Study of winter fog over Indian subcontinent : Climatological perspectives
Keywords:Visibility, Fog, ICAO, METAR
Fog is localized phenomenon where horizontal visibility is reduced to less than 1000 m due to suspension of very small water droplets in the air. In fact Fog is cloud at surface level. This phenomenon matters to us in our daily activities due to its effect on life, public health, road safety, and economic prosperity. Its knowledge is essential to meet critical societal needs. In India, radiation and advection fog are most common which occur mostly over north India in winter. This study presents results of spatial variability of average number of days having visibility less than 1000 m at 0300 UTC during winter season over Indian subcontinent (0° N-35° N and 60° E-100° E) using synoptic hour daily surface data. Analysis of Fog events based on half hourly METAR observations for winter months (December 2010-February 2011) at selected ICAO stations of India is also presented.
The monthly spatial variability of average number of days with fog having visibility less than 1000 m indicates presence of fog over Indo-Gangetic plains on an average of 7 to 10 days during December and more than 8 days during January. South Gangetic West Bengal is susceptible to fog during February. Seasonal variation of fog shows that there is prominence of fog over Indo-Gangetic plains on an average of more than 6.5 days. Significant incidences of foggy days occur over South Interior Karnataka and Coastal Karnataka has incidence of foggy days on an average of 6.5 days. North India is susceptible to dense fog on average of 2.5 days during the season while Gangetic West Bengal, West Rajasthan and adjoining parts of east Rajasthan and East Uttar Pradesh have occurrence of fog on an average of 3.5 days. Thick fog occurs on an average of 3 days over northern India during the winter season. Analysis of time series of air temperature, dew point temperature, dew point depression, visibility, zonal and meridional components of wind and its magnitude at selected ICAO stations indicate that visibility reduces to below 1000 m while light winds are southeasterly at Ahmedabad, northerly to northwesterly at Amritsar, northerly to northeasterly at New Delhi, westerly to southwesterly at Kolkata, Guwahati and dew point depression is below 3 °C during such conditions. Long period 1971-2010 analysis of visibility conditions less than 1000 m over Indian subcontinent shows Indo-Gangetic plains region to have average wind speed to be 0.6 m/s, air temperature in the range 9 °C to 15 °C during December-February at 0300 UTC. Long period 1971- 2010 analysis of relative humidity shows Indo-Genetic plains region to have relative humidity in the range 72% to 84% at 0300 UTC during December-February.
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