The numbers don’t really matter
Grams per meter squared over a 24-hour period may be a scientific way to describe breathability, but most folks eyes will glaze over if we try to explain the complex theory of waterproof/breathable fabrics this way.
In buying a waterproof/breathable shell (W/B shell), try to stay away from the numbers: they don’t really matter. In fact, they tend to become confusing unless you allow yourself an hour for an explanation of the size of water vapor molecules, interstitial spaces in fabric coatings and laminates, and a long section with drawings on the theory of driving force and temperature differential. Yaaaawn. As in most purchases from an outdoor specialty retailer, you should be most interested in your comfort, convenience, and safety when shopping for a W/B shell. Leave the scientific stuff for the gear geeks who have nothing better to do with their time.
To begin, you need to know where you are planning on going
You cannot properly buy a W/B shell unless you know how and where you will likely use it, and discovering that begins with answers to a series of questions:
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