I'm trying to understand what the waterproof rating means in cycling clothes. I understand the "science" behind the test, that is if a jacket is rated at 5000mm waterproof it means if you put a column of water over the fabric the water will leak only over 5 liters of water or above that pressure. What does pressure have to do with rain intensity? Are we talking about about the pressure the rain hits the fabric? But even then it doesn't add up because light rain shouldn't be expected to hit a lower pressure than a storm. What about the liters? Will ever get 5 liters of water over my jacket all the same time?
This isn't really cycling specific, but I'm not sure it's worth migrating to Outdoors.
What does pressure have to do with rain intensity?
The rating is indeed from the hydrostatic head test, and doesn't translate directly into rainfall, but there are approximate conversions like this. The conversion is indeed related to the average peak impact force, but it's probably a fairly rough heuristic rather than a formula, and it doesn't directly address issues like whether the seams are taped.
Are we talking about about the pressure the rain hits the fabric? But even then it doesn't add up because light rain shouldn't be expected to hit a lower pressure than a storm.
The impact force is probably correlated with droplet size, and light rain will tend to have smaller raindrops. Strong winds can obviously also drive raindrops harder into the fabric. Finally, peak force isn't reached until the surface layer is completely wetted, which will happen more slowly if at all in light rain.
Most waterproof fabrics have water-repellent surface treatments which will encourage droplets to simply roll off - this is the system that breaks down when the surface layer is completely wetted: if incoming drops hit a stationary layer of water they impart most of their momentum (creating an effective hydrostatic head on the underlying membrane), instead of beading off the fine surface structures and rolling away.
as Criggie says, the water-repellent surface treatment (or DWR = Durable Water Repellency or whatever) needs maintenance. Some are refreshed by cool ironing, and some need chemical re-application.
After-market treatments by Nikwax or Granger are also useful, and do note that washing with detergent (rather than soap or Nikwax TechWash or whatever) can further impair proper function.
- I mentioned the effect of surface wetting on the hydrostatic head, but it also impedes evaporation through the membrane. They're only breathable while there is air outside the pores to absorb vapour.
Breathable membranes are anyway unable to keep up with hard efforts, at least in high-humidity rain conditions.
IMO for low effort activity like walking or slow cycling, you don't need an expensive breathable fabric anyway, as mechanical ventilation is sufficient (ie, wear a poncho or use an umbrella).
For high-effort activity, you're just choosing between being drenched in sweat and drenched in rain. A water-repellent windproof and anything underneath that stays warm when wet, without getting heavy or uncomfortable (ie, not cotton) is generally lighter and cheaper.
The conditions where breathable waterproofs are actually beneficial are roughly
- when you don't want to carry an umbrella or mechanical venting is for some reason unsuitable (maybe high winds)
- low external humidity conditions such as snow rather than rain
TL;DNR - It is is a nice number that a manufacturer can put on a label. At best, its a guide.
The hydro-static head (HH) is nothing more than a measure of the pressure it takes to push water though the fabric. As far as rain coats it is a poor measure of suitability for a purpose, except to use as a guide - a low HH is probably not suitable for hours at a time out in the monsoon season, and an exceptional high HH is probably not particularly breathable, so not suitable for extremely active wearers.
HH does not specify how much water is let though once that pressure is exceeded. Some materials will let a lot more though than others.
The measure is not an indication of durability. Cling Wrap probably has a very high HH but would make a poor rain coat. Many fabrics are highly waterproof when they leave the shop floor, and degrade depressingly quickly when used.
HH is not a measure of the design of the rain jacket. It takes effort and skill to design a jacket that is both comfortable and water proof for a long time, especially with an active person. Any twit can make a rain coat that keeps a dummy in a shower dry, few succeed at making a rain coat that keeps an active sport person dry for hours in the rain.
HH is not measure of the other materials used in the jack construction, particularly water proof zips, but also linings. The edges of opening can wick water, a poor choice of design and materials can lead to a lot of water entering by flowing up hill and around corners.
As far as pressure - large rain drops are heavy and fall faster than small ones due to weight/surface area increasing with size so less impacted by air friction. The pressure is related to your speed as well as the speed of falling rain - so a fast roadie in light drizzle will get saturated in a shower proof jacket even though the 'rain' drops are tiny and the shower proof jack keeps him dry watching sport from the side lines.