Duncan, our in-house gear nerd, takes you through what hydrostatic head means. He's got a wealth of knowledge around products after working for a few outdoor retailers and is an active member of the outdoor community.
Hydrostatic Head is the measurement of how waterproof a piece of fabric is, be it tent fabric or jackets. The way it's measured is quite simple. A column of water is placed over the fabric, the more water, means more pressure is applied to the fabric. Once the water goes through the fabric, the test is stopped and the column is measured. On waterproof garments and tents, you will always find a Hydrostatic Head (HH) measurement, and from this you can see how waterproof something is. In the UK, for something to be called waterproof, the fabric needs to have an HH of 1500mm, that's a column of water a metre and a half high resting on the fabric. But, water never really just rests on fabric, high winds can add pressure too, A heavy rain shower will be around 2000mm when you take into account wind and the sheer amount of rain coming down, so a 1500mm waterproof will leak. Most waterproof fabrics will be around 5000mm or more with Goretex around the 35,000mm mark. Obviously, the more waterproof something is, the more expensive it will be, so you need to factor that into your choices.