A waterproof jacket is a membrane, a face fabric and a coating that wears off

This works through waterproof outerwear construction in the order the parts actually depend on each other.

The two numbers on the label

Hydrostatic head measures the height of a water column a fabric resists before leaking, and it is quoted in millimetres. Breathability measures how much water vapour passes through a fabric in a day, and several incompatible test methods are in use. Because the test methods differ, breathability figures from different manufacturers frequently cannot be compared at all.

Both numbers are measured on flat new fabric rather than on a garment with seams, zips and a wearer inside it. A very high hydrostatic head is easy to achieve with a thick coating, and the difficulty lies in achieving it while remaining breathable.

How a membrane works

Membranes stop liquid water because a droplet cannot pass through a pore smaller than itself, while vapour molecules can. Other membranes work by absorbing and transmitting moisture chemically rather than through pores, and they behave differently in cold conditions.

On the bench, breathability depends on a vapour pressure difference, so a membrane moves moisture well when it is cold and dry outside and poorly when it is warm and humid. This is why a jacket that works on a winter hill feels clammy in mild rain, which is physics rather than a defect. No membrane transmits sweat as fast as hard exercise produces it, so ventilation openings remain the practical answer.

The finish that wears off first

A durable water repellent finish makes the outer face fabric shed water so that it beads and rolls off. When that finish wears, the face fabric wets out and saturates, which blocks vapour transmission and makes the jacket feel like it is leaking.

On the bench, the membrane is usually still working at that point, and the wearer is feeling their own trapped perspiration. Repellent finishes are renewable at home with wash-in or spray-on treatments, and heat from a tumble dryer or iron often reactivates the existing one. Chemistry in this area has changed substantially as regulations restrict certain fluorinated compounds, and the newer finishes generally need more frequent renewal.

Construction details that decide dryness

Seams must be taped, since a needle hole is a hole regardless of how waterproof the fabric is. Fully taped construction covers every seam, while critically taped covers only the shoulders and hood, and the distinction is usually stated.

Zips are either water resistant with a coated tape or protected by a storm flap, and neither is fully waterproof under sustained rain. Hood design, cuff closures and hem adjustment determine where water enters in wind-driven rain, and no rating captures any of them.

Two-layer, two-and-a-half-layer and three-layer constructions differ in how the membrane is protected, which affects durability and comfort against the skin.

Where the money goes

Higher prices buy more durable face fabrics measured in denier, better membrane laminates, and more careful pattern cutting. They also buy features such as pit zips, better hood adjustment and hardware that works with gloves on.

They do not reliably buy a higher waterproof rating, because a modest rating is sufficient and the difficulty is breathability and durability. Lightweight jackets sacrifice abrasion resistance directly, and a very light shell used under a pack will wear at the shoulders. Fit is a performance feature, since a jacket that binds when you reach up will pull the hem out of position in wind.

Where a claim here has not been tested directly it is inference, and it should be read as inference.

Judging one before it rains

Establish the construction layer count, whether seams are fully taped, and the face fabric denier, all of which are usually published. Treat breathability figures as incomparable across brands and read them only within one manufacturer's range. Check the care instructions before buying, because a jacket needing regular washing and heat treatment is a maintenance commitment.

The measurable part is this: match the jacket to the activity, as high-output exercise and standing about in rain have opposite requirements. This site does not test fabrics under rain, so laboratory comparisons and long-term field reviews are what turn these ratings into evidence.

The takeaway

Learn to renew the repellent finish, because the fabric rating is rarely what stopped working.

Buy for the failure you can live with, not the feature you will use twice.