ActivitiesEnvironmentForest School

How wet is wet?

By September 14, 2017 No Comments

Waterproof clothing is a bit of a minefield. It’s particularly fraught for us because if a child doesn’t have adequate waterproofs then they can get wet if the conditions are serious enough, and wet can equal cold, and cold can equal danger. It’s also problematic because we have no control over the quality of waterproof clothing that parents send their children in – and if you thought that ‘waterproof’ was a case of ‘it is or it isn’t’ then read on.

Waterproofing a fabric really means dealing with two opposing issues

  1. Preventing ingress of water from the outside to the skin; and
  2. Encouraging egress of water (in the form of perspiration) from the skin

Too little of 1 and you get wet and cold; too little of 2 and you get wet and hot. Finding a balance between these two factors, and the fact that the phrase ‘it’s raining’ can (in this country in any season) cover anything from light drizzle to full-on biblical torrents, is why taking time over waterproof gear is essential if you and your children are to be comfortable in the wet.

Waterproof ratings for fabrics
How waterproof a fabric is, is measured with two numbers. The first of these numbers concerns  how resistant to water from the outside (rain) the fabric is; the second how breathable it is. I’m going to concentrate on the first number. It is measured in mm (millimetres) and these numbers will commonly be displayed on a disposable tag somewhere on the coat/trousers when you buy them.

It’s not tricky: the higher the number the more waterproof the fabric is. Fabrics are tested via a ‘hydrostatic head test’. It’s a simple test where a long 1 inch diameter tube is placed over the fabric and filled with water until the water starts leaking through the fabric. A fabric classed as 5,000mm can support 5,000mm of water in the tube (5 meters) before the water penetrated the fabric. Or, put another way, you could stand out it in it in the rain and stay dry until 5,000mm of water had fallen on you.

What is waterproof?
The important point here is that the British standard for a fabric to be allowed to be described as waterproof is just 1,500mm. Take a look at the chart below. You will see that 1,500mm of test head pressure equates to ‘light rain or dry snow’. What that means is that you can buy your child a coat and trousers that can legally be described as waterproof, which in anything more severe than persistent drizzle will be well outside their design tolerance, and will consequently let water through. Such clothing is waterproof only to a limited degree, and it’s a degree that a brief cloudburst in an English Summer can very easily exceed; let alone the persistent torrential rain that we sometimes encounter. Children’s waterproofs, especially at the cheaper end of the market, tend (as you might imagine) to the least degree of waterproofing they can get away with and still carry the ‘waterproof’ description. It is almost certainly legal in terms of trades description for the fashionable light jacket you sent your son or daughter in to Wild Learning to have a ‘waterproof’ label. Notwithstanding that label I will leave you to imagine how well it will cope with a 3-hour summer storm, or late October heavy showers. we don’t search out this weather on purpose, or avoid shelter where its available, its just that sometimes its unavoidable, and that’s when you really need to be protected by your clothing.

This inevitably leads to wet children, and the emails we get which say (in varying degrees of astonishment/disappointment/outrage) “my son/daughter got wet through his/her waterproofs”. The implication seems to be that I order wetter rain than usual for Wild Learning days. I promise you I don’t have a rain dial in my office; it’s just that you’ve bought waterproofs that…aren’t.

Waterproof head pressure rating vs real-world conditions

 

Waterproof Rating (mm) Water resistance Conditions
0-5,000 No resistance to some resistance Light rain, dry snow, no pressure
6,000-10,000 Rainproof and waterproof under light pressure Light rain, average snow, light pressure
11,000-15,000 Rainproof and waterproof except under high pressure Moderate rain, average snow, light pressure
16,000-20,000 Rainproof and waterproof under high pressure Heavy rain, wet snow, some pressure
20,000+ Rainproof and waterproof except under very high pressure Heavy rain, wet snow, high pressure
admin

Author admin

More posts by admin