Context:

  • As winter peaked in northern India, a thick blanket of fog descended on several States in the last days of 2023 and also first of 2024.
  • This thick layer of fog created challenging conditions for residents and travellers alike.

Why is northern India prone to fogging?

  • The entire Indo­Gangetic plains are prone to the formation of fog during winter season, as all the conditions like low temperatures, low wind speed, moisture availability and plenty of aerosols etc are present in this Indo-Gangetic region.
  • Moisture incursion into this area can happen once a Western Disturbance which is a pattern that brings rain to north India during winter months that moves across northern parts.
  • Sometimes, moisture incursion can happen also from the Arabian Sea also.

What is fog?

  • In General terms, Fog is nothing but a thick cloud, but this cloud is very close to the earth’s surface.
  • A fog is a collection of small droplets of water which are produced when evaporated water has cooled down and condensed.
  • For a thick fog to form, the condition should be such that temperatures should be lower and abundant moisture should be available near the surface.
  • Fog forms whenever there is a temperature disparity between the ground and the air.
  • This happens frequently during Indian winters that fog is created when the temperature drops at night and in the early morning and aerosols present in the atmosphere condense.
  • High humidity, combined with an ample presence of water vapour or moisture, encourages fog formation conditions.
  • The process by which it cools plays a key important role in the formation of fog.

Infrared cooling:

  • One primary mechanism contributing to fog formation is called as infrared cooling.
  • It generally occurs when the weather is transitioning from summer to winter.
  • In the summer, the ground absorbs radiation from the sun, becomes warmer, and moistens the air passing over it.
  • When cooler weather kicks in, this mass of warm, moist air comes in contact with processes that cool it.
  • The ‘collision’ prompts the water vapor in the air to condense rapidly which give rise to fog.

Radiation fog:

  • Another type of fog which is known as radiation fog, is prevalent and occurs when an unseasonably warm day with high humidity is followed by rapidly dropping temperatures.
  • The specific type of fog, its duration, and its effects are dependent on various environmental conditions.