But outerwear isn't really waterproof (the way, say, thick rubber and plastic are). Technically speaking, "waterproof" outerwear is extremely water-resistant.
What the Hell Do Those Ratings Mean?
Static-column testing is the most widely used waterproof test. A 1-inch-diameter tube stands vertically over a piece of material. The tube is filled with water, and the water's height in millimeters when leakage begins becomes the waterproof rating. A piece of fabric that can withstand 20,000mm of water pressure will have a rating of 20,000mm or 20K.
What do these ratings mean for real-life situations? Here's a general run-down:
How Does Construction Affect Waterproofing?
Short answer: Greatly
Ratings offer a vague notion of waterproof protection, but understanding construction is a much better tool for comparing and choosing outerwear.
A 2L (two-layer) garment consists of a face fabric bonded to a waterproof breathable membrane.
2.5L garments protect the membrane with a partial protective spray or coating.
There's not much point in springing for a high-tech waterproof breathable membrane if water can seep in past the stitching.
Welded seams join panels of fabric via gluing or ultra-tech sonic bonding for a seam that's stretchier and less bulky than a taped seam and significantly more resistant to water pressure.
PU laminates move moisture via an absorption-diffusion-evaporation process.
This is the same stuff used to coat non-stick frying pans. PTFE membranes work by causing water to bead and roll off.
How to Care for Your Waterproof Outerwear
Want to get the most out of your outerwear? Notice a decrease in waterproofness?
Washing your jacket removes oils and grime that can compromise the waterproof breathable membrane.
An outerwear-specific detergent, like Nikwax TechWash or Granger's Performance Wash, will help protect the DWR and the waterproof breathable membrane.
Tumble drying helps redistribute DWR treatment, restoring your garment's water-resistance.
If after step 1 and 2 your jacket still isn't beading properly...
Repeated wear and washing will eventually deteriorate the face fabric's DWR treatment.
Read the care label. Almost all waterproof shells benefit from washing and drying, but some garments have insulation, linings, or trims that require special attention.