In News
- Recently, dense fog has enveloped northwestern India, including Delhi, Punjab, Haryana, parts of Uttar Pradesh, and parts of Rajasthan.
About
- The fog episodes are likely to recur over the Indo Gangetic Plain for the next three days.
- It will keep visibility poor in the hours before and after daybreak.
Fog Formation
- Fog forms like clouds do — when water vapour condenses.
- Formation:
- With the land surface cooling down at night, the air close to the surface also cools down.
- Since cooler air cannot hold as much moisture as warm air, the water vapour in the air condenses to form fog.
- Fog begins to form in the early hours of the morning, when the temperature is at its lowest.
- Favorable Conditions:
- The presence of moisture and a fall in the temperature are key factors for the formation of fog.
- Fog can have “high spatial variability”, and its intensity can depend on factors like humidity, wind, and temperature.
- Areas near water bodies, for instance, may see denser fog because of the higher humidity.
- The Indo Gangetic Plain is most vulnerable to fog occurrences, with major, weeks-long spells of dense fog in the months of December and January.
Image Courtesy: media.bom.gov.au
Types of Fog
- Radiation Fog (or ground fog):
- These fog episodes last for a few mornings on account of calm winds and western disturbances, resulting in localised fog formation.
- Once the temperature increases during the day, the fog dissipates.
- Advection Fog:
- Advection fog is larger in scale both in terms of the area covered and duration.
- Advection fog forms when warm, moist air passes over a cool surface, causing water vapour to condense.
- Advection fog mostly occurs where warm, tropical air meets cooler ocean water.
- If the wind blows in the right direction, sea fog can be transported over coastal land areas.
- Valley Fog:
- It is the result of mountains preventing dense air from escaping.
- The fog is trapped in the bowl of the valley and can last for several days.
- Freezing Fog:
- It is the result of liquid droplets freezing on solid surfaces.
- Cloud-covered mountaintops often see freezing fog.
- These are not applicable to the Indo Gangetic Plain.
Conditions Favouring Fog over Northwestern India
- Decreasing Temperature:
- Temperatures have begun to dip over northwestern India.
- Recently, Delhi recorded the lowest minimum temperature of the season so far — 6 degrees Celsius.
- Cold wave conditions, in which the minimum temperature is significantly lower than normal, have been recorded recently over Punjab, Haryana, and parts of Rajasthan.
- Dense Fog:
- The fall in temperature along with moisture and light winds over the Indo Gangetic Plain has resulted in dense fog over the region.
- Effect of Western disturbances:
- Western disturbances, which are storms that originate in the Mediterranean Sea, bring moisture-bearing winds to northwest India.
- This can result in increased moisture levels over the region.
- In the absence of western disturbances, local moisture sources like water vapor from rivers and soil moisture can also cause fog.
Characteristics of the fog over Delhi
- Warmer winter:
- Delhi saw a warmer start to the winter this year, with maximum temperatures remaining above normal till around mid-December.
- Scientists attributed this to a lower number of western disturbances affecting the city.
- This means that northwesterly winds did not bring much moisture and did not lead to any significant fog formation until mid-December.
- Highly Variable Fog Episodes:
- Season to season, it shows very high variability with extreme fog of 25 to 35 days (200 to 285 hours) of dense fog” like in 2017-18.
- On the other hand, in some years — like in December 2021 — Delhi hardly sees any dense fog events.
- Radiation fog:
- An update from the SAFAR forecasting system categorised the recent fog episode in Delhi as “radiation fog”.
Link between Pollution Levels and Fog
- More Fog at Polluted Places:
- According to the India Meteorological Department (IMD), Delhi being more polluted, records more fog days compared to others.
- Fog increases Pollution:
- As temperature declines, local wind speed also falls. The inversion layer comes down and vertical mixing reduces.
- This results in fog formation and particulate matter hangs on the boundary layer, increasing pollution levels.
- Recently, Delhi recorded a spike in pollution levels with AQI in the ‘severe’ category.
- Rapid Growth of Pollutants:
- Advection fog episodes last longer and secondary particulate formation then begins leading to rapid buildup of pollutants.
Source: IE
