An account of fog over Chennai

Climatologically (based on 1951-1980) the annual fog frequency of Chennai airport is 4.3 days. But, the operational aviation meteorological forecasters often experienced more number of foggy days during the past decade. Hence the fog frequency has been critically analysed based on current weather observations made by aerodrome meteorological office, Chennai during 1981-2002 (barring 1984 for which data is not readily available). It has been found that the annual frequency based on the present study has shot up to 21.5 days. The most favourable period for fog over Chennai airport has been identified as January followed by February and March. The formation of fog has been mostly observed during 0000-0200 UTC although in good number of cases it was during 2200-2400 UTC. The most common duration of fog is 60-120 minutes albeit duration as high as 540-570 minutes are also probable. The low level (surface) nocturnal inversion frequency has alarmingly increased during 1990s and the inversion is almost a day-to-day phenomenon during 2000s. Rapid urbanisation, vehicular traffic and industrial growth could be the cause for the increased  atmospheric pollution which has  increased the nocturnal stability conditions as well the fog frequency. Visibility as low as zero had been recorded on a number of cases and their causes  have been analysed. Neutral or absolutely unstable stratification at 1200 UTC coupled with high relative humidity and high concentration of pollution cause the fog to form from 2200 UTC onwards and the nocturnal surface inversion / isotherm at 0000 UTC maintains the fog. Though the low level inversion maintains the fog once it is formed already, inversion alone is not a sufficient condition for the formation of fog.