Frozen light switches, and science

With scary visions of house fires raging in my head, I decided it was best just to go for it and crack open (not literally) the frozen light switch. What I found on the inside was both relieving and puzzling. There was absolutely no ice, or even moisture whatsoever inside the wiring box. My first thought was “Yay!”, followed closely by my second, which was “How can that be?”. How could there be moisture and ice on the cover, but not inside the box?

Our carpenter friend is at the house helping us prep the fireplace, so I presented the puzzle to him. He immediately, and nonchalantly provided the answer. First, there is cold air coming into the house through the box (a project for another day) and cover. The air in the house, along with the surface of the cover, are much warmer, so when the cold air meets the warm air, they interact on the outside of the cover to form condensation (or more correctly, “dew”). The dew eventually freezes because the outside air coming in cools it to below freezing (keep in mind the air outside as I am writing this is around 0°F).

The air and the metal surfaces inside the wiring box, however, are much closer to the outside air temperature. The temperature differential of the outside air and the air and metal inside the box is not great enough to initiate condensation inside the box. No condensation means no dew, and no dew means no ice.

Tada.