March in Reykjavík, Iceland, brings average daily highs of 4°C and overnight lows of 0°C in the 2016–2025 historical record. Warm layers deserve space in the bag. Keep longer outdoor plans easy to adjust, and allow places to stop between walks so that a colder day does not determine the whole itinerary.
In the record, at least 1 mm of rain or snow falls on 17 of 31 days on average, a daily chance of 56%. 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.
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.
April has an average daily high of 7°C, 3°C higher than March. Its historical rain-day chance is 50%, compared with 56% 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.