How do you factor in heat to calories burnt?

I'd like to know this too.

I don't. I'm also in Phoenix - and figure I may lose more fluid, but not necessarily burn more. Don't know if that's true or not, but I've never seen research that supports a higher calorie burn in hotter weather. (Not even in those fancy hot yoga classes!)

First I want to give you congrats on your tremendous weight loss! I also questioned this when going for a 2 mile walk this morning. I guess an experiement is in order with a HRM to see if there is a difference.

You don't.

Heat does not affect calories burned. If anything, it will result in FEWER calories burned because the excess heat can cause you to work at a lower intensity than usual and/or shorten the duration of your workout.

HRMs will dramatically overestimate calories burned in a hot environment.

Logic would dictate that you would burn more calories if the exercise is done at the same intensity since your heart is working harder to cool your body. For example, the last two easy long runs I did were both at a 7:18 pace, one was 14 miles in 70 deg weather at night and the other was 10 miles in the middle of the afternoon in sunny 85 deg weather. My average heart rate was in the upper 150's for the 14 miler and upper 160's for the 10 miler. It's not possible that my calories burned could be the same if my average heart rate was about 10 beats higher. Therefore, the hotter it gets, the more calories you burn at a specific level of intensity.

You can use "logic" all you want, but the basic principles of exercise physiology are pretty well established.

Increased heart rate ONLY indicates an increase in calories burned when it is accompanied by an increase in cardiac output (and thus oxygen uptake). Under conditions of thermal stress, the increased core temperature and decreased plasma volume lead to a DECREASE in stroke volume (amount of blood pumped with each heart beat). Heart rate increases in order to maintain the same cardiac output. Since there is no increase in cardiac output, there is no increase in calories burned.

Now the myocardial tissue itself has to work harder, and the oxygen demands of the heart itself increase (one of the reasons why thermal stress can precipitate incidents in those with heart disease), but the amount of muscle mass involved is very small and, when looking at the body overall, the effect is trivial.

The disconnect between heart rate and oxygen uptake is the main reason why HRMs dramatically lose calorie counting accuracy under conditions of thermal stress--whether that is running outdoors, "hot" yoga, whatever.

You don't..