I am going on a road trip around the north of Thailand driving on a vacation during the trip i will be looking at some were else besides bkk to reside when im not working like buying something like propert. I want to stay in a cooler area when I am not busy in bkk

I was told Mai hong son as an area Pai udon tani , chaing rai . I will be visiting these areas on my travels,

What area has the coolest climate usually year round

I think you'll find that the coolest temperatures 'year round' are at the coast.

The North which gets a cold winter is also cursed with a hot summer and lately smog

Depending on where you come from, you'll likely find Thailand hot - plain hot - just about everywhere, all the time.

In the mountains of Chiang Rai for a few months, it sometimes gets cool enough to require long sleeves or even a sweater, particularly at night.

However anywhere in the country, being somewhere in the countryside, especially raised up a bit from ground level exposed to the breeze it's much more bearable than being in a built-up area surrounded by countryside.

Up on a hill overlooking the ocean is best IMO, ideally in a shaded hammock with a pitcher of iced lemonade within reach. Sometimes a fan is necessary but for me that spoils the effect.

If you're here long-term and enjoy spending time outdoors, then definitely wean yourself off aircon, only sleep with a fan, maybe turn the A/C on for an hour or two say if you're working at a desk in the hottest part of the day.

The worst is spending most of your time in A/C and then trying to walk around outdoors, sure formula for misery.

I think you'll find that the coolest temperatures 'year round' are at the coast.

The North which gets a cold winter is also cursed with a hot summer and lately smog

The north does have a hot dry season, but it cools down at night. I can deal with a slightly hotter day if it cools down more at night. Lately in Bangkok it's too hot even at night, it's still in the 80s at 3am.

The air pollution is a big problem though

I think Phu Rua has the coldest temperatures... and Umphang has the reputation of being the coolest spot overall. For places to live, you have not much choice outside the provincial capitals, unless you want to set up house miles away from anywhere. Mae Hong Son is nice for a visit, but there's very little to do. Pai is Khao San Road writ rustic.

You can try Doi Inthanon...

"Temperatures at the summit in winter have been known to drop to around zero, and even in summer it remains relatively cool on the peak." (http://www.chiangmai-chiangrai.com/doi-inthanon-national-park.html).

Unfortunately, don't think you can live there as it's a National Park and all that...

Most of the year Northern Thailand is not too uncomfortable. We have relatively few days where it is cool during the day but it is usually not too bad after the sun sets or in the early morning.

That said I do us the A/C most of the year. It helps to filter the air during the burning season and at night it filters the village, animal and bug sounds.

Just take your trip and keep an eye out for a place that clicks with you. Only you know what you like. There is a lot of development up here so expect things to change rapidly.

I think Phu Rua has the coldest temperatures... and Umphang has the reputation of being the coolest spot overall. For places to live, you have not much choice outside the provincial capitals, unless you want to set up house miles away from anywhere. Mae Hong Son is nice for a visit, but there's very little to do. Pai is Khao San Road writ rustic.

Don't know if it's fact or not (about Phu Rua) but spent a few months there in Phu Rua. During the summer, it was nice hot days, cold cold nights. During the winter, it was just incredibly cold. Remember thinking "Really Thailand, REALLY?!" There's also Phu Luang that's similar. Or just Loei lol (though I know you said North, would you consider Isaan?)

The worst is spending most of your time in A/C and then trying to walk around outdoors, sure formula for misery.

When I first got here, most expats tried to avoid air-con to save money and we were mostly miserable. Eventually, it became less expensive and most people that I know use it when it gets really hot. Personally, i feel terrible during hot season if I do not use it at all, but am OK with using it only part of the day.