I’ve found that my Max heart rate, which I usually hit during a sprint finish of a 5 or 10K, in hot weather is is about 5% higher than in cooler weather.   No matter how hard I go, I cannot approach my hot weather MHR when it’s below 75 degrees F or so.  When zone training, do I base my zones  on my MHR  potential on that day (the way I do for SUP, which is also about 5% lower than running),  or stop being a Nancy?

This is very normal.  You are not being a Nancy.

This is a tough concept to explain but it is something we see in Ironman athletes in Kona all the time. On race day it is sweltering out there and a failure to understand this concept often leads to race day disasters.

What is going on is that your perceived effort in hot climates (at least until you are heat acclimated which can take weeks) does not always correlate with your heart rate. Stated another way, you won’t feel like you are going as hard as your heart says you are when it is hot out. Because your perceived effort is wonky, unless you take this into consideration, if you feel like you are maxed out at a race the reality is that you are probably already blown. When you race in extreme heat you need to leave something in the tank and feel like you have a few more gears just to make sure you don’t blow up.

On the flip side, when it is very cold out, it can also be difficult to hit your max heart rate because of the cold.

So what do you do if you are training according to your threshold. For example, shooting to stay below your threshold, going above threshold on an interval, or trying to pace your race based on your threshold heart rate?  My recommendation is to just use your normal threshold as you measuring during your AT test.

Practically speaking that means if it is hot out, you are going to feel like you are going a little bit easy and if it is cold out you are going to feel like you are going a little harder than expected.

Again, without getting too deep into physiology this is a really common concept that is kinda difficult to explain but if you use your normal threshold number and be smart about correlating your perceived effort with your heart rate you can manage this phenomenon. All the (successful) pros in Kona account for this and this is something I consider every year in Molokai.