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Rainproof fabric. Coloured Scanning electron micrograph of the undersurface (worn next to the skin) of a waterproof fabric used in outdoor clothing. Here, the tightly-woven base fabric is seen. The outer surface is covered with a polyurethane layer with many tiny pores. These allow humid air and water vapour to come out of the garment, but the small size prevents liquid water from entering due to surface tension. The humid air permeates between the fibres on the inside face to reach the pores. This type of coating is known as 'breathable waterproofing'. Magnification: x30 at 6x7cm size. x105 at 10x8ins (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)