Bangkok can be hot and humid throughout the year, with temperatures often reaching 35°C (95°F). The best time to visit is between November and February as April is generally the hottest month and October the wettest.
The Thais often say: We have only two seasons, hot and hotter. This is true as far as it's never cold in Thailand. Officially however there are three seasons:
Cool (from November to February) with temperatures in the mid twenties to thirties (Celsius).
Hot (from March to May) with temperatures always above 30 and often around 40.
Rainy (from June to October) with temperatures in the mid twenties and regular tropical rain and thunder showers.
In Bangkok and the South of Thailand it is never cold but in the North temperatures can come close to 0 during the night in the cool season.
Fondest memory: bring a small umbrella always when travelling in october like we did
Favorite thing: When I was in Bangkok, I got caught in a horrible downpour. Having an umbrella didn't help me at all. I was wet up to my waist. Sure my head and shoulders were dry, but that was it. Unfortunately, I can't think of anything that will keep you dry short of wearing full body rain gear. My suggestion is go inside somewhere and wait it out. The rain that I was showered in only lasted a short while.
The weather has been wierd, not just in Thailand but many other parts of the world as well. Last year, Nov in Chinag Mai up in the mountains, the weather should've been cool but it was pretty hot, so....
"Normally", first part of April you may get a shower but even that is rare. It may cloud up some but rarely rains until end of April, mid May. As John said, if it does rain it'll be hard and short and the hot and humid. April is known as the last part of the dry season. I think you'll experience more cloud action than anything.
I say, "never pass up a chance to go to Thailand!"
As in many of the questions asked on VT, it is really a matter of one's opinion.
For us it is Novemeber. The rainy is behind you, you could still get a nice shower periodically lasting maybe 15-30 minutes but nothing like peak season! The tourists have not gotten there in high numbers yet like in Dec and Jan and it's not dry like it starts to get in Feb. However, should you wantto do some diving the best season is during the dry seasons of Jan through April, best visibility, conditions etc... We like November also because of the lushness of the forests and jungles after the rainy season. If you are only staying in Bangkok as you mention,then I would say any of the months are not too bad. The closer you get to Feb the hotter it's going to get. Depending on where you are coming from, you may not even notice the difference.
Good luck and have fun!
You are going to have a lot more heat, more heat and a lot of humidity this time of year. Actually it's the dry season just before the rains. If it does rain you are probably going to like it! If not to cool off for the change in weather. But heat and humidity is somewhat relative depending on where you are coming from, the climate you are used to and like and how fast you can acclimate if you have to. Leaving this week, lucky YOU!
Bangkok has a tropical monsoon climate under the Köppen climate classification system.
Bangkok is said to have the highest average temperature of any city in the world.
Average temperatures in the city are about 2°C higher than the ones shown for the Don Muang Airport at 1960-1990 period. Absolute maxima is 40.8°C and absolute minima is 9.9°C. The coldest temperatures were recorded in January 1924, January 1955, January 1974 and December 1999. The coldest daytime maximun temperature was 22.3C, recorded in December 1999. Hailstorms are virtually unheard of in the city, with only one having been recorded in the past fifty years.
I visited Bangkok in September 2007. My guidebook suggested that my visit would coincide with one of the wettest months of the year. The highest peak on my guidebook's rainfall chart was in September and below the chart was a warning that parts of the city can be flooded after torrential rain in September and October.
The online weather forecasts painted a similarly wet picture. For a few weeks prior to my departure I checked the 10 day forecasts on a number of weather websites. Without exception, they contained little cloud and lightning icons and predicted "scattered thunderstorms". Every day, every week.
So, it's fair to say that I expected to get a bit wet during my week long stay in Bangkok, and I even wondered if I should pack a light coat.
The reality however was very different. During my week in the city, I saw it rain for a grand total of 10 minutes! My first 5 days were a mixture of bright sunshine, light cloud cover and, at times, dark overcast skies. But no rain. Then, on Friday night, I felt a raindrop, then another and a few more. It rained lightly for 10 minutes or so and then stopped.
On a few occasions, I awoke in the night and thought I could see flashes of lightning outside. But each morning the roads were dry and there was no sign of any overnight rainfall.
So, when visiting Bangkok in the rainy season....don't assume that rain is guaranteed (even if all the weather forecasts tell you that it is!)
In my humble opinion the best time to visit Bangkok is January - February. In these two months the temperature is around 24C (75F) and mostly dry.
April - May are the hottest months where temperature gets to be 38C (100F).
September-November you get a lot of rain; which in itself is not a problem but the road gets flooded and it compounds to the horrendous traffic jams that is an inseparable part of Bangkok.
Favorite thing: One afternoon we looked at the horizon from our hotel window and it was just black and gradually the city started to disappear and we had a massive storm which last for a couple of hours, it was very scary, afterwards we were supposed to go out for dinner but the streets were flooded so we ate in the hotel.
Average temp is 35C.
Hot and humid: March - May
Wet and rainy: June - September
Cool: October - February
Fondest memory: Being drenched by street revelers on Songran Festival during the hot month of April.