In all rain repellent clothing and gear, there is a certain grade of measuring waterproofness. To test the waterproofness, garments undergo a testing process called the Hydrostatic Head Test (HH for short).
HH is measured in laboratory environments using hydrostatic pressure. The test is fairly straightforward: First, the fabric to be tested is pulled taut and sealed in a controlled chamber. Water is then slowly poured into a graduated cylinder placed on the fabric (which adds pressure to the surface). Once this pressure becomes too great, water will begin to permeate through the fabric. Depending on how many milliliters of water is able to penetrate will determine how it scores on the HH test.
For additional information on waterproof ratings and testing, please refer to our blog post: https://triplefatgoose.com/blogs/down-time/how-waterproof-fabric-is-tested