Relative humidity (RH%) measures water vapour saturation in air at a given temperature. High RH (>70%) impairs the body's evaporative cooling; fabrics must wick and breathe effectively to compensate. At 100% RH, no sweat evaporates — breathability becomes the primary fabric selection criterion. FashionFactory uses WMO surface RH normals (30-year averages), normalised 0-1, to weight moisture-management properties in its scoring function.