First, a word to the wise. For a few dollars at most real sporting goods stores, you can get a Space Blanket. They are pretty much a thin sheet of plastic (Usually PET) coated with a thin layer of aluminum. They have a lot of caveats to their use that means that you can't go out into a snowstorm and trust that a space blanket will save you from danger, but they do offer a very effective way to increase your margins of survival without adding much weight. This is especially useful for cyclists!
The basic idea is that they act as a radiant barrier. They prevent your heat from escaping from convection and evaporation. They also prevent you from getting wetter. Furthermore, they are silver and can therefore reflect light upwards for a rescue or let you gather more heat from a fire. Anyway, they saved me from a few tactical errors dealing with the rain recently.
The first 200km brevet of the year is next weekend. Sunday, I took an experimental ride through the rain. I have my commuter raingear well characterized because I do ride rain-or-shine to work, but I was wondering about my long distance riding gear. And I can't do the allegedly optimal retro-grouch thing and wear wool because I'm allergic, so a lot of the advice I hear from people who don't just call for their crew car when they get wet doesn't apply to me. I'm glad I did a trial run because it's predicted to rain for the 200km and now I know a few things what not to do.
Overall, I think that you can view raingear in terms of two basic qualities. I think that you could probably derive a method of measurement, but I think the entire industry would much rather market all products as being the best in both qualities.
First, different materials have different levels of much downpour it can handle before being saturated. Cotton T-shirts rank very low. Technical fabrics are in the middle. Plastic sheeting is on top. Second, how breathable the material is. Plastic sheeting is on the bottom. Cotton T-shirts are only in the middle. Technical fabrics can be fairly good. Allegedly, so is wool.
The way a technical rain jacket is supposed to work is that it causes water to bead up on top of it and run off. At the same time, water vapor will evaporate through it. However, it's not perfectly, so eventually it will let water through. Which is what happened, so I had water soaking all the way to my skin.
The usual reminder is that, if you are riding on a bike, too much waterproofing means that you will end up drenched in your own sweat. I think it's a little more subtle than that. I think there's some careful compromises that need to be made. See, if you are wearing a tight-fitting waterproof outfit, you will be drenched in your own sweat and uncomfortable. But looser fitting waterproof gear can prevent the rain from soaking you but still let adequate amounts of water vapor through.
For example, the first part I was wearing bike shorts and legwarmers, following some advice that I'd been given that nothing is going to make your legs more comfortable. The rest of the time, I wore my loose fitting commuter rain pants over top my bike shorts. That turned out to work much better. Likewise, I left my plastic rain jacket that has very few pores at home even though that was exactly what I really needed.
It also depends on how much of a downpour. When it was really coming down hard, that was when I was the most uncomfortable and that's when the amount of water I had in my clothing gave me the signs of hypothermia. But it was not raining nearly as hard on Tuesday, so I ended up being able to just use the new red jacket with factory-fresh DWR coating that hasn't started to wear off without getting wet because its saturation limit had not been reached.
In this modern era of oversold technical clothing, we keep having these silly opinions that you can go out in the rain and exercise and not get the slightest bit wet or cold. I mean, it works pretty well when you are just casually walking.
But I was wet for most of the ride. This was not the problematic part. The problematic part was that I was wet and my core temperature was decreasing, which gave me the shivers.
Finally, paper. This seems to be a longstanding thing with the cyclists. See, if you throw some loose papers underneath your jersey, it's an ersatz layer of insulation. It also serves to accumulate moisture. So one of the ways I managed to not get the shivers on the way down was to crumple up some papers and stick them underneath my jersey and then threw them out when I got to the bottom.
I should note that, while I never spent that much time in Boy Scouts, I spent a while in Cub Scouts and read the Boy Scout Manual cover-to-cover many times. So I had all of the knowledge about hypothermia right there in my head. I knew that the shivers weren't bad and were a way my body was trying to keep warm. But I also knew that the rest of hypothermia was dangerous and unpleasant and found the idea of paradoxical undressing to be a little unnerving. I also knew that if I couldn't stop the shivers on my own that I'd best call the wife and get a rescue. Or get help from the park rangers. But I also wanted to make sure I knew what to do in an emergency, so I managed to get myself out on my own accord.