The most important thing to know about the weather in Bangkok is that it is hot and steamy almost all the time.  Those in the know say there are three seasons: hot, very hot, and absolutely unbearably hot. 

Actually, there are three slightly definable seasons.  The rainy season begins in late May and lasts until October or November.  It is warm and humid, and most days have a few showers, especially in the afternoon.  At times -- but not all the time -- the rain comes down in buckets, and the thunderstorms can be dramatic.  But if you are visiting during this time of year, don't bother to bring a raincoat -- it will be too hot to wear it.  Carry an umbrella with you at all times and be prepared to take your shoes off and wade, and you'll be fine.

The very best time to visit is late November to mid-January.  The Thais call it the cool season, but that really stretches the truth.  It's a tropical cool, still humid, but comfortable.  Still, you do not need to bring warm clothes.  "Cool" does not mean you need to change your tropical wardrobe; it just means you sweat a little less.

The time of year you do not want to visit is from March until the rainy season begins in May.  It is hot, hot, hot.  It doesn't seem to faze the Thais at all, but those of us who were born in more hospitable climates start to get a little cranky.  It is very hard to get out and see the sights during this time of year; the heat drains you of energy and makes it hard to keep going strong all day.