As a side note to all the talk of the rainy season, be aware that there is some flooding in part of Thailand. Even here in Bangkok, the rains keep coming and the water keeps rising. There is talk of possible serious floods in the city in October is the tides rise, rains continue and river levels overflow. Even the train service north to Chiang Mai may be affected. Areas along that route, such as Uttaradit and Phitsanulok, have been affected by floods recently, and when it gets really bad the trains cannot operate. Bear in mind, this in only a warning about what may happen in October. Right now things are under control. But if you are visiting Thailand in the next month check news and weather reports for potential flood problems.
Thanks for the update.
It's not very good news for people like me who are planning to fly to Bangkok at the end of October. Do you think airports could be affected?
Good question about the new airport. It was built on land that used to called "Cobra Swamp" so it's safe to say that it's not a high elevation area. The rain and flooding should not have any affect on the airport or roads serving it, but nearby housing estates might suffer.
I read in the Bangkok Post yesterday that the main canal boat operation (in Klong Saen Saeb) in Bangkok had to shut down for a few days this week because water levels were too high and the boats could not go under some of the bridges. There is standing water in some parts of Bangkok (but not everywhere; it's not that bad yet) so don't bring your good shoes!
No worries about Bangkok yet, it has been raining a bit the last couple of days, but there's no floodings yet, and it doesn't seems to happend during this week.
About the airport and Chiang Mai, everything in the aiport keeps going smoothly, don't worry about that.
Chiang Mai have no clue how the is up there, do a weather forecast occasionly and you'll be updated through that.
Have a great trip to Thailand