December in Alajuela, Costa Rica, brings average daily highs of 26°C and overnight lows of 18°C in the 2016–2025 historical record. Light clothing is a useful starting point, with room in the plan to pause indoors. Decide how much walking to fit into each day using the local forecast, rather than treating the monthly average as a limit.
In the record, at least 1 mm of rain or snow falls on 11 of 31 days on average, a daily chance of 34%. 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 12 hours. Sunshine averages 10 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. 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.
The following month, January, has a similar average daily high of 26°C. Its historical rain-day chance is 15%, compared with 34% 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.