Standing on a wood deck might feel warm on a hot day, but a metal one would be unbearable. A casual look at wood and metal won’t tell you why one gets hotter than another. You have to examine microscopic features, then see how the atoms in these materials conduct heat.

Heat causes molecules in a material to vibrate. As they vibrate, they jostle their neighbors, transmitting the energy of their motion. When one group of molecules sets another to vibrating, heat conducts through the material.

Heat conduction between materials depends partly on how their surfaces meet. If a surface is rough and uneven, contact and heat conduction are interrupted by gaps. Wood is full of microscopic gaps at its surface. Metals are smoother and have fewer gaps.

In metals, the outer electrons in its atoms are more loosely bound than in wood. Metal atoms are packed more densely and can transmit heat vibrations more readily.

On an atomic level, metals arrange themselves in networks of crystals, which tend to be stiff. Wood is made of tiny fibers, which are both softer and more randomly organized. Heat vibrations are conducted less efficiently though these fibers.

Wood has gaps internally as well as at on its surface. It’s riddled with microscopic air pockets left when the living wood dried out. Molecular vibrations from heat move through these pockets slowly. Metals have far fewer voids.