Hot Season: March-May (Avg. High 90sF)
Cool Season: Nov-Feb (Avg. 78F)
Rainy Season: April-November
Thailand has two distinct climate zones: tropical in the south and tropical savanna in the north.
The northern, and central areas of the country (Bangkok, Chiang Mai, Chiang Rai) experience three distinct seasons.
The hot season from March to May, brings temperatures averaging in the upper 90sF and sporadic rain.
The cool season, from November to February, has temperatures around 80F with infrequent showers. Daily temperatures can drop as low as 60F in Chiang Mai and 41F in the hills.
The rainy season, from April to November, brings average temperatures of around 84F and 90% humidity. While heavy downpours are common, all-day rain is quite rare. Trekking in the north is not recommended in June, when floods are more likely. 
The Southern Thai Peninsula (Phuket or Koh Samui) has intermittent showers year-round, but daily downpours during the rainy season. Temperatures average in the low 80sF. Optimal weather on Phuket occurs between November and April, when the island welcomes the highest numbers of travelers. Koh Samui's high season is from Feb. to Oct.