Thailand is a tropical country so it is never cold during the day. However, at night in the winter, the temperature can drop to zero degrees at high elevations. But, even on the coldest of nights, by 10 a.m. the next day it will be at least 15C/59F by mid afternoon it will be warm enough to wear shorts and a T-shirt.
There are three seasons, the hot season, the rainy, and the cool. The hot season is from late February until July, the rainy season is from July until October, and the cool season is from November until mid-February.
You will notice in this portion of the gallery people are dressed in warmer clothing. Chiang Rai has the coolest weather in the country, as the city is located at over 350 m. elevation. Mornings are usually comfortably cool. When we visited the night bazaar, many of the locals bundled up in warm wool clothing giggled and pointed at me. I was dressed in short pants and a short sleeve shirt. Back home I put on a coat when the temperature drops below 20/68F.