How do meteorologists classify a heat wave?

There is no single global temperature that defines a heat wave, because what counts as extreme heat depends on local climate and season. Meteorological agencies usually define it as a spell of unusually hot weather lasting at least a few days, sometimes combined with humidity or nighttime heat. In many places, heat waves are measured against historical percentiles rather than one fixed number. This is important because regions adapted to cool climates can be affected by temperatures that would be ordinary elsewhere.