Fog Early Warning System for Egyptian Desert Highways A Five-Year METAR Climatology and Multi-Source Remote Sensing Validation

Road fog constitutes one of the leading meteorological hazards for vehicular safety on Egyptian desert highways, with the Cairo–Alexandria Desert Road (210 km, ~90,000 vehicles/day) recording recurrent multi-vehicle collisions attributable to sudden zero-visibility conditions. This study presents the first five-year systematic fog climatology for HECA station (2019–2023), integrating half-hourly METAR observations (68,146 records), MSG SEVIRI satellite imagery, and SRTM terrain analysis to develop a prototype Egypt Fog Early Warning System (E-FEWS). Twenty-eight fog events were identified over the study period, overwhelmingly concentrated in the November–February winter season (75%). The Dew Point Depression (DPD = T_air − T_dew) emerges as a robust, operationally viable early warning signal: validated against the complete observational record (22,332 winter half-hourly observations, with no estimated values), DPD < 2°C preceded 26 of 28 confirmed fog events at least 1–3 hours before onset (Event-level Recall = 92.9%), with a mean lead time of 2.1 hours (range: 0.5–5.5 h) and a directly measured False Alarm Rate of 11.4% (morning windows). Validation against MSG SEVIRI Night Fog RGB imagery confirms a clear spectral signal for the most intense event (8 February 2024, VSI: Extreme), with blue channel values increasing from background ~170 to ~190 DN and a diagnostic green/blue ratio of ~0.9, compared to values of 3–30× for concurrent dust/sand events. A Visibility Severity Index (VSI) is introduced (Extreme / High / Moderate / Low), with 32% of events reaching Extreme status (visibility ≤ 0.12 statute miles). A 7-day Consecutive Fog Nights (CFN) sequence in January 2021 — the longest in the record — highlights the risk of driver complacency during persistent fog episodes. The study demonstrates clear phenological and spectral separation between fog (DPD < 5°C, wind < 10 kt, low G/B ratio) and dust/sand hazards (DPD > 8°C, wind > 10 kt, high G/B ratio), supporting a three-class (Fog/Dust/Mixed) operational classification framework for Egyptian highway warning systems. Full ground-truth validation across the entire winter record yields observation-level Recall = 89.2% and event-level Recall = 92.9%, comparable to or exceeding established fog warning systems in temperate climates, with the single-parameter DPD criterion identified as an effective screening layer that benefits from multi-parameter refinement (wind speed, time-of-day) for operational false-alarm reduction.