Heat acclimation

"Heat acclimation" hasn't provided me with any useful information. It seems to be yet another psuedo-scientific metric dreamt up by Garmin's marketing people in the hope of selling more watches, with only the vaguest of explanations as to what it actually represents. Now and again it'll report something like "37% acclimatized", or some other suspiciously-precise number, but I've no idea what that means in practical terms. I suppose it's trying to tell me that I'd be faster if I were better acclimatized (or maybe that my heart rate would be slower), but how much faster or slower is anyone's guess.

My understanding is that the real "use for the acclimation figure is that the watch automatically compensates the effect of heat and altitude to the vo2max estimation. In my old Fenix 5+ (without acclimation estimate) it was annoying to see the vo2max etc. drop when exercising in heat. No such problem with my 965, so maybe there's some use for the acclimation estimate.

Okay, but in my example what does "37% acclimatized" mean? 37% of what? For a given temperature, would my VO2Max estimate be any different if I was 0% or 100% acclimatized instead of 37%? And by how much? It can't possibly be a 37% difference.

I'm sure the underlying concept has some validity, but Garmin treats its customers with borderline contempt when it comes to explaining what these numbers actually represent.

The percentage is described as "Your heat acclimation will be displayed as a percentage of how acclimated you are to the temperature". So when the temperature rises above 22°C, the watch starts compensating for the temperature in the training status and VO2max estimates. When the percentage reaches 100 %, heat compensation is no longer applied even if the temperature is above 22°C, since the watch thinks you are fully adapted. And if you don't work out in the heat for 3 days, the percentage starts to decay: support.garmin.com/.../

(I personally don't pay any attention to the percentage, I'm just happy the watch compensates for my lack of acclimation when it occasionally gets warmer here in Finland. )

Well, that's certainly good to know. Thank you!

The conclusion that I have come to, based on all replies, is that heat acclimation is (most likely) based on the temperature at the start of the activity. In the case of my device, this seems to be a degree lower than is reported in other places. There is a second theory, that the device will interrogate various weather outlets over time, depending on your location. I have no reason to believe that this is true. I looked at my fit file to see if there was any evidence of this and found none. This is disappointing, as temperature can range a lot on a long bike ride, and just because you start early when it is cool does not mean that you do not spend a considerable amount of time in the heat.

Secondarily, it does not seem that weather is consistent from device to device. While we can discuss the absolute correct way to get the weather to work, simply setting it to the device location should produce good results in an urban environment. Also, the current temperature on my watch does not match the consensus of the official temperature, nor does it match the temperature offered by my Pixel phone. In contrast, Apple watches match the Apple iphone exactly. (Yes, I understand the difference, and the challenges.)

As for weather, while the current temperature is close, forecasted temperatures, even an hour out, are all way low, but about 5 or more degrees. Something is wrong here. 

I would hope that Garmin would address these issues.

1. Implement more frequent temperature checks, based on location. 

2. Fix the current temperature if possible. 

3. Fix the forecast.

I will also offer you a slightly different point of view.

It works terribly badly for me, but as has been said here, the importance of this feature (for me) is mainly because of VO2Max compensation, and therefore I don't actually need to be 100% acclimatized. Otherwise, it turns off VO2Max compensation, and I don't trust Garmin to accurately determine that I'm fully acclimatized at 100 procents, so it would rather be under for long periods of time and not mess up my VO2Max on hot days, of which there aren't that many in my area.

So I guess