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There’s no such thing as waterproof.
I was taught this by a Scottish mountain guide. As much as I wish it wasn’t true, it is.
Waterproofing clothing is tricky. If you make a jacket out of plastic, it will not be breathable. If it is not breathable, then the sweat you generate on the inside of the clothing condenses and you get wet anyway.
The way that most modern waterproof garments are made is with a very thin membrane of PTFE (polytetrafluoroethylene – no, I can’t pronounce it either).
This is a material which allows water vapour to pass through freely but will only allow liquid water through under extreme pressure. This pressure is measured as if you got a piece of vertical drainpipe with
the membrane covering the bottom and poured water in the top until it started to force its way through.
As a general rule, anything less than 10,000mm is not suitable for mountain use.
If you use Gore-Tex, depending on which type, it can be rated at 28,000mm meaning that there’s 28 metres of water before it forces its way through. This seems unnecessary but when all-day rain is considered
and a heavy rucksack is worn or you sit against something, those pressures can be created.
Other technologies are available and suit different uses. Ultimately, the best thing I can say, is find out what works for you, ask staff in store to explain your options … or just don’t go out when the weather is bad.