Heat acclimation

Is that hotter than where you've normally been doing activities lately? If yes, acclimation should go down, as your body is not yet accustomed to that kind of conditions.

Yes, hotter. But then I don't understand how this is supposed to work. Until now I had the impression that it worked as my intuition: when I excersized on a hot day it increased. 

If what you say is true, then what if not an excursion on a hot day makes my body accustomed to heat? Should I go to ski?

It does make you more acclimated. But that shows only the next time when you do an activity in the same temperature. 

100 % means your body has got used to the current temperature. If you do activities in hotter, the acclimation percentage first drops because the reference temperature changes.

And yes, if you do an activity in colder, your acclimation percentage grows, because your body has it easier in cooler. But growing the percentage is not a "goal", the percentage tries to tell you how accustomed your body is too the current temperatures.

It is not just doing an activity in the heat that matters, it is the amount of time at a aerobic heart rate or more during that activity.  I am not sure what that HR limit is, but from my experience on hikes/walks or easy bikes, it is somewhere around HR > 60-70% of max... but not sure. I think the documentation online mentions it, it is a lot like vo2max detection... HR needs to get above a certain amount before it will give you a result.

It is not just doing an activity in the heat that matters, it is the amount of time at a aerobic heart rate or more during that activity.  I am not sure what that HR limit is, but from my experience on hikes/walks or easy bikes, it is somewhere around HR > 60-70% of max... but not sure. I think the documentation online mentions it, it is a lot like vo2max detection... HR needs to get above a certain amount before it will give you a result.