Indira Gandhi International Airport (VIDP/DEL), one of the largest aviation hubs in India, is frequently affected by dense fog during the winter season, causing significant disruptions to flight operations.
Due to the generally dry atmospheric conditions, the airport’s proximity to desert regions, and the pronounced presence of smoke from residential areas and industrial activities during the cold season, visibility-reducing phenomena occur almost daily throughout the year. This article analyzes the formation mechanisms, occurrence patterns, and key operational considerations of radiation fog at VIDP from a meteorological perspective.
Radiation Fog at VIDP
Airport Overview
| Item | Description |
|---|---|
| ICAO Code | VIDP |
| IATA Code | DEL |
| Official Name | DELHI Indira Gandhi International Airport |
| Operator | Delhi International Airport Private Limited（DIAL） |
| Operating Hours | 24-hour operations |
| Location | South West Delhi, National Capital Territory of Delhi, India (approximately 16 km southwest of central Delhi) |
| Latitude / Longitude | 28°34′07″N 077°06′44″E |
| Elevation | 777 feet / 237 meters |
Occurrence Period
Fog observed at VIDP Airport is predominantly radiation fog.
The onset of radiation fog typically occurs from late October through December, when moist air associated with the passage of Western Disturbances is cooled by strong nocturnal radiative cooling, leading to fog formation during the early morning hours.
The fog season generally extends from late October to late March, lasting approximately five months. Within this period, January experiences the highest frequency, with fog occurring on more than 25 days on average per month. This is followed by December, with approximately 15 fog days. In total, fog occurs on more than 60 days annually. Furthermore, when mist (MIFG) is included together with fog (FG), it is not an exaggeration to say that fog-related visibility reduction occurs almost daily in January.
Causes of Formation
The airport is located on a broad, largely flat inland plain, where strong nocturnal radiative cooling frequently occurs, contributing to fog formation.
In general, during seasons when nocturnal radiative cooling is pronounced over the plains of India, a strong temperature inversion develops during the pre-dawn hours, extending from the surface to approximately 200 m above ground level. Within this layer, air temperature can increase by 10–15°C with height, creating a highly stable stratification that favors the development of dense fog.
Fog persists until this temperature inversion is eroded; therefore, accurate identification and monitoring of the inversion layer are critical for assessing fog duration and dissipation.
Favorable Conditions
In general, fog is said to form when the temperature–dew point spread falls below 1°C, indicating near-saturation of water vapor. However, at VIDP, fog has been observed to develop even when the spread is around 3°C.
Based on past cases, fog formation at VIDP can be expected when the following conditions are met.
＊SKC (Sky Clear) / NSC (No Significant Cloud) / FEW (Few) / SCT (Scattered)
＊HZ (Haze) / FU (Smoke) / BR (Mist)
| Parameter | Conditions |
|---|---|
| Season | Late October through late March |
| Time of Occurrence | From approximately 3 hours after sunset or after 1900 local time (1330 UTC) until around 0500 local time (2330 UTC) |
| Dissipation Time | Fog typically begins to dissipate around 1100 local time (0530 UTC) |
| Cloud Conditions | SKC/NSC, or FEW/SCT clouds around 10,000 ft |
| Temperature Trend | Prior to fog formation, weather conditions such as HZ, FU, or BR are often observed, with a temperature–dew point spread of 5°C or more. After sunset, rapid surface cooling causes the spread to decrease; fog formation begins once the spread falls below approximately 3°C. Continuous monitoring of surface air temperature is critical. |
| Wind | Calm winds are most common. Fog occurs under various wind directions, with no dominant directional preference. Wind speed is generally 6 kt or less. |
Operational Decision-Making Challenges
Radiation fog at VIDP presents several challenges for operational decision-making, as outlined below.
To address these challenges, it is essential to compare multiple forecast models and establish a framework that enables objective evaluation of forecast reliability.
| Challenge | Details |
|---|---|
| Difficulty in predicting onset timing | Fog can form even when the temperature–dew point spread is around 3°C, making it difficult to apply conventional fog prediction thresholds. |
| Uncertainty in dissipation timing | Fog duration depends on the timing of temperature inversion breakdown and may persist until around 1100 local time in some cases. |
| Forecast discrepancies among models | Significant variability exists among numerical weather prediction models in forecasting both fog onset and dissipation. |
| Reliance on individual experience | Operational decisions often depend on the judgment of experienced personnel, making it difficult to standardize criteria across teams. |
