December in Queluz, Portugal, brings average daily highs of 15°C and overnight lows of 11°C in the 2016–2025 historical record. Bring layers that work for a longer spell outside as well as a short walk. A warm extra layer gives you options when the day's weather feels cooler than the monthly average suggests.
In the record, at least 1 mm of rain or snow falls on 9 of 31 days on average, a daily chance of 27%. Leave some freedom to swap an outdoor day with a sheltered one. There is no need to read the average as a fixed number of lost days; it says nothing about how long rain lasts on each occasion.
The calculated day length, sunrise to sunset, averages about 10 hours. Sunshine averages 8 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 a culture-focused visit, group the places you want to see into manageable areas, with time between stops. Check each venue's opening dates before fixing the order.
January has an average daily high of 14°C, 1°C lower than December. Its historical rain-day chance is 26%, compared with 27% 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.