A study to improve night time fog detection in the Indo‐Gangetic Basin using satellite data and to investigate the connection to aerosols

Fog is a socio‐economically important weather hazard in the Indo‐Gangetic Basin, Northern India, disrupting public life and incurring massive economic loss because of delays in road, rail and air traffic every winter (December–January). Accurate detection of the spatial extent of fog and forecasting of its persistence and dispersion are critical in minimizing this economic loss. In the present study, a satellite based bi‐spectral brightness temperature difference (BTD) technique has been applied to detect night time fog over this region during the fog episodes of 2010–2011. The method, validated against ground‐based observations from four urban centres (Amritsar, New Delhi, Lucknow and Varanasi), yields 83.9% accuracy in detecting fog for a total 393 satellite overpasses using the operational 2.5 °C BTD threshold. The accuracy further increases to 88.3% with a reduction in the BTD threshold to 1.5 °C. A minimum fog droplet number concentration of 3.23 cm−3 is required to degrade visibility below 1 km in the presence of aerosols at 95% RH in the Indo‐Gangetic Basin. The results emphasize the need to improve treatment of aerosols in order to improve forecasting of fog occurrence in this region.