Is it true that Bangkok can be very cold in winter? If so,when would it be very cold and how cold can it be?
Over the course of a year, the temperature typically varies from 23°C to 35°C and is rarely below 20°C or above 36°C.
The cold season lasts from November 18 to January 31 with an average daily high temperature below 32°C. The coldest day of the year is January 6, with an average low of 23°C and high of 32°C.
Which for me is 'not very cold', but your internal thermostat may be set differently...
This link tells me that Bangkok has cool weather between December and February. (not cold or freezing). This link has preserved the temperature data over the last few years in Bangkok city. Since you did not mention which month you will be going, I suggest you feed in the month data and choose the year and see how it was.
Not very cold. As a Thai living in Bangkok I would say that central Thailand rarely has cold winter. Normally, it is above 15°C throughout winter and above 25 downtown (last year was an exception as downtown Bangkok reached 16). I think the weather won't be your problem if you plan to visit central Thailand in the winter.