Increase doesn’t require a certain calculation---depends on context.

Thus, even in Petitioners' chosen examples, how an "increase" is calculated depends on the purpose of the calculation. A meteorologist interested in long-term trends will calculate an increase differently than one interested in day-to-day events. The owner of a classic car will calculate appreciation differently than if the car is just basic transportation. The term "increase" has no fixed unambiguous meaning in normal discourse, and there is no basis for the Court to read one into the Clean Air Act. To the contrary, EPA must exercise its discretion to determine how an increase is determined, and EPA has done so here in a way that maximizes facilities' flexibility to implement efficiency-enhancing projects while also requiring NSR permitting for projects that result in an increase of emissions above a facility's normal range of operations.