Although Bangkok has a tropical savannah climate, it has hot, cool, and rainy seasons. Average yearly temperatures range from 22.0 °C (71.6°F) to 35.4 °C (95.7 °F). The humidity averages between 68% and 79% year round.
The rain begins late April to mid-May with the southwest monsoon season.
From this time until October heavy storms can pummel the country side with several inches of rain per day. The southern regions of Thailand definitely get the most rain (up to 2400 millimetres or 7 feet and 10.49 inches).
April is definitely the hottest month of the year in Bangkok and should be totally avoided as far as trip planning is concerned. From July to October is the absolute heaviest rainfall time, so unless you don’t mind the rain this may be a bad time to visit as well.
Perhaps the best time to visit Bangkok is between November and February. It is the cool season and at night temperatures can drop below freezing, especially in the outlying areas of higher altitudes.
March to June is the summer season and temperatures can go as high as 37.7 °C (100 °F). It is the hot and dry season of Bangkok.
When the rains begin in the city, the residents react like Americans in a hurricane or snowstorm. There is much bustling for provisions and leaving work early is a daily occurrence between July and October.
These rains come suddenly and unannounced. All of a sudden water dumps from the sky in torrential downpour. If you are trapped in your room and you can’t get to food, its best to order some food by phone. The delivery guys have special ponchos for this type of delivery. Convenience can be pricey, and you will want to tip the delivery person well for his/her bravery. There are also lots of malls and theatres that can be visited while the rain is pouring down. Taking a swim is also okay as long as no thunder and lightning are happening. Accept the rain and don’t let it ruin your trip. Just remember, it can happen at any time.