Abstract
The Zero-Degree Level (ZDL) serves as a critical boundary for aircraft structural icing in the presence of supercooled droplets. Understanding its spatiotemporal evolution is crucial for evaluating mid-tropospheric thermal shifts and aviation safety. Using ERA5 reanalysis data (1990–2024) across 38 Greek airports, this study analyzes trend dynamics of mean freezing levels and lower-tail cold extremes (Q25) via Mann-Kendall and Sen’s slope methods. Overall, the 0°C isotherm exhibits a systematic upward shift within a 10,000–14,000 ft Mean envelope (with Q25 values extending down to 6,500–8,500 ft), showing statistically significant warming trends across over 80% of stations. Crucially, the Q25 threshold displays a lower rate of long-term shifting than the mean, governed by strong seasonal asymmetries. November acts as the primary warming hotspot (Q25 up to +63 ft/yr), indicating delayed early-winter cold-air outbreaks, whereas September exhibits trend stagnation. Winter shows strong intra-seasonal contrasts, transitioning from January cold-tail stationarity to widespread February warming. Spring functions as a thermal bridge toward summer’s elevated freezing levels (>15,000 ft Mean). Spatially, maritime sectors benefit from sea-surface thermal buffering, whereas enclosed northern inland basins display persistent cold-air pooling. Operationally, these shifts displace the primary structural icing window (0°C to -20°C) to higher flight levels.