as a avid bowhunter, I stand hunt for apx 3 hours in the morning. Here in Connecticut it can get real cold, So far I have not tried these in anything below 28 degrees. But I can tell you in all honesty, for activities such as mine where you dont move for 3 hours, they dont get the job done. They dont last and they dont get hot enough. The temps I tried them at is 29 to 38 degress. I have gone to the Grabber brand and have been quite happy.