Hydrostatic Head Explained
Hydrostatic Head (HH) is a way of measuring how waterproof a fabric is. The test is completed using a long tube that will hold water - the piece of fabric under test is fastened across the open end at the bottom. The tube is then filled with water until water can be seen to penetrate the fabric - at which point the length of water in the tube is read off - for the best fabrics it can be as high as 30,000mm i.e. 3metres high.
Applying this to Garments
In the UK manufacturers are allowed to claim a fabric is waterproof if the HH is 1500mm but most jackets exceed this comfortably and figures of 10,000 to 30,000mm are not uncommon.
Fabric for use in garments require higher levels of waterproofing because garments are subjected not only to driving rain but also to pressure applied by straps and hipbelts of rucksacks.
Gore-Tex and eVent fabrics typically achieve figures of HH upto 30,000mm making them the most waterproof garments around, however some fabric manufacturers, most notably Polartec, aregue that these figures are not necessary, and by accepting a figure of 20,000mm or thereabouts you can make a fabric considerably more breathable - they introduced 'Neo Shell' which is becoming more popular and is a proven technology.
A higher HH does not guarantee a better garment since breathability is usually a compromise when water proof performance is maximised.
Softshell garments come in a range of different types from lightweight highly breathable that won't put up with much rain, to heavier more water resistant garments that are often made from waterproof fabrics - the garment is not called 'waterproof' because the seams aren't 'taped'.
Applying this to Tents
In order for a tent to resist light showers the HH needs to be around 1000mm, heavy rain and driving wind will create more pressure on the fabric and require a higher HH - around 2000mm. Anything above these figures and the tent will keep out water being pushed through by something physical, like a person leaning on it.
A groundsheet need to have a higher HH figure because of the pressure of people pressing down on it, and should be around 3000mm or higher ideally.