Weather in Thailand
One of the most common questions about living in Thailand is the heat, humidity, sunshine and rainfall. "What part of Thailand has the best climate?"
The answer to all of the questions, "It all depends".
Weather Is As Weather Does
Okay, so I stole that line from Forrest Gump. What it actually means is that what you expect from your research may be completely unrelated to what you may experience and where you experience it.
A perfect example is the Southern peninsula of Thailand. If you read about heavy rainfall in the South you are reading a gross generalization. Ranong has the heaviest rainfall in the Southern provinces year in and year out with a gully washing 4,000 mm of annual rainfall. Yet down in Songkla they only get about 1,600. That's less than the 2,200 of Nakorn Phanom in Thailand's "dry" Issan region.
If you check Prachuap Khirikan and Petchaburi you might expect somewhere around 1,000 mm. Yet if you lived in a condo on a stretch of beach between Cha am and Hua Hin in 2008, you probably didn't get enough rain to keep a goldfish alive.
So what's this all about? Microclimates. Thailand has lots of microclimates, some of them influenced by mountain chains. To find out for sure about weather in a region you need to get information from the locals, whether expats or Thais.
Strange Weather Events in Thailand
Who knows what influences mother nature to occasionally create a stunning display of weather. "Thailand doesn't get typhoons", yet in 1989, Typhoon Gay completely uprooted the eastern beach and coral reef at Koh Talu and heaved it onto the land. Now there is a coral beach there.
Whaddayamean hailstones in April. In Thailand? Quite frequently in Chaing Rai, Thailand; Songkran cooks up a hailstorm. I have been exposed to one at close range. Shivering in an open sided restaurant, pelted by near freezing rain driven by 40 km/hr winds as hailstones on the corrugated metal roof created such a din that no one could hear any of us ask, "What the hell?"
Flooding Happens Often in Thailand
One aspect that ties house hunting / building to weather is the potential for flooding. Every year, it seems, floods devastate sections of Thailand.
"The water level reached on August 15  at Vientiane was the highest recorded since records began in 1913." - Mekong River Commission
Certain areas of Thailand seem more prone to flooding more often, but predicting when and where is difficult.
After living in one house at a resort that was inundated when a dam overflowed, I can tell you that devastated the area. My house was elevated and I happened to be in Bangkok when it happened. I returned to see entire sections of the coastal road washed out to sea. Roadside salas and more than one house suffered the same fate.
Don't live in an area that floods on a regular basis. Believe it or not, many seem to flood annually yet people continue to live there.
Hot & Hotter Thailand
In the overall scheme of hot and cold regions of the world, Thailand has some pretty good choices. Even on the hottest Day of May in Chiang Mai, Doi Intanon will be pleasant. Not only is Northern Thailand in a more temperate zone, many of the cities and communities are at advantageous elevations. Sea breezes will also cool most days along the coastlines. You actually have to work at it a bit to find a place where it seems to be unpleasant 24/7.
Yes, there is Bangkok. The steel and concrete jungle that is Thailand's capitol has an additional issue called smog. The air is terrible there.
But Thailand is a "long" country, somewhere around 2,000 km from top to bottom. Google earth show the northern border to be 20.27° north latitude and the southerly most tip reaching 5.35° north latitude.
That leaves lots of room to find a climate that suits your taste and high ground as well. You're just not likely to find all that on a white sand beach.
General Information About Thailand Weather
So now that you're convinced that you really do need to do more than cursory research on the weather in various regions, you do need a starting point. We also have some opinions and information about air quality.
So have a look at these Thailand temperature and rainfall tables.