Northern Thailand is the coolest part of the country. Thailand’s tallest mountains surround this part of Thailand. In the mountains at night during the coolest part of the year the temperature can dip below freezing. During the day it will go up to 25C or 77F.
The northern region does have three seasons. Hot season runs March through May, rainy season starts end of May through November and the cool season runs December through February.
During the rainy season, the northern region will get a lot of rain, Chiang Mai will get any where from 2 inches a month at the start and end of the rainy season, to over 6 inches to 8 inches during the heavy times.
Central region of Thailand with Bangkok in it also has the three seasons. Hot season runs March through June, rainy season starts July through October and the cool season runs November through May.
The temperatures in Bangkok range from 35c/95f during the day to 21c/70f the night on average. The hottest month in Bangkok is April. It has been know to get close to 40c/104f some times but not very often.
Southern peninsular region Malay Peninsula only has two seasons. Hot and wet. There are several mountain ranges along the length, 1835 meters at the highest. Because of the mountains, the weather is different on the sides on the peninsula. East coast rainy season is September through December. West coast side rains come April through October.
Yearly temperatures are more even than in the northern parts of Thailand. The more south you go on the Malay Peninsula the shorter the rainy seasons.
