December in Reykjavík, Iceland, brings average daily highs of 3°C and overnight lows of -1°C in the 2016–2025 historical record. Pack for cold conditions, with warm layers for time spent outside. Overnight averages fall below freezing; check the forecast and local conditions before committing to an exposed walk or a long outdoor day.
In the record, at least 1 mm of rain or snow falls on 16 of 31 days on average, a daily chance of 52%. 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 4 hours. Sunshine averages 1 hour a day. Keep the shorter daylight window in mind when arranging longer excursions, and check local sunset times before setting out. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
For nature trips or walks, keep the longest outings movable and check route conditions locally; a monthly climate average cannot establish whether a particular path is accessible. If the coast is part of your plan, check water conditions separately. Air temperature and sunshine do not establish swimming conditions, surf quality or diving visibility.
The following month, January, has a similar average daily high of 3°C. Its historical rain-day chance is 58%, compared with 52% 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.