Rainfall vs rainy days: what hikers need to know
Salkantay, Cederberg and Jeju receive similar monthly rain totals in selected months, yet the rain falls across very different numbers of days.
8 min read
Monthly rainfall is easy to picture as a container: 85 millimetres of water collected over a month. A hiker experiences time rather than a container. The same total might arrive through light precipitation on many days, several concentrated storms, or a mixture of both. This is why BestTimeToHike publishes both precipitation total and wet-day probability. A matched example makes the difference visible. Salkantay in September averages 85.1 mm and Cederberg in June 86.9 mm at their selected representative cells. Their wet-day shares are 85.33 percent and 28.10 percent. The volumes are close; the calendar pattern does not match.
| Destination and month | Monthly precipitation | Wet-day share | Approximate reading |
|---|---|---|---|
| Salkantay, September | 85.1 mm | 85.33% | Wet-day signal on most days |
| Cederberg, June | 86.9 mm | 28.10% | Similar volume across fewer wet days |
| Jeju, October | 81.3 mm | 24.42% | Lower volume and wet-day frequency |
The matched-pair experiment
If the only question is how much precipitation falls during the month, Salkantay and Cederberg tie. If the question is how often a day crosses the dataset's wet-day definition, they are far apart. The 59-point gap matters for itineraries. Frequent precipitation can affect repeated starts, drying opportunities and the number of days with at least some rain signal. A lower wet-day share with the same total suggests that more water is concentrated into fewer days, but it does not tell us the exact storm duration, time of day or trail impact. That final inference needs hourly weather or local evidence, which the monthly summary does not provide.
A near match confirms the lesson
| Metric | Salkantay, September | Jeju, October | Difference |
|---|---|---|---|
| Monthly precipitation | 85.1 mm | 81.3 mm | 3.8 mm |
| Wet-day share | 85.33% | 24.42% | 60.91 points |
| Hiking-hour mean | 9.2 °C | 17.0 °C | 7.8 °C |
“Rainfall total describes volume; wet-day share describes frequency. Neither one describes timing or trail condition.”
Jeju in October sharpens the point. Its 81.3 mm monthly mean is 3.8 mm below Salkantay's September total, yet its wet-day share is 24.42 percent rather than 85.33. Temperature adds another difference: 17.0 °C during the sampled hiking hours at Jeju versus 9.2 °C at Salkantay. A traveller choosing between the two is not choosing between totals alone. The practical comparison includes frequency, temperature, elevation, route surface and current forecasts. Close rainfall totals are therefore a reason to open the detail view, not to declare the destinations climatically equivalent.
How to read the two metrics together
Use wet-day probability first when the planning concern is how often precipitation may interrupt a multi-day itinerary. Use monthly total to understand the overall water volume and to distinguish a lightly damp pattern from a much wetter climate. Then inspect heavy-rain probability if available, because frequency alone does not describe intensity. Always keep the definitions visible: the figures are historical shares and means from 1991–2025, aggregated at a representative grid cell. They are not a forecast probability for a booked week. A month with fewer wet days can still contain severe events; a month with frequent wet signals may include many modest events.
The final step is route context. Soil, rock, river crossings, drainage, vegetation and maintenance determine what a given amount of rain does to a trail. Eighty-four millimetres in an arid, fast-draining landscape is not the same experience as the same amount on steep humid terrain. BestTimeToHike deliberately stops before making that claim. The data helps identify the shape of the historical climate, while official route sources and current local weather explain the likely consequences for the intended hike.