Fitness trackers and summer weather

Last week, in one day, my VO2max dropped from 60 to 57. Or so my Garmin told me.
Those of us in Wisconsin have had a very strange spring. It was cool to the point that it felt like we weren’t getting spring. Then, suddenly, things changed very rapidly until, on Tuesday, I found myself running in 85° weather with extremely high humidity, direct midday sun and pretty much no wind.
After one run in that weather, my Garmin said I lost a lot of fitness.
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