GuardTrails
What it means
You read that a coast sees rain almost one day in two, and you give up. But a day with a forty-minute shower counts as a "rainy day" — and it spoils nothing.
Scandinavian rain is frequent and brief. Counting days measures its frequency; counting hours measures what it costs.
How to read this figure before you book: Rain days or rain hours?.
And where those hours fall matters as much as how many there are. Three hours of rain in the evening block nothing. The same three between nine and midday kill the day.
How they are counted
Not "did it rain that day?", but "did it rain for several hours inside the window when you are out walking?". The window shifts towards the morning when the day is long or the effort high. The count uses recent hourly records rather than climate normals, which already lag behind the current climate.
Wind is a separate blocker, which only counts for exposed days — ridge, arête, summit, cliff. On a ridge you turn back in strong gusts under a dry sky, and you set off in drizzle when it is calm.
Heat, for its part, is counted in usable hours: What is your heat ceiling?.
An example
Our stays in Iceland and Norway had very little effective rain, while rainy-day statistics predicted far worse (our trips).
Scotland is the counter-example: there, rain genuinely cancelled planned hikes (our trips). It is not the same rain — and that is exactly what counting in hours brings out.
Consequence: an extra week cannot be bought as insurance against rain. We had written that it could; we have corrected it.
What it does not say
The count says nothing about intensity, nor about how long a weather system lasts. It treats each day separately, whereas a front often lasts several days (meteorological order of magnitude). It is therefore optimistic.