Breathability

A waterproof shell traps your own moisture as readily as it keeps rain out, and a jacket that soaks you from the inside is only technically dry. Breathability ratings describe how much water vapor passes through in a day: 5,000 g/m² is modest, 10,000 to 15,000 is comfortable for active use, and above 20,000 is high performance.

It matters most when you are working. Walking a bank, rowing, or casting hard all generate vapor faster than a low-breathability shell can shed it. Sitting still in a boat, it barely matters.

Breathability also depends on conditions. The mechanism needs a humidity difference between inside and outside, so it slows right down in warm, humid rain, which is exactly when you most want it.

There is no single agreed test either, so numbers from different brands are not directly comparable. Treat a rating as a rough tier rather than a precise figure.