Chiang Mai, Chiang Rai and much of northern Thailand are likely to be cool and dry, with little chance of rain (avg temp: 22-25°C). These favourable weather conditions make February an ideal month for the outdoor activities that the north is so well known for and provides a welcome break from the heat further south.
Dry conditions can also be expected in Bangkok, although temperatures will be warmer than further to the north due in part to the city's lower altitude, and tend to hover in the high 20’s°C.
If you plan on exploring the rainforest of Khao Sok National Park, February is one of the driest and warmest months of the year, however being the rainforest, showers can be experienced even during dry season.
In the south, dry, sunny weather is the order of the day with plenty of blue skies and little chance of rain. Average daily temperatures of 26-31°C can be expected. The islands of Koh Chang & Koh Kood (on the eastern side of the Gulf of Thailand) may experience the occasional rain shower but this shouldn't deter visitors to these islands.
Favourable diving conditions continue off the west coast around Phuket, Khao Lak & Krabi, with good underwater visibility and water temperatures around 27°C.