The weather in Bangkok is dominated by a tropical monsoon climate which means there are three main seasons: hot season from March to June, rainy season from July to October and cool season between November and February. As Bangkok holds claim to being the hottest city in the world, expect daytime temperatures to be in excess of 30 degrees centigrade throughout the year (with the possible exception of late December and early January).
Read more at: http://www.bangkok.com/weather.htm?cid=ch:OTH:001