Developing a cold threshold scale and index

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I’ve talked previously about defining “cold” when it comes to water, and organizations that put classifications on water temperature, which is helpful in describing swim events in a very factual way. I just swam in 2.7C water, and yes, yes it was very cold. But it says very little about how we experience cold, which is individual, personal, relative.

I’m been pondering this for some time, here I’ll to try my best to put these thoughts into the right words.

Developing a cold threshold scale

As I’ve been writing articles on cold swimming, I’ve come to realize that I tend to talk about my own cold water experiences in three simple categories:

  • Pleasantly cool:    Could we stay in a bit longer, please?
  • Moderately cold:  Gahhhh, oh that nips… 
  • Very cold:              Flipping’ shit that hurts!

It’s good. Having a way to categorize my different swims makes it easier to explain to others what I do, how I prepare, and so forth. So naturally, I put these sentiments into a colourful scale, relative to actual water temps, this is what swimming in cold water feels like to me at this point in my training:

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I don’t have any recorded swims between 3C and 6C, this year’s Canadian winter came on late but it did so pretty fast.

My experience of cold also seems to be defined by a few key things:

  • How both entry and cold shock felt;
  • The duration I’m able to stay submerged; and
  • The event of frostnip on my extremities.

To more hardened cold swimmers, an hour at my “pleasantly cool” water temps is nothing, but keep in mind this is my first year swimming cold. Very few people here swim past early fall, most hop out when the water hits 20C, and most that go lower are fully outfitted in wetsuits and other neoprene regalia. Cold water swimming isn’t a thing here yet, like it is in Ireland or the UK where water temps are generally colder for much longer in the year, where it’s simply a fact of life if you’re an open water swimmer. Our little Britannia Beach will likely never be mistaken for Sandycove. But I can dream.

Back to business. The scale above is helpful in a couple of ways. It gives me a general point of reference at this point in my training for planning – in very cold swims, I must bring my tent for shelter and my heat packs for rewarming afterwards, no question. More importantly, it also helps identify my thresholds, at the end of each range, I know to expect – below 3C, I will get some frostnip on my fingers if I stay in more than 2 minutes, it’s a given.

As each season passes, and the more I swim and adapt to cold, the scales for each category and thresholds will change. The scales might move down, and moderately cold will start at 6C. Or perhaps the scales will stay the same, but I’ll be able to stay in longer. This is good information to track, as I’m trying to get better at being in cold water. I sill enjoy my swims, but now I have a point of reference to use before getting in.

But this threshold scale feels very one-dimensional, as it’s based on the assumption of the differential between water and air being pretty constant. Where I swim, if the water is one temperature, the air will almost always be about 3-4 degrees higher. Sure, on some wintery days the air will drop down to create a higher differential (one swim was: water 2.8C and air -5C, a 7.8 degree differential), but generally speaking our experience has been pretty consistent temps, with air and water being only a few degrees apart.

On a recent swim, however, I learned a surprising lesson about how the importance of factoring in wind and air, and suddenly the scale took on different shapes and meaning…

Hong Kong Swim, January 24th 2016

On a work trip to Hong Kong, I found myself with a bit of free time on the weekend. I woke up at 4am that Sunday morning, exhausted from jet lag but still unable to sleep. I noodled around on the internet for awhile, worked on a few blog posts, and started to plan out the day. A quick check of the weather was disheartening – HK was having record low temps, it was rainy, windy. Screw it, I thought, no sightseeing today, I’l just head to the pool later and call it a day.

I took one last read of my Facebook feed at 7am, sleepy and just about ready to close my eyes again. In an incredible coincidence, I noticed a post in a swim forum from someone in HK, mentioning how busy the pools had become since the weather outside had dropped. I responded, explaining that I was in town for a couple more days and asked if she could suggest the best time of day to go. Connie and I messaged, and an hour later she picked me up at the hotel and treated me to a lovely swim in the outdoor unheated pool at the local university. I was thrilled! I’m practically obsessed with swimming, she wrote to me, possibly the most shared secret in swimming. I am too, I replied, I could swim every day all the time. Yep, I knew we were going to get along very well indeed…12553004_10156492701000352_7151932700146539851_n

When we got to the pool, we checked temps: water 15.5C, air 3.4C, wind 25km/h with gusts up to 64km/h. I felt confident, I’d swum in water temps far lower, 15.5C would be a breeze, I was more worried about swimming sloppy from fatigue than anything else.

It was indeed a wonderful swim, I really enjoyed it, but I was very wrong about a few things:

  • I failed to take air temp and wind speeds into consideration when judging where my thresholds lay;
  • I’d never swum with highly varied water and ambient temps (water warm, air cold), I had no idea of its significance anyhow; and
  • I misread physical signs of impending frostnip as “just really cold tippy-toes”.

So the point is, even at water 15.5C, the air and wind made it cold enough that it became an acclimatization swim for me, just a different kind than what I was used to. Too bad I didn’t realize it at the time.

This swim did make me realize that we can talk about cold relative to actual water temps, but cold swimmers (especially newbies) need to explore how cold feels to them and what it means to them, relative concepts learned over time and linked to understanding one’s own personal thresholds. Here’s how the swim felt to me while I was in the water (yes, with the colourful visuals again):

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The air and wind made me a little slow getting in (whereas Connie just hoped right in), but I had no cold shock. As usual, it took only a few laps to feel settled. Sweet, 15.5C is a peach, I thought.

After 45 minutes, I actually started to feel cold. I was stopping at the wall a little more often, my skin felt chilled, I knew I felt differently than the first 45 minutes. This feeling of cold lasted for 30 minutes, getting progressively colder feeling but not so much that I was worried – no shivers, no claw (I’ve actually never had it full on), no difficulties speaking, clarity of thought and function, I chatted with Connie at the wall, by all accounts I was just fine.

So I pushed on past the 30 minutes, and swam for another 15 minutes. Towards the end of this last bit, the tips of my toes felt really cold, not painful, and I could wiggle them comfortably, so I kept going and finished up the swim.

We jumped out, walked to the showers, and got changed. Connie dropped me off at the metro, we said our goodbyes, and I headed back to the hotel. Within about an hour, I could tell the tips of my toes were slightly frostnipped, just a little bit and covering a small area. No big deal, I knew it would be gone in a few hours. But later that day, I felt discomfort under both toenails, the left one much more that the right. By nightfall, it throbbed. The next day, I noticed a purple spot on the inner corner of each toenail (gross-out pics in this post). Hmmmm.

Frostnip is a risk to cold water swimming that I understand and accept – to a point. The interesting thing is this: when I swim in very cold water (0-3C), my hands are quite painful all over and I know the fingertips will get some frostnip, but I can’t really tell how much until later on, after I’ve gotten out and rewarmed. But with this longer mixed-temp swim, my toes weren’t painful at all, just really nippy, and yet I still ended up with minor nerve damage, albeit very superficial.

Why didn’t I see the frostnip coming, how could I have known better? I could have gotten out immediately when my toes felt cold, and declared my threshold reached (I might not have though, knowing me). But I learned something new; forewarning signs of the same problem can take different forms and send different signals, if any at all, don’t assume anything – be aware, decide if the risk of discovery is worth it. It would have still been a good acclimatization swim, even without cold toes. And never underestimate the impact of air and wind temps, they contributed far more than I understood at the time.

Calculating a cold swim index: why air and wind matter

Let’s talk about why 15.5C was an issue. It certainly wasn’t the water itself, I’ve swum at that temp many times with a big smile and no issues whatsoever. The obvious answer is the added impact of (a) much colder air temp and (b) significant winds moving the cold air around. Both of these factors had clearly offset what should have been an easy and comfortable swim.

I thought back to Loneswimmer’s reference to the concept of “combined” temperature (water + air), and thought of how interesting it would be to develop a scale or an index that better reflected the experience of the temperature I’d swum in. It sure as heck didn’t feel like a 15.5C swim. As heat index is a combination of air temperature and humidity, and is meant to give a description of how a temperature feels…I thought, let’s do the same with the feeling of cold.

Not a scientific proposition, of course, just an idea I’ve been toying with. My starting point of reference is weather here in Ottawa/Gatineau, where for the most part, air and water during winter and fall swimming is consistently about 3-4 degree apart. If water is 0.6C, air will be about 4C (not counting occasional days with sudden changes in air temperature).

Factor A: actual water + wind temperature

What if we took the combined air and water temp, and divided it by half for an averaged number? Did the swim “feel” like a 9-10C swim? Perhaps at the beginning, so close enough I suppose, but 9-10C still seemed too warm to describe the overall feeling of the water, based on my previous training experience…

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Factor B: wind, humidity and evaporative cooling

Moving air moves heat. A chilly 5C winter’s day feels much colder if wind is present. Same with water, stick your hand in a cold bath. Now move it around quickly, so the water moves around and over your skin…the sensation of cold changes very quickly. And, moving air and water both cause you to lose heat faster.

Let’s talk evaporation, which is a cooling process (I’m not gonna get all sciency here, but will do my best to make the point correctly). When dry air moves over a wet surface (your skin), it carries away moisture, causing it to evaporate. As a result, the surface (your skin) cools much faster.

Now what about humidity? Higher humidity means there’s a larger concentration of water in the air. Water conducts cold more easily than air, which is why we often hear water can feel 25-30 times colder to the body than air. It rained a little as we swam that day, and more importantly, consider the environment immediately close to my body – the strong wind was churning up droplets of the water from the pool.

So we have both wind/evaporation and cold high humidity. Let’s try to apply those to the scale. I have no real way of determining exactly how much rain and 25km/h winds with gusts of 64km/h affected my feeling of cold, so I dropped it down the scale by a couple of degrees:

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Did the swim feel like my previous 7-8C swims? Yes, actually, it did. Perhaps a little colder here and there when the wind really picked up (maybe even 5-6C in those moments?), but overall, it felt like a 7C swim – cold, enjoyable, but still physically challenging.

And yet this time I got frostnip. What gives?

Factor C: duration and intensity

The longer the swim (duration), the colder I got (intensity). Based on my previous swims, I’d say the last 7-8 minutes felt to me like water closer to 4-5C. My toes were cold, my pinkie finger was numb, I hugged myself for warmth when stopped at the wall.

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I had no signs of hypothermia during or after the swim, no shivers, but as I explained, I did have frostnipped toes.

So what does all this get me?

Well, I now have a different understanding of the swim, and consider it be cold indexed at approximately 7C, with the exception of the last few minutes.

I also learned the following:

  • Dividing air and water into an averaged temp gives me a reference point for future swims where there’s a big variance between both. I’ll have to test the theory more to determine it’s legitimacy, mind you, we’re just playing with ideas here;
  • Taking note of the wind speed and humidity give me a second point of reference by which to be more aware of how cold I might get towards the end of the swim;
  • Taking these factors into consideration will allow me to plan accordingly in future swims, and;
  • Scoffing at the actual water temp was silly of me, I should have been more vigilant about signs my body was giving me. I might have still pushed my limits a bit by staying in the water as long as I did, but I would have better understood the risks I was accepting by doing so.

Back to my toenail damage. What happened there? More stupidity. When I got into the change rooms, I broke another cardinal rule – never get into a hot shower right away, it accelerates after-drop and can also damage cold skin. Always rewarm slowly. But the shower head was wonky, I could only get very hot water, so I didn’t stand under it right away but instead let it slowly heat up the shower stall. But I did stick my feet directly under the hot stream of water while the rest of me slowly warmed up. Felt great. Was really stupid. Cold toes + hot water = skin thaws before circulation has returned, causing the cells to burst. Always use passive, gently rewarming on cold skin.

As for Connie, her toes seemed just fine, better than usual, probably. She’s used to swimming very regularly in cold water, so even as the wind picked up it probably didn’t impact her nearly as much as it did me. I’ll get there too, once the rivers and lakes started unfreezing and we get the opportunity to head back out again.

I truly hope my big toenails don’t fall off. Gross.

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