December in Binjai, Indonesia, brings average daily highs of 30°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 28 of 31 days on average, a daily chance of 90%. Make flexibility part of the trip from the outset. Keep time outside easy to rearrange and choose plans that still work if it rains. The monthly figures do not establish whether rain is brief, prolonged or concentrated at a particular time.
The calculated day length, sunrise to sunset, averages about 12 hours. Sunshine averages 7 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.
If food is a reason for the trip, leave time around meals rather than filling every gap with another journey. Check the places you choose for current opening hours.
January has an average daily high of 31°C, 1°C higher than December. Its historical rain-day chance is 82%, compared with 90% 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.