WINTER SCIENCE

Hydrostatic head (mentioned a few times here) is the measure of waterproofing. Put simply: a strip of the fabric is tested by taping it to the bottom of a tall perspex tube, which is then slowly filled with water. The height of the water when the fabric begins to leak then becomes its ‘hydrostatic head’; ie, if the water got up to the 10,000mm mark when the fabric leaked, its hydrostatic head will be 10,000mm. The bare minimum for a product to be considered waterproof is 2000mm, but most waterproof jackets seen in Country Walking will have much higher hydrostatic head ratings (check out the Berghaus one).