In hilly areas, the lower ambient temperature affects the efficiency of heat engines, particularly in relation to the cold sink. While a Carnot engine may appear more efficient in colder environments due to a lower cold sink temperature, real heat engines operate differently. They expel expanded gas to the environment and intake new air, which can be colder in hilly regions. This means that the hot reservoir must work harder to maintain the necessary temperature for efficient operation. Consequently, heat engines may actually be less efficient in hilly areas compared to plains, especially during colder months.