Waterproof fabrics are rated using Hydrostatic Head (HH), which measures the pressure a fabric can withstand before water seeps through. Light rain requires a minimum of 1,500 mm, while heavy storms with high winds can exceed 10,000 mm. Backpack straps can create localized pressure over 20,000 mm, which can compromise even high-rated rain jackets. Understanding these ratings helps in choosing the right gear for specific conditions.