When purchasing a new shell-layer, the waterproofness of it is one of the main functions you search for - this is where this mystical term “water column” shows up and we’re gonna explain what it means.
First some basic facts. If you’re looking for a 100% waterproof garment for yourself or your child, classic rubber rain clothes may come first to mind. The downside of these types of rain clothes is that breathability is non-existent. This means that you will get sweaty and damp even if you’re doing low impact activities, and as you increase activity level this problem will only worsen.
The alternative to these classic rain clothes is shell garments. Shell garments are breathable since they have a laminated membrane on the backside of the outer fabric. The outer fabric combined with the membrane creates a shell-like material. This material releases air but not water - that’s why we say that shell materials are breathable and waterproof, but the degree of waterproofness depends on the quality of the membrane and the outer fabric.