December in Porto, Portugal, brings average daily highs of 14°C and overnight lows of 9°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 14 of 31 days on average, a daily chance of 45%. Give wet-weather alternatives a proper place in the itinerary. Arrange outdoor plans so you can move them between days, and pack a rain layer rather than relying on a completely dry run of weather.
The calculated day length, sunrise to sunset, averages about 9 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.
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. 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.
The following month, January, has a similar average daily high of 14°C. Its historical rain-day chance is 38%, compared with 45% 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.