This specialized switch has an eye that detects moisture and an internal thermometer that detect sub freezing temperature. When both conditions are met a 120V current flows to snow melting cable (not included) in driveway, roof lines, exposed steps, drain spouts. I've installed these in all of those conditions.
Works very well.
I don't do this type of work anymore so this useful switch is available at about 1/5 of new cost.
Includes installion instructions.