As we have well and truly moved out of the Summer season, and the weather will become a little more testing and tempermental; you might start to look at new or existing options for your outerlayer. With so much choice of brands to start with, it is easy to get overwhelmed with the different styles, materials, and that strange number – Hydrostatic Head. Deciding upon a brand/style is often a case of personal choice and/or budget and is never the confusing part of deciding on a new waterproof jacket. Again, there are an ever increasing number of waterproof material/membranes to choose from, but what largely sets them apart from each other is the hydrostatic head.
So what is Hydrostatic Head? Put simply; it is a lab tested means of arriving at a numerical indication of how waterproof a material is. Generally, the higher the hydrostatic head, the more waterproof a material is regarded to be. The test itself is relatively straight forward, in that a column of water is placed on a sample of the particular waterproof jacket or fabric and the height of the column of water in mm determines the hydrostatic head. In terms of what these lab achieved figures equate to in understandable terms; 1500mm and above is regarded as “waterproof”. Looking at that in real terms; 2000mm is the rating of an average tent flysheet, 10,000mm is around the measurement of most good, breathable waterproof jackets. Moving upwards, 20,ooomm is about right for a solid four season hardshell. There are some jackets that reach up to 30,000mm but it is argued that this is far too high a figure to retain any breathability and will possibly lessen how comfortable a jacket is.
The thing with a HH rating is, that it isn’t all encompassing. The tests dont take particular things into account that are very relevant in an outdoors situation.
Extreme Weather – As we well know, the weather can change quickly on the hill and generally HH ratings are accepted to relate to only 35mph winds and driving rain. If the wind is stronger than this, and the force of the rain water hitting the jacket is stronger then its theoretical “waterproofness” is lessened.
Useage Outside Test Variables – Tent floor fabrics are given a hydrostatic head rating, as they are needed to be waterproof of course. One thing that a hydrostatic head test does not take into account in the case of tent floors is the extra force exerted on the fabric when it is being knelt on, or heavy rucksacks are thrown onto it. Similarly to stronger winds, this can lessen the theoretical “waterproofness” of the fabric as the variables are not constants, as in a standard hydrostatic head test. Another example of this in waterproof jackets is the ‘stress’ or ‘high wear’ areas such as the elbows, shoulders and waist (where rucksack sacks will sit), wrists (where straps are cinched).
Longeivity/Robustness Of Fabric – Whilst hydrostatic head tests let us know how waterproof fabrics are at the time of testing, there is no rating of how long this level of waterproofness will last. A fabric with a tested hydrostatic head of 20,000mm may degrade quickly with useage and the rating deplete rapidly – in which case, the fabric is not overly ideal for extensive outdoor use. The fabric may be susceptible to abrasion, tearing, being worn through. All of which will of course superceed the fabric’s apparent hydrostatic head rating.
Garment/Gear Design – Not directly talking about the fabric itself, external factors/ the way in which the garment or item of equipment is designed may affect how waterproof it ultimately is. So hydrostatic head is not the be all, and end all. If zips are not designed well for example, or there is no option to cinch hoods or wrists, the hydrostatic head rating can be pretty redundant.