How heat travels

You stir your tea, and suddenly the metal spoon is hot at the top. The air shimmers above the radiator, and at a campfire your face gets warm even though you are not touching anything. All three times heat is on the move, each time by a different route. Once you know the three routes, you understand why a down jacket keeps you warm and why a pot has a plastic handle.

Route 1: conduction

In solids, heat is passed on from particle to particle. The hot particles at the bottom end of the spoon jiggle strongly and bump into their neighbours, which then jiggle more themselves. That is how heat travels up the spoon without the spoon itself moving. This is called conduction.

Routes 2 and 3: convection and radiation

In air and water, heat travels along for the ride: warm air expands and becomes less dense than the cool air around it. That is why the warm air rises, while cool air slides in from below. This creates a loop above the radiator that warms the whole room. That is convection. The sun and a campfire, on the other hand, send their heat as radiation: it needs no material to travel and even warms across empty space. When it hits your skin, you feel it immediately.

Good and bad conductors

Metals conduct heat extremely well, which is why a metal railing feels ice cold in winter: it drains the heat from your hand. Wood, plastic, and wool conduct poorly, they are insulators. The best everyday insulator is still air. That is exactly why a down jacket keeps you warm: the down traps countless tiny pockets of air in which the air can neither flow nor conduct well. Your body heat simply stays with you.