What Temperature Triggers The Deicer To Turn On?

Why does the deicer turn on in the 40°F range and not in the mid 30°F range?

As water cools it get denser and sinks to the bottom of the tank, but once the temperature drops below 40°F, water actually gets lighter and remains at the surface. Thus, the surface can get colder and start freezing even though the water just a few inches below the surface is still at 40°. Therefore, to make sure the deicer protects the tank, all deicers are set to turn on above 40°F.