Fenix 7s heat acclimatization

JoLindell said:

I have been hiking 5 days in a row in 25-35 Celsius temperatures

For the activity to count against the acclimation, the temperature not only has to be above 22°C, but especially the watch has to know it - it means the weather widget has to be up to date, and has to show minimally that temperature during the activity. If the weather widget was not up to date, or it acquired data from a remote weather station with lower temperature, the acclimation algorithm did not kick in.

... see more details in the document What Is Heat and Altitude Performance Acclimation? | Garmin Customer Support

How Does Heat Acclimation Work?

• The heat acclimation feature is available only for GPS activities and requires weather data from your connected smartphone.

• Location access in your smartphone settings for the Garmin Connect app must be set to "Always" or "Allow all the time". For help updating this setting, see - Why Does the Garmin Connect App Need Location Data from My Phone?.

• Compatible watches and cycling computers provide acclimation notifications and corrections to your VO2 max estimate and training status when the outside temperature is above 22ºC (72ºF). Temperature readings for this are based on the weather data from your connected smartphone rather than sensors in your Garmin watch or cycling computer.

• Full acclimation takes a minimum of 4 training days.

• Heat adaptation will begin to decay after 3 days of no heat exposure training.

To add to what Trux said, the amount of heat acclimation you acquire from an activity depends on the intensity/effort of the activity, as well as temperature.

In other words, you could gain a LOT of heat acclimation from a short high-intensity running session when it's 40c, or very little from a long low-intensity hike when it's 25c.

Hiking is a low-intensity exercise, and I find that generally it does not add very much (if anything) to my heat acclimation, even if it's quite warm outside. But when I run a 5k or 10k race in similar weather, it adds quite a lot to my heat acclimation.