It's not the heat, it's the humidity.
Prov. When the air is damp, hot days feel even hotter and more miserable. Jill: I hope the air-conditioning is fixed soon; the heat is unbearable in here. Jane: It's not the heat, it's the humidity. Alan: I thought the summers were hot when I was growing up in New Mexico, but they're even hotter here in Iowa. Jane: The climate is moister here; it's not the heat, it's the humidity.