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• a 10,000mm hydrostatic water column rating equates to about 14psi
• the hardest wind-driven rain only exerts about 2psi
• you don’t need a 20,000mm membrane to stay dry.
There are far more forces at play on your garment than just the rain falling. Your shit gets wet, and then you move around and put pressure on the wetted out fabric. Sitting down on the chairlift, wiping out, resting your arms on your legs on the lift ride, hell even just moving your arms around all put far more than a couple PSI's worth in pressure on your laminates.
I have a closetful of sub-20k outwear that I have at one point or another ridden in the rain, and I absolutely get wet through them. On the other hand, I've spent full days happily riding around in heavy downpours in goretex and other high end laminates, and come away bone dry. At my local mountain, many of the park kids will ride in garbage bags with arm holes cut into them, because their 10k rated gear just can't handle being in the rain.