Context:
For two consecutive mornings, dense fog has enveloped northwestern India, including Delhi, Punjab, Haryana, parts of Uttar Pradesh, and parts of Rajasthan.
Relevance:
GS I: Geography
Dimensions of the Article:
- How does fog form?
- What’s been happening over northwestern India?
- What is the link between pollution levels and fog?
How does fog form?
- Fog forms like clouds do — when water vapour condenses.
- The presence of moisture and a fall in the temperature are key factors for the formation of fog.
- 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.
- 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.
What’s been happening over northwestern India?
- Temperatures have begun to dip over northwestern India.
- Cold wave conditions, in which the minimum temperature is significantly lower than normal, have been recorded recently over Punjab, Haryana, and parts of Rajasthan.
- The fall in temperature along with moisture and light winds over the Indo Gangetic Plain has resulted in dense fog over the region, according to the India Meteorological Department (IMD).
- 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 vapour from rivers and soil moisture can also cause fog.
- According to IMD, 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.
What is the link between pollution levels and fog?
- Delhi being more polluted, records more fog days compared to others.
- Delhi recorded a spike in pollution levels with AQI in the ‘severe’ category.
Radiation fog:
- 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.
- Once the temperature increases during the day, the fog dissipates. This is the radiation fog that we are seeing in Delhi.
Advection fog:
- This happens when the humidity is much higher.
- These fog episodes last longer and secondary particulate formation then begins leading to rapid build up of pollutants.
- Lower temperatures across the Indo Gangetic Plain in January can cause such fog episodes.
-Source: Indian Express
