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Membrane (Gore-Tex and others)
Rating
A membrane is a thin waterproof but breathable layer in jackets and footwear (the best-known brand is Gore-Tex) that keeps rain out while letting body vapor escape from inside.
In detail
The membrane works thanks to microscopic pores that are smaller than a drop of water but larger than a water vapor molecule — this lets clothing "breathe" while staying waterproof. On long trail races in the mountains (including in the Caucasus), a membrane jacket is often part of the mandatory gear list for safety reasons.
How to use it
Even the best membrane "suffocates" under very intense effort in warm weather — on hot, long climbs it's sometimes more comfortable to temporarily take off the membrane jacket than to put up with condensation building up inside.