Analysis and quantification of contrasts in observed meteorological fields for foggy and non-foggy days

Abstract

The genesis and dynamics of fog take place under certain meteorological conditions. Thus quantification of the differences in different meteorological variables between foggy and non-foggy days can provide insight into the dynamics of fog; such contrasts can also help to identify and quantify precision and the accuracy required in the meteorological forecasts for driving fog prediction models. However, systematic evidence and quantification of differences in meteorological variables for foggy and non-foggy days are missing. Based on analyses of composites of differences between meteorological variables (temperature, relative humidity, wind and dew-depression) for days with no fog, mild fog and dense fog (based on visibility) days over Delhi, a fog-prone metropolis, we show that consistent and appreciable distinctions exist between days with fog and no fog (characterized in terms of hourly visibility) for both December and January. An important finding is that the differences persist not only near the surface, but also at 850 hPa level. Expectedly but consistently, the contrasts were stronger for long-duration (duration ≥4 h) than those for short duration (duration <2 h) fog; the contrasts were also stronger for dense (visibility <500 m) fog. The average differences between non-foggy and dense fog (duration ≥4 h) in composite temperature, relative humidity and wind are of the order of 3 °C, 10 % and 5 m/s, respectively; these findings can help to assess and validate meteorological forecasts for driving fog prediction. Fog is a high-impact event over many locations world-wide; while the present analysis is for a specific location, the methodology is generic to be applied over any fog-prone location.