A well-made waterproof jacket is a beautiful thing. A miracle of engineering. It's a barrier between human and the forces of nature, a tiny tent for your torso. It'll keep you bone dry in inclement weather and shield you from the wind, all while allowing perspiration to escape from within. But only if you buy a decent one.

Cheap out on a poor-quality waterproof jacket and you will get wet. If not from rainwater when it eventually breaches the zips and stitching, then from sweat collecting on the inside of the garment due to poor breathability.

To help you avoid the soggiest of fates, this guide will equip you with all the information you need to make an informed purchase.

All waterproof jackets are water-repellent. But not all water-repellent jackets are waterproof. Waterproof means that liquid will not pass through the jacket from the outside under normal circumstances (ie in rain or snow). Water-repellent, on the other hand, means that the outer of the garment has been given a hydrophobic, durable water-repellent (DWR) treatment that allows water to bead off its surface without soaking through.

A water-repellent jacket may shrug off showers, but it will become saturated in heavy rain, letting the water soak through. A waterproof jacket will usually be treated with DWR, too. The fabric's multiple-layer construction will generally be less breathable, but it should completely seal rain out, even in downpours.

It's important to ensure your waterproof jacket has taped seams and waterproof zips (or, failing that, a storm flap protecting the main zip). These are both potential water-ingress points that need reinforcement.

Look specifically for the word "waterproof", which should be clearly stated, either on the swing tags if buying in-store, or in the product description if buying online.

Why have a DWR coating if the fabric itself is already waterproof? Well, keeping water out is only half of a waterproof jacket's job. The other half is to make sure sweat vapour can escape from within.

"The DWR coating allows water to bead off the surface fabric quickly," says Andy Laycock, an expert on waterproof materials from performance-fabric brand Pertex. "Without it, the material will 'wet out'. This is when water soaks into the outer layer of the fabric and prevents moisture from being transferred to the outside. When this happens, you end up with a buildup of sweat on the inside of the jacket."

There are hundreds of waterproof fabrics, many claiming to be the best of the bunch. The important thing is to check the numbers. Buckle up, because it's about to get nerdy.

The "hydrostatic head" of any given waterproof fabric will be shown as a figure between 1,500mm and 20,000mm+. The higher the number, the more water pressure a fabric can withstand before it fails. Only those with a rating of at least 1,500mm, meaning they can withstand a vertical water column of 1.5 metres, are classed as "waterproof" by the British Standards Institution.

So, isn't it always best to pick the fabric with the biggest number on the label? Not quite. A high hydrostatic-head rating can sometimes mean lower breathability. It all depends on how and where you intend to use your outerwear.

A mountaineer in UK winter conditions, for example, might want maximum rain protection with breathability being less of a concern. A trail runner in mild, drizzly conditions, on the other hand, would probably prefer something lighter and more breathable, perhaps at the expense of waterproofness.

For most of us who walk, hike and run errands in UK conditions, aiming for somewhere in the middle of these two extremes is a safe bet. A jacket with a rating of about 10,000mm should be sufficient for most casual users.

Waterproof jackets can cost between £50 and £1,000 depending on the make, model, features and intended use. Canadian outdoor brand Arc'teryx, for example, sells its flagship extreme-weather jacket, the Alpha SV, for about £800. While this shell has long been the gold standard for alpine climbers in high-altitude environments, it's probably excessive for walking the dog around the park.

For general use, including wet-weather hiking and everyday wear in the UK, £150 to £350 is a good range to aim within. If it's going to get only occasional, light use, you may not even want to spend that much. Thrifty individuals can get more for their money by hitting the sales.

For most people (such as those not pushing their jackets to the absolute limits on a regular basis), I'd say about £350 is the point of diminishing returns. A jacket retailing at this price might be noticeably better in terms of quality and performance than one costing £200 less, but once you cross over that threshold, the improvements become less apparent to the average consumer.

Buy a waterproof jacket that's too large and you'll effectively be wearing a giant sail that will flap around on gusty days. Go too far in the opposite direction and you may find it uncomfortable with layers underneath.

As a rule of thumb, you should be able to comfortably fit a nice warm mid-layer underneath your waterproof without it hindering your range of motion. This is important because most waterproof jackets are nothing more than shells designed to be worn with multiple layers that can be added or removed depending on the temperature.

It's also important to make sure the jacket is long enough to cover whatever is underneath and that the hood is sufficiently sized to keep the rain out of your face when worn. In terms of adjustability, I'd advise avoiding anything that lacks a hem drawcord, an adjustable hood, and either Velcro or elasticated cuffs. Armpit zips are nice to have for ventilation if you run hot.

Packability and weight are worth considering, particularly if you're likely to be travelling with your jacket. And, speaking of travel, it's a good idea to look at the placement of pockets. Are they easily accessible even when wearing a backpack? If not, you may want to reconsider.

Now, to address the elephant in the room: environmentally speaking, waterproof jackets aren't all rainbows and unicorns. The chemicals used in traditional DWRs are not biodegradable; they can enter the food chain and water supply, and there's an increasing body of evidence linking them to a heightened risk of developing certain forms of cancer.

"These are generally being phased out now and replaced by DWRs based on hydrocarbons or silicones," says Pertex's Laycock. "These can provide the same, or even better, water-repellency. The downside is they're more susceptible to degradation from oils produced by the skin. This means it's more important than ever to wash waterproof jackets properly and frequently."

Regular washing and re-proofing (reapplying the DWR treatment) are essential to keep a waterproof jacket performing at its best. "For casual use, I'd say you should be washing a jacket every five to 10 uses," says Laycock. Pay attention to the care instructions inside the garment, and wash it with an appropriate technical wash solution, like Nikwax or Grangers."

Laycock stresses the importance of using a specially formulated "performance" wash - one designed specifically for cleaning waterproof fabrics. Using your regular detergent can strip the DWR coating and leave your jacket susceptible to wetting out. If you notice that the fabric isn't beading water as effectively as it used to, you can buy a wash-in or spray-on solution to reproof it as and when necessary.