How does waterproof rating translate into rain intensity? 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. 



Footnotes:


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