On a winter day the temperature on the inside surface of a wall is much lower than room temperature and that of the outside surface is much higher than the outdoor temperature. Explain.

Energy that flows from one object to another because of a difference in their temperature is called heat. Heat energy always flows from higher temperature to lower temperature. Because the hot air rises and cold air sinks, the temperature difference between the hot air inside surface of the wall and the cold air outside surface of the wall can become quite large during the winter. Thus the thermal energy will flow from inside surface of the wall to outside surface of the wall. Therefore, on a winter day the temperature on the inside surface of wall is much lower than room temperature and that of the outside surface is much higher than the outdoor temperature.