How is heat flow defined in relation to temperature differences?

Heat flow is fundamentally defined by the principle that it moves from areas of higher temperature to areas of lower temperature. This is based on the second law of thermodynamics, which states that heat energy will naturally transfer until thermal equilibrium is reached. Thus, when a hot substance and a cold substance come into contact, heat will flow from the hotter substance, where the molecules have a higher kinetic energy, to the colder substance, where the molecular activity is slower. This transfer continues until both substances reach the same temperature.

In this context, the other options do not align with the established principles of thermodynamics. For instance, the notion of heat flowing from cold to hot contradicts the natural direction of heat transfer as recognized in physics. Saying that heat does not flow in a specific direction overlooks the critical aspect of temperature gradients driving heat movement. Lastly, while environmental factors like humidity can impact heat transfer rates in practical situations, they do not dictate the fundamental direction of heat flow in relation to temperature differences.