chilly. 

On the gulf coast around the islands of Samui, Phangan and Tao, the seasons are different with the best weather being from about June-October (July & August are peak season), then the weather slowly changes towards the local monsoon season in the area. 

Whenever you’re visiting be sure to bring plenty of breathable and light clothes, and a lot of good sunscreen. The sun can be quite intense. But at least there is typically a lot of it to enjoy!

Real-Time Weather Reports

 You can use this link to check on the current weather conditions anywhere in Thailand.. Updated in real time. You can select regions and then cities from the left navigation. http://www.tmd.go.th/EN/ - for Phuket click here: Phuket Weather