Delhi fog is notorious for causing major delays of trains and air traffic. At times it is so dense, that it leads to last minute cancellation of flights and trains. Fog is typically a winter phenomenon i.e. it is observed in the period 15th November to 15th February. In the month of November, generally shallow fog is seen, which gets thicker as minimum temperature drops. Many a times, it will persist for whole day & in extreme cases it even persists for few days. Forecasting fog duration on a given day is quite difficult & forecasting duration of fog in a season is not within the realm of possibility.
For fog formation, air should saturate, so that it condenses on ground. Therefore, either the temperature should be low, or humidity increases. Both, these conditions are met when a Western Disturbance (WD) moves across Indian latitudes, after its passage, when the sky is clear, cold dry air penetrates the Northern plains and where it has rained due to WD, those areas become prone to Radiation Fog.
The duration of fog, varies a great deal because, not only it is city specific, it is site specific. Meaning it may last for two hours in Safdarjung, but it may last for six hours over Palam. Also, it depends upon how long the cold spell lasts and how much was the precipitation.
Climatologically speaking, the mean ‘Foggy’ days in winter season are:
November 4.6
December 11.2
January 12.1
February 5.0
Total 32.9 = 33 days
On an average, fog should last for a minimum of 3 to 4 hours, which means 99 to 132 hours in a season. But, fog is equally notorious for persisting for longer duration especially in the month of December and January. Duration in these months can easily go up to 6 – 8 hours which will push ‘Fog Hours’ to more than 150 hours. Thus, it is nearly impossible to predict the exact duration of fog or ‘fog hours’ in any season. At present, there are no forecast models which predict season wise ‘fog duration’. Therefore, it will be more of a guess work than a calculated forecast.
