International standards for waterproofing state a minimum of 150cm hydrostatic head. FurTech type garments don't pass this standard but do remain waterproof in heavy rain as demonstrated by the Leeds University Rain Room Test and vast amounts of users, including Mountain Rescue Teams, the Search And Rescue Dog Association and numerous guides and outdoor instructors.
So why this standard?
Many decades ago someone decided to approach the problem in terms of water pressure. This may be very relevant to some applications but isn't a good measurement for rain wear.
It is far better to think of rain in terms of droplets crashing into the fabric. Each one has a mass and velocity depending on how "heavy" the rain is. This gives it a penetrating ability and is proportional to its kinetic energy... a bit like a bullet from a gun: heavier, faster bullets have more energy. (Kinetic energy = 1/2 mass times velocity squared.) Unlike bullets, rain is always made from the same material with the same properties... luckily we don't get armour piercing rain.
The ability for a fabric to absorb the energy of the rain is dependant on a number of factors:
- Fabric weight
- Fabric porosity
- The angle of collision
- How much the fabric can move
The porosity may have been what the standards scientists were trying to measure. The angle of collision can be just about anything, with perpendicular being the worse case scenario. The energy absorbed from the collision depends on how the fabric moves and how the rain drop distorts. It is the force the fabric exerts on the water acting over a certain distance (energy = force times distance)... or the work done by the fabric.
This is similar to a Kevlar bullet proof vest which distorts to slow down the bullet. (Ceramic plates defend against armour piercing rounds.)