What is the primary cause of heat loss for a diver underwater?

The primary cause of heat loss for a diver underwater is conduction. When a diver is submerged, their body loses heat to the surrounding water through direct contact. Water is a much more efficient conductor of heat compared to air, which means that the rate of heat loss can be significantly heightened under water. As the diver's body temperature is higher than that of the surrounding water, heat is transferred from the body to the water molecules in contact with the skin, leading to a rapid decrease in body temperature.

This emphasizes the importance of wearing appropriate thermal protection, such as wetsuits or drysuits, to minimize conductive heat loss. These suits work by creating a barrier and retaining a thin layer of water that is warmed by the body, thus reducing further heat loss.

While convection, radiation, and evaporation can also play roles in heat loss, especially in specific scenarios, conduction is the dominant factor for divers in a typical underwater environment due to the nature of heat transfer through direct contact.