Hi Urp, Buzz here. We get some ice and snow here at PDX, so i'm familiar.
Let's see, The Fokker F-28's (Horizon air is a local carrier) and DC-9 series 10 didn't have Leading Edge Devices, slats and Kruger flaps. Any frozen crud on the wings would act like a stall strip and spoil the lift on that part of the wing. So if the pilot didn't want to suddenly become a test pilot and discover when one wing (or maybe the other?) would quit flying, they make sure the wings are clean before takeoff.
MD-80's are deiced in the summer too! Seems that the center wing area collects cold fuel during a long cruise (over 3 hours). When it lands somewhere, it starts icing up. Then, when the MD-80 takes off, the wing flexes, the ice flies off the upper surface of the wings, right into the hard working engines. Swissair had it happen right after liftoff, and had a sudden twin engine failure. A sad story. American Airlines has heat blankets inside to get the area above freezing. I guess it works.
So that airplane gets a clean wing check / deice in the summer too.
The A320's ice sensor is only made to detect inflight ice. Snow / freezing rain before she flies down the runway won't work.
The 727 / 737 have a similar, but low tech visual ice detector: there's a nut on both windshield wipers that gets inflight ice build up. If you can't see it, it's icy. (grin) UAL's pilots will turn on anti-ice heat (engine inlets and wing leading edges) if it's 08c and visible moisture. g'day
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew chief by choice.