Radiation: exchange of electromagnetic energy waves emitted from one object and absorbed by another. Conduction: two surfaces with differing temperatures are in direct contact. Still air is an excellent insulator, while water is an excellent conductor. Heat conduction in water is about 25 times greater than in air. The rate of conductive heat exchange is inversely related to the thickness of the insulating substance - air trapped in goose down clothing, body fat, layer principle of dressing for the cold. Convection: one of the media is moving with a fluid or gaseous medium. For example, heat transfer from skin to moving air or water. Wind chill index: gives the equivalent still air temperature for a particular ambient temperature at different wind velocities. The temperature gradient between the skin and air and the air velocity are important factors in determining convective heat loss. Convective heat loss during swimming is more than twice that during rest in still water.