Why must you derate a breaker for temperature?
Breakers are temperature sensitive. A breaker works on the thermo dynamic (heating and cooling) properties of metals. When a breaker is exposed to hot or cold conditions, the circuit will not operate properly. For instance, if a short circuit happens when it is really cold, the metal will not bend far enough to break the circuit. On the other hand a circuit running at 60% under hot ambient conditions will also fail. To get a better understanding as to why temperature plans such a large role in breaker operation, let's look at the construction of that bi-metalic strip. Click here to understand the bi-metallic strip and how that works.
To the left you will see the actual ratings from Siemens for their range of circuit breakers. Notice how the range of temperatures, the accuracy of the breaker, and the actual trip amperage varies greatly. For instance a 50 amp breaker can deviate 10% on the high side at low temperatures, and pop at only 85% load on a bit hotter day. This is why electricians are taught to derate their circuit breakers depending on the application, installation, and intended usage.