This is a climate classification based on human comfort, described by Werner H. Terjung in 1966 in his article "Physiologic Climates of the Conterminous United States: A Bioclimatic Classification Based on Man."
He envisioned that it would be useful to planners, vacationers, and military quartermasters to know what kind of clothing to prepare for which places.
I think it's pretty interesting.
https://www.jstor.org/stable/2569310 (if you can get on JSTOR)