Working outside on days vs. cool days

Broven

Hello,
When I mow my lawn, I usually put on my heart monitor (a Polar FT4) to get an idea of how many calories I'm burning, etc. But today I noticed something. During the hotter days of the month, when temperatures were around 90 F, my monitor would tell me that I burned 1200+ calories by the time I was finished. Today, with a much milder 65 F, it says I burned about half that amount. The time it took to finish the yard was pretty much the same.

I know it certainly FEELS like more work when the temp is really high. But now I'm wondering... is it really more work? My heart rate is definately much higher on the hot days. I guess I am confused as to why this is. It seems like the amount of effort to push a mower around my yard should stay pretty much the same from day to day.

So, what gives?

Thanks,
Tim