In theory there are 3 seasons in northern Thailand:

  • the cool season from November to February
  • the hot season from March to June
  • the rainy season (some want to call it the green season... more marketable) from July to October

In practice:
El Nino is a bad boy, his sister El nina is no better, they're messing up with the weather... well they have an excuse, humans feed them with carbon dyoxide, but still, it is becoming difficult to predict the weather. This year it's been raining in December and during a large part of May.
So you'll understand that the following is not 100% foolproof.

Usually the seasons change quite abruptly during the transition months, October/November, February/March, June/July.  The landscapes and nature can look quite different, and this might be an important factor for some travellers. For a visual idea visit  When is the best time to visit northern Thailand ?  

During the cool season , the max. temperatures reach a comfortable 25° C, nights and early morning are cool in the city (15°), cold in the countryside (10°), possibly freezing in the mountains (ice is reported from time to time at Doi Inthanon). Though you'll probably find it great when you walk around the city, you'll really feel the cold if you decide to ride a motorbike, then the air turns crispy and it's not uncommon to need jacket, gloves and scarf (before noon or after 6 PM). Otherwise the sky is blue, without clouds. The good thing is, you don't need to turn on the air conditioning to sleep.

During the hot season, the max. temperatures can reach 40° C and won't drop below 25°C at night. Many rooms keep the warmth accumulated during the day so you'll definitely need a full blast fan or a proper air con to sleep well. The sun is scorching, play the lizard between noon and 2PM or you'll soon take on the color of a boiled lobster (with a burning sensation to match). Another must is to drink loads of water, 4 or 5 liters even if you don't feel thirsty (no you won't spend your time in the toilet, the water evaporates beforehand, through every pore in your skin). Local talcum powder (with cooling effect) works wonders to keep a fresh look.

During the raining season, the max. temperatures stay over 30° C, evenings can be hot or cool depending if it  has been raining or not. But what you lose in temperature, you gain in humidity, it doesn't make it more comfortable. Bad weather usually comes in 3 to 4 day periods, and alternates with fine weather. So, those fortunate ones can spend a week under the sun most of their time, while others will have rain, rain and more rain... it's a bit of a lottery. Rain comes in every form, at anytime, day and night: heavy downpour for an hour or two , small drizzle all day... better to bring your own raincoat, the plastic ponchoes sold here are real sweat factories.

When to come then ?

  • January weather is almost perfect, but it is considered tourist high season and prices for accommodations are at their highest. It may seem exotic to spend the new year's eve in Thailand but there is not much celebration. International restaurants offer traditional meals from their home country. Quite popular with the expats, maybe not for the tourists.
  • February, almost like January weather wise, but with less tourist and prices back to normal. Yearly burn off (crops and forests) can affect the pollution levels in Chiang Mai, usually late Feb.
  • March, you start to feel the heat but it is nothing unbearable. If you like mangoes, it's the right time to come and enjoy the fruit traditionnaly served with sticky rice and coconut milk. Yearly burn off (crops and forrests) can be quite bad during March, take care to look up air pollution levels and wear facemasks when walking around or riding motorbikes.
  • April, temperatures are at their peak, but it is also the time for the craziest celebration in the country: Songkran, the water festival also called Thai New Year. Thais come from all the provinces for the occasion. It may prove difficult to find transportation/accommodation during the festival.  Festivities last about 5 days, and you don't feel the heat because you're drenched from dawn to dusk with buckets of water (not always the cleanest) thrown by everyone. Could be called the alcohol festival, too, as there is a huge consumption of various liquors. Consequently there is a lot of road casualties, beware if you have to drive during this period. The yearly burn off (crops and forrests) usually ends early April.
  • May, the accumulated heat leads to regular evening thunderstorms, much welcome. This could be called the month of Buddha, because he was born, enlighted and died in the month of May. There is alot of genuine celebrations in the many temples. Don't be shy, you can join even if you don't know the local customs. 
  • June, usually very similar to May, unless the rainy season comes early. If you're a fruit lover this is your month. Don't miss the Mangosteens and above all the 'jumbo' Lychees. The landscape takes brownish shades after so many months without proper watering. Maybe not the best time for jungle trekking.
  • July, due to dry soils, the first heavy rains of the 'green' season can lead to spectacular floods. In Chiang Mai, the night bazaar area can easily suffer, though it never lasts very long. You should still get a majority of dry days (not always sunny). If you're a photographer, skies can be very dramatic, especially when the sun goes down behind Suthep mountain.
  • August, dry days, rainy days are even its really up to your luck during which you end up. Rivers fill up, it's time to enjoy some rafting, in the Pai area, or to take nice photos of the many waterfalls (great in Doi Inthanon park).
  • September, statisticaly the month with the most rain, not necesserally true. But floods can occur during this time, due to the soils being saturated with water.
  • October, the period when the rainy season ends, you just wake up one day, the sky is bright blue and will remain like this for months. Prefer late October, if it's your choice.
  • November, if I had to name the perfect time to visit Chiang Mai, it would be November. The weather is nice, the temperatures in the low 30°C, it's not the high season yet, prices are fair. But most importantly, it is the time for the beautiful Loy Krathong Festival. To be honest, it's in Sukhothai that you'll see it under it's most traditional aspect. In Chiang Mai fewer and fewer women put on their best sarong and jewelry, still it is great close to the Mae Ping River. Be careful, there are a lot of firecrackers around (many kinds, they can be bought anywhere), maybe not the best for young children, but kids would definitely love it.
  • December, great weather, high season, prices on the rise, the Night Bazaar is crowded to its maximum, but Christmas in Thailand that sounds so cool... have never seen Santa Claus in shorts, T-shirt & sandals though.
 Overall, it really depends on a variety of factors and for every traveller the importance of these factors will differ.