The frost beneath the cup

A copper cup can melt the frost beneath it, changing how easily heat escapes.

Put two matching cups in the same freezer and their surroundings seem identical. Watch what happens underneath them.

In an experiment by physicist James Brownridge, copper cups holding equal amounts of hot and cold water stood on a bed of frost. The hot cup melted the frost beneath it. The puddle soon froze, leaving that cup in better contact with the freezer floor. Heat had an easier route out through its base. In this comparison, the hot water began freezing first.

The frost adds a complication to the question of whether hot water can start freezing first. It looks like part of the freezer’s coldness, yet it also acts as insulation. A cup warm enough to melt it can change its own cooling conditions.

Brownridge repeated the comparison with an insulating support beneath the cups, keeping them off the frost. With that route to the floor interrupted, the cold water began freezing first.