The study drew on an unusually rich observational foundation. The researchers compiled routine and special aviation weather reports issued every thirty minutes from three airports serving Istanbul: Sabiha Gökçen, identified in the study by its ICAO code LTBA, situated on the city’s Anatolian side; Istanbul Airport, coded LTFM, on the northwestern shore near the Black Sea; and Atatürk’s successor regional field LTFJ in the eastern inland part of the metropolitan area. These half-hourly reports, which record visibility, cloud base, temperature, dew point, wind, and pressure, were supplemented by radiosonde soundings that profile the vertical structure of the lower atmosphere and by microphysics satellite imagery capable of distinguishing cloud properties at night. From this combined dataset, the team applied an objective physical typing algorithm that sorts every fog event into one of five mechanistic categories: advection fog, radiation fog, precipitation fog, cloud-base lowering, and evaporation fog.

The classification matters because each fog type carries a distinct physical signature and a distinct forecasting challenge. Advection fog forms when moist air flows horizontally over a cooler surface, chilling the air to its dew point; it is the classic fog of coastal regions where warm, humid maritime air drifts across cold water or cold land. Radiation fog, by contrast, is born on clear, calm nights when the ground radiates heat skyward, cooling the air layer in contact with it until condensation begins. Precipitation fog arises when rain falls through drier air and evaporates, pumping moisture into the boundary layer. Cloud-base lowering describes stratus decks that descend until they effectively become fog at the surface, while evaporation fog forms when water evaporates from a warm surface into cooler air above. Sorting events into these categories allowed the researchers to trace each airport’s fog regime back to its thermodynamic roots.

The spatial contrasts the analysis uncovered are striking. Over the extended historical period from 2012 to 2023, the eastern inland station LTFJ logged an average of 17.2 fog days per year, more than five times the 3.2 fog days recorded annually at the southern urban station LTBA. The dominant fog mechanism also differed fundamentally between the two sites. At LTFJ, advection fog accounted for 53 percent of events, a signature of moist air being transported into the area and chilled over locally cool surfaces. At LTBA, radiation fog dominated at 55 percent, and the researchers attribute this to the interplay of urban heat island effects and topographic shielding around the airport. The urban heat island raises nighttime minimum temperatures, making it harder for the near-surface air to reach saturation, while surrounding terrain can block or channel the drainage flows and moist intrusions that feed fog elsewhere in the city.

When the analysis was restricted to the common period from 2019 to 2023, during which all three airports provided overlapping records, the three-way comparison sharpened the picture further. LTFJ recorded 52 fog-affected days over that window, LTBA just 12, and LTFM, the northern airport closest to the Black Sea coast, 57. Advection fog again led the statistics at both LTFJ, where it made up 50 percent of events, and LTFM, where it reached 56 percent. The elevated fog frequency at LTFM fits its coastal geography: proximity to the Black Sea supplies abundant low-level moisture, and the temperature contrasts between sea and land provide the cooling mechanism that advection fog requires. That two airports separated by a few dozen kilometers within the same metropolitan area can differ by nearly a factor of five in annual fog days underscores how strongly microclimate and siting control fog occurrence in complex coastal terrain.

The temporal structure of Istanbul’s fog is equally distinctive. The analysis demonstrated a pronounced winter peak, with fog activity concentrated in November and February, and a complete absence of fog during the summer months. This seasonality reflects the underlying thermodynamics. Winter brings longer nights, weaker solar heating, lower sea-surface and land-surface temperatures, and a greater supply of moist air masses moving across relatively cool waters, all of which favor the saturation of the boundary layer. Summer, with its strong insolation and warm surfaces, keeps the near-surface air well above its dew point even under maritime influence. For aviation planners and forecasters, the message is that Istanbul’s fog risk is not evenly distributed through the year but clusters sharply in the cold season, with November and February emerging as the critical months for low-visibility operations.

Perhaps the most consequential finding concerns spatial coherence. Between 2019 and 2023, the researchers evaluated how often fog occurred simultaneously at all three observation nodes, and the answer was remarkably small: only four days in five years saw all three airports shrouded at once. In other words, Istanbul’s fog is profoundly fragmented, appearing at one airport while the others remain clear. This localized fragmentation has direct operational implications. Flight diversions, holding patterns, and ground delays are typically triggered by visibility conditions at a single destination airport, so a fog climatology built on city-wide averages would badly misrepresent the actual risk. A pilot approaching one Istanbul facility may encounter dense fog while another facility across the city operates normally, a pattern that only station-specific, mechanism-aware forecasting can capture.

The study’s methodology also highlights the value of combining conventional surface observations with remote sensing. Radiosonde profiles reveal the depth and structure of the temperature inversion layers that cap and nourish radiation fog, while night microphysics satellite imagery allows researchers to track fog patches across the metropolitan area between routine observation times, confirming events that a single surface station might miss or mischaracterize. Supplementary analyses in the paper examined the distribution of fog-affected observations by cloud cover, cloud base height, runway visual range, and surface pressure, along with satellite composites of notable fog episodes in February 2019 and November 2021, building a multi-layered empirical baseline for the region.

Istanbul is far from alone in facing the forecasting puzzle of urban fog, and the study situates itself within a growing international literature. Comparable event-based climatologies have been constructed for New York City, Casablanca, Abu Dhabi, Cape Town, Belgrade, Lisbon, Perth, Shanghai Pudong, and airports across South Korea, Turkey’s northeast, and Argentina, each confirming that fog regimes are tightly bound to local geography. Recent work has pushed further, exploring machine learning approaches to low-visibility prediction, Bayesian decision networks for airport fog forecasting, and the dramatic forecast improvements that come from including irrigation effects in numerical weather prediction over northern India. The Istanbul analysis contributes a critical empirical foundation for such efforts in the eastern Mediterranean, where complex topography and competing maritime influences make statistical or machine-learning fog prediction especially dependent on high-quality, mechanism-resolved training data.

The broader significance of the work lies in what it says about fog as an urban boundary-layer phenomenon. In a megacity, the surface is a patchwork of heat-retaining concrete, vegetated hills, cool water bodies, and engineered drainage basins, and the atmosphere within the lowest few hundred meters responds to that patchwork in ways that standard synoptic-scale models struggle to resolve. By demonstrating that fog frequency varies fivefold within a single metropolitan area and that simultaneous city-wide fog is a rarity, the Istanbul study makes a compelling case for high-resolution, station-specific fog forecasting grounded in physical typing rather than raw visibility statistics. For the aviation sector, which loses millions of dollars annually to low-visibility delays worldwide, and for the growing community of researchers developing fog decision support systems, the message from Istanbul is clear: to predict fog in a megacity, one must first understand that the megacity is not a single climate but an archipelago of microclimates, each with its own fog story written in temperature, moisture, wind, and terrain.

Subject of Research: Long-term fog climatology and physical typing across three airports in the megacity of Istanbul

Article Title: A comparative long-term analysis of fog characteristics in the megacity of Istanbul

Article References: Yoğun, B., Aktaş, C., Bakır, G., Bozdemi̇r, R., Çeli̇k, Ş., & Özdemi̇r, E. T. (2026). A comparative long-term analysis of fog characteristics in the megacity of Istanbul. Theoretical and Applied Climatology, 157(10), Article 685. https://doi.org/10.1007/s00704-026-06617-w

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06617-w

Keywords: fog climatology, Istanbul, boundary-layer meteorology, advection fog, radiation fog, urban heat island, aviation meteorology, METAR observations, urban microclimate, low visibility, Türkiye, Theoretical and Applied Climatology