Rain jacket check
Hydrostatic head is on almost every label, but which figure is enough for your trip? The check asks how long and how heavily it is likely to rain. You then get a minimum figure to compare jackets with.
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How the calculation works
Hydrostatic head is measured in the lab to ISO 811: water presses down on the fabric until drops come through. The height of that column in millimetres is the figure on the label.
The check puts your answers into three levels: up to two hours of light rain 10,000 mm, up to six hours 15,000 mm, anything longer or heavy rain 20,000 mm. The levels are our recommendation with some headroom, because rucksack straps and knees press the fabric harder as you walk.
How to read the result
The figure is a minimum. Higher is not automatically better if breathability suffers.
Taped seams, a well-adjustable hood and vents matter just as much, so you do not get wet from the inside.
Frequently asked questions
Why do I get wet despite a high rating?
Often it is sweat. A jacket can only let so much moisture out. Open the vents on climbs and avoid cotton underneath.
What does reproofing do?
It makes water bead off so the outer fabric does not soak through. The membrane stays waterproof, but a wet outer fabric breathes worse.
Is a poncho enough?
For short walks and over a backpack, yes. In wind and rough terrain a jacket is safer.