The concept of off-season heat waves

Heat waves are becoming not only more intense, frequent, and longer-lasting due to climate change, but they are also increasingly occurring outside their typical seasonal window. A recent example is the March 2026 heat wave in the United States, during which temperatures exceeded 30 °C at numerous stations and broke all-time March records. Similar events have been observed across mid-latitudes; for instance, Europe experienced unusually high, summer-like temperatures in late March and April 2024. Despite these emerging trends, a consistent methodological framework for analysing such events is still lacking.

Heat waves are traditionally defined as a summer (June–August) phenomenon, while warm spells are typically identified using relative temperature thresholds. However, this approach does not adequately capture events in which absolute temperatures reach hazardous levels outside the summer season. As a result, potentially high-impact early- or late-season extremes have not yet received sufficient scientific attention.

In this study, we introduce and evaluate the concept of off-season heat waves. This concept is based on two key principles: (i) such events occur outside the summer period, with timing that is region-specific and reflects local climate conditions; and (ii) the absolute temperatures reached during these events are comparable to those observed during typical summer heat waves. The applicability of this concept is assessed across Europe using E-OBS gridded temperature data for the period 1950–2025. Given the diversity of climatic zones across the continent, we first determine the timing and duration of climatological seasons for each region, allowing for a consistent distinction between off-season and summertime heat waves. We then identify corresponding absolute temperature thresholds and examine their spatial variability. Finally, we present the spatial and temporal patterns of the most prominent off-season heat waves.