Columna de agua
The grade of textile water proofness can be measured by the height of an imaginable water column (in mm) standing on the material without leaking. A 1,000 mm water column is equivalent to a water pressure of 0.1 bar. Rainwear is regarded as being water proof when blocking a water column of 1,300 mm (DIN standard). This is not very much: High quality tent cloths resist 4,000 - 8,000 mm, and tent bottoms up to 10,000 mm.