The breathability rating measures how much sweat vapor can escape through the fabric over 24 hours, shown in grams per square meter (g/m²/24hr). Higher numbers mean more moisture escapes, so you stay drier from the inside instead of getting clammy and cold.
For casual resort riding with frequent lodge breaks, 5,000–10,000 g/m² is plenty. Active resort riders who ski hard all day should look for 10,000–15,000 g/m². If you're heading into the backcountry or touring, aim for 15,000–20,000+ g/m² since you'll sweat heavily on the skin track.
One common mistake is prioritizing waterproofing while ignoring breathability—leaving you soaked in your own sweat. Also, don't assume a high breathability rating means you can skip leg vents; they still make a big difference when you're working hard.