The proportionality factor between energy flux and temperature gradient (see conduction). Thermal conductivity is to an extent an intrinsic property of a medium but may depend on temperature. The thermal conductivity of air is about 50% greater than that of water vapor and that of both increases (approximately) as the square root of absolute temperature. The thermal conductivity of liquid water is about 25 times that of air. Thermal conductivities of solids, especially metals, are thousands of times greater than that of air.