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When I sit by a campfire, how does its hot air heat me?
Category: Physics
Published: February 26, 2015
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
When you sit by a campfire, most of the heat you are receiving from the fire does not come from hot air. It comes from thermal radiation.
Generally, there are three ways that heat can travel: radiation, conduction, and convection. Thermal radiation consists of electromagnetic waves (mostly infrared waves and visible light) emitted by an object due to its temperature. This radiation carries energy which is converted to heat when it hits another object and is absorbed. In contrast, conduction involves the direct movement of heat through an object. Since air is a good thermal insulator, conduction is the least effective way for heat to travel through air. Lastly, convection involves the bulk movement of pockets of heated fluid. In convection, a pocket of fluid such as air or water gets heated and then moved somewhere else by fluid currents.
Thermal radiation tends to spread out in all directions. Similarly, heat being transferred via conduction tends to travel in all directions, and dominantly flows along object parts with higher thermal conductivity. Heat being transferred via convection can also generally travel in any direction if there is a current traveling in that direction (e.g. a current of air created by a fan). However, in typical situations on earth, heated fluid tends to convect upwards. This is because heated fluids tend to be less dense than the surrounding colder fluid, and therefore are pushed up and out of the way as gravity pulls harder on the colder, denser pockets of fluid.
With these basics in mind, let's turn to the campfire. The campfire is emitting heat through all three modes. Since air is a poor thermal conductor, you won't receive much heat from the campfire via conduction unless you stick your hand in the fire. Therefore, assuming you are sitting several feet away from the campfire, we can ignore conductive heat transfer. The thermal radiation from the fire spreads out in all directions and is able to reach you. This thermal radiation is mostly in the form of infrared waves and visible light. In contrast, the campfire heat transferred via convection shoots straight up into the sky and never reaches you (i.e. hot air billows upwards). Therefore, when you are sitting beside a campfire, almost a hundred percent of the heat that you receive from the fire is transferred through thermal radiation. This is why the side of your body facing the fire gets hot while the side facing away from the fire stays cold. Thermal radiation is a form of light, and light travels out in straight lines. The side of your body facing away from the fire is literally in shadow and cannot receive the thermal radiation.