December in Bang Kapi, Thailand, brings average daily highs of 31°C and overnight lows of 23°C in the 2016–2025 historical record. Allow room to shorten long outdoor plans when the day's forecast is hotter than you want. A schedule with easy breaks and less travelling between stops will give you more choices.
In the record, at least 1 mm of rain or snow falls on 2 of 31 days on average, a daily chance of 8%. Rain has been relatively infrequent in this record, which leaves more scope for an outdoor plan. Still keep an alternative for a wet day: a low monthly average cannot tell you when the rain will arrive.
The calculated day length, sunrise to sunset, averages about 11 hours. Sunshine averages 10 hours a day. Use the daylight window to plan the length of excursions, and check local sunrise and sunset times for the dates of your stay. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
For time in town, cluster a few stops together and leave the rest of the day open. Check transport and opening hours for any plans that run late.
January has an average daily high of 32°C, 1°C higher than December. Its historical rain-day chance is 12%, compared with 8% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.