Northern Thailand
Chiang Mai's location at the foot of Doi Suthep in the North of Thailand makes for varied and sometimes surprising weather patterns. Situated in a valley at the foothills of the Himalayas, Chiang Mai, the "Rose of the North", is often referred to as Thailand's most liveable city. From the hot, humid days of Songkran -- the exhilarating water festival every April -- to the Cool January evening's by the Ping River, one can enjoy seasonal contrasts not typical of Southeast Asia.
For those trekking into the mountains, be sure to carry along a cosy sweater and a pair of cotton socks. Light, casual attire is fine most days of the year, though one should bear in mind that Thais -- and most other people as well! -- appreciate respectful clothes and are offended by revealing or unseemly dress. Although you won't run across any snowmen in the hills of northern Thailand, it sometimes feels that cold because of the contrast with the heat of the day. The cool season, between November and February, sometimes sees temperatures drop to near freezing at the higher altitudes. The hot season is between February and May, with roasting days around the end of March and the beginning of April. These are the days of mad dogs and Englishmen. The rest of us seek cool shelter in the heat of the day.
If you are a rain lover, you can certainly get your fill in the rainy season, between May and October.Although the mornings are often clear and bright, the clouds begin gathering and darkening in the early afternoon, typically releasing their loads later in the day, before retiring until the next day. The evenings can often be fresh and enjoyable after the cleansing downpour. It is a well-kept secret that this is a fine time to visit Thailand (at least it was).
The Northeast
In the northeast, which includes Ubon Ratchathani, Udon Thani and Nong Khai it rains less than in the north and central plains,and droughts in the region are not uncommon. The wettest months are August and September for when the rains are joyfully welcomed. Some of the hottest areas inthis region have been known to easily reach 39°C in the daytime, dropping only a few degrees at night during April. December is the coolest month but still averaging 29°C.

Central Thailand including Bangkok
Central Thailand including Bangkok is hot throughout the year, but the coolest and driest months to visit are November to March.April is hotter but dry and the rains begin in May, continuing through to October. Travelling during the rainy season needn't be unpleasant, since rain fall is usually short and heavy showers with some days cloud free and dry. The worst months for flooding are often September and October when the ground is saturated.
The South and The Southern Islands
The South has a tropical climate that is warm the whole year round averaging temperatures of around 29°C. There are two distinct seasons with a few minor variations in-between.
Many people visit in the high season (November to March) because of pleasant temperatures, blue skies and low rainfall. As a result the islands are quite busy and everything is a little more expensive. Hotels and tours can be quite crowded and the islands are not quite so laid back.
During April till mid-May, it begins to heat up to around 34°C  and there are often heavy thunder-showers in the late afternoons. The Thai New Year (Songkran Festival) occurs around April 13th and everyone sprinkles (or throws) cool water on each other. Welcome relief!
This quickly gives way to cooler temperatures and rain showers from late May until November (the monsoon season).
Visiting the Southern Islands in the monsoon season has lots of advantages. Temperatures average a comfortable 29/30°C and light warm winds make sightseeing tours very pleasant. During the monsoon season, it doesn't rain every day! The usual weather pattern is for short dramatic showers, with plenty of sunshine in between, providing an opportunity to chat with the locals while you're waiting for the rain to pass. June, July and August are particularly pleasant.
Activities and trips (apart from Similan diving trips) remain totally unaffected. You can still go swimming or kayaking, (falling off a canoe in warm water will not leave you shivering!) The islands are less busy and more relaxed - and also less expensive.
There are some great deals on hotel rates and much better shopping bargains. Better value for money - get the hotel room you want at the price you want! Check out the Special Offers! . Less people, more room on the beaches and it's easier to get a good table at your favourite restaurant. One point to watch, however, Is that swimming off the West coast beaches can be dangerous due to strong currents and rip-tides. Always observe the warning flags on the beach.
Don't be put off by the long-range weather forecasts. There could be a shower on one Beach, and bright sunshine on another a few kilometres south!

Finally, remember that this is after all a tropical climate, so its never cold and the sun is never far away!!!.