Companies go to great lengths to ensure that their rain gear is water resistant to withstand wind and rain, but how can you tell if it will repel as much incoming precipitation as you need it to?
In all rain repellent clothing and gear, there is a certain grade of measuring waterproofness . The process is called the Hydrostatic Head Test (HH for short). To be classified as waterproof, all garments must undergo this test for a waterproof rating.
Essentially, 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 begins 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.
Becoming familiar with HH grading numbers is a handy tool to use when deciding what outerwear grade best suits your needs. The range is generally from 1,500mm (low) to 40,000+ (highest).
HH Grading Scale (mm)
“Best Used In” Weather Conditions
1,500mm to 5,000mm
Light to average conditions: rain showers and light snow dustings
5,000mm to 10,000mm
Moderate Conditions: steady rain and snowfall
10,000mm to 40,000mm+
Extreme Conditions: heavy rain and snowstorms
From a consumer perspective, these numbers are what will determine the level of waterproof rating you need in a garment.
If you live/work in an area of infrequent/light precipitation, you’ll be better off with a jacket or coat that has an HH grade between 1,500mm and 5,000mm. This grading is considered ideal for urban and travel wear.
On the other hand, if you live/work in an area prone to frequent and steady precipitation levels, anything ranging from 5,000mm (water-resistant) and 10,000mm+ (true waterproofness) is closer to the mark.
There are a few instances where HH results may vary:
Remember, numbers never lie. Whether it’s ratings for HH grading, waterproofness , down , or layering , for a consumer it’s important to be wary and informed of all the technical information available on product labels.