Why can't I ski in a rubber suit?

The answer lies in waterproofing and breathability. Unlike rubber, good skiwear must have the right balance of both

The colour is probably the first thing that attracts you. Then, you can't fail to notice the logo, because it's usually emblazoned across the chest. Giving the fabric a fondle, you search out the price tag - and, if it's within your means, you take the jacket off the rail to try it on. Plenty of pockets and vents? A good fit? If it passes those tests, there's one more thing you need to know. Is this better or worse than the other skiing and snowboarding jackets in the shop?

The colour is probably the first thing that attracts you. Then, you can't fail to notice the logo, because it's usually emblazoned across the chest. Giving the fabric a fondle, you search out the price tag - and, if it's within your means, you take the jacket off the rail to try it on. Plenty of pockets and vents? A good fit? If it passes those tests, there's one more thing you need to know. Is this better or worse than the other skiing and snowboarding jackets in the shop?

That's a difficult question to answer. True, there is a forest of reading material hanging off each jacket: booklets proclaiming the virtues of the hi-tech wonder-fabric of which it is made (Gore-Tex, Entrant II, Triplepoint Ceramic or whatever); tags "explaining" its design in terms of energy, dynamics and so on. But you would need to have a doctorate in materials technology and be fluent in marketing bull to work out whether all that amounts to anything.

In an ideal world you could just ignore it: there are industry standards which, although expressed in intimidating formulae and phraseology, permit direct comparisons to be made between different garments. Unfortunately, few manufacturers state on the labels how well (or badly) their products perform. Bonfire is one that does, so with a pile of its snowboarding jackets and the help of George Curleigh, UK planning manager for Salomon (which owns Bonfire), I was able to crack the codes of skiwear's technical specifications.

Beyond its obvious ability to provide shelter from the wind, the two most important characteristics of a jacket (and also trousers) are waterproofing and breathability. For snow sports you need a top layer of clothing that stops water coming in but allows water vapour to escape. The importance of the former is obvious, but what about the latter? "Imagine wearing a rubber ski-suit," says Curleigh. "That would be 100 per cent waterproof, but it would give you the coldest day you've ever had on the mountain. When you are skiing down you sweat, and if that moisture cannot escape - as would be the case in a rubber suit - it will start to freeze when you are up on a chair-lift, exposed to the wind."

The standard measure for waterproofing involves stretching a piece of fabric over the bottom of a tall tube and pouring water in at the top. The more effective the fabric's waterproofing, the greater the depth of water it can retain. The industry standard allows a fabric to be described as "waterproof" when it can retain 2,000mm of water. As Curleigh says: "That sounds impressive, until you realise that some fabrics will withstand more than 30,000mm. Three years ago, the basic Bonfire jacket had a 2,000mm rating, but fabrics have improved so much since then that the entry-level 'Silver' jacket now has a 6,000mm rating."

For the breathability test, fabric is stretched over a vessel of boiling water, and the amount of vapour which penetrates it is measured (in grams of water per square metre) over a 24-hour period. A rating of 9,000 grams per square metre is a good one for skiwear, but for top-of-the-range Bonfire jackets it falls to 6,500 as fabrics have to compromise between waterproofing and breathability. The tiny "holes" in their surface permit vapour to escape but are too small to let water droplets in. Increasing waterproofing on jackets designed to be used in extreme conditions - by the use of smaller holes - obviously reduces breathability. Choosing a jacket, says Curleigh, involves "deciding which of the two criteria is more important, and that will depend on how much you sweat and on the kind of activity - snowboarding or climbing, say - for which you will wear the jacket".

In absolute terms, the ratings don't mean much to the buyer: "No one is going to conduct these tests at home," says Curleigh. "But they make it possible to compare the performance of the different fabrics from which jackets are made." Just how the jacket is made is important, too: even when it uses the most waterproof fabric known to man, it will still leak if the seams are not sealed. Bonfire uses a hierarchy of terms to describe the extent of the seam-sealing on a particular garment, which range ungrammatically from "select" through "critically" to "fully" - the last even includes a sealed patch behind where the logo is sewn on.

The final criterion by which to judge a jacket's performance is its "beading" properties. "When you get a new jacket it's great the way water falling onto it turns into beads and just rolls off," says Curleigh. "But after a while the beading effect wears out, and water begins to soak in." The thing to look for on the label is the jacket's DWR figure. "That stands for 'Deluge Water Resistance', although Durability/Wash Ratio would be more illuminating." An 80/20 rating, for example, means that the garment will retain 80 per cent of its beading properties even when it has been washed 20 times. A coating is commonly used to create the beading effect, which is why the washing instructions for many jackets recommend the use of a hot iron: it helps to restore the surface's beading properties.

If that is all the Bonfire label has to say, you may feel it has forgotten something. Once you have checked its DWR, seam-sealing, breathability and waterproofing, you might wonder: Will the jacket be warm enough? That is, however, a rather old-fashioned question, dating back to the days of the Puffa jacket. There are still lined jackets, of course, and they often specify how much insulation they contain, measured - rather elusively - in grams. But Curleigh is dismissive about warmth. "Insulation is secondary," he says. "You can vary what you wear underneath a jacket, according to the temperature."

The most expensive Bonfire jackets have no lining, because serious skiers and boarders use them merely as a protective outer skin. They wear layers of "wicking" fabrics (Bonfire's range is described as a "dynamic layering system") underneath to keep warm.