Air quality turns ‘severe’; BJP requests L-G to intervene, call an all-party meeting

Subject: Environment

Context:

For the second day in a row, dense fog hit Delhi Tuesday morning with visibility falling sharply in parts of the city.
Experts attributed the increase in the city’s AQI to meteorological factors.

Meteorological factors behind the dense fog in delhi:

There is a radiative fog due to colder temperatures and higher relative humidity.
This tends to increase the pollution level as particles are suspended with water molecules in the atmosphere.
Additionally, mixing layer height is significantly low, restricting the vertical dispersion of pollutants, which also increases pollution.

Fog:

When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles.
- During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. One can see fog because of these tiny water droplets.
So, the fog is a cloud with its base at or very near to the ground. Because of the fog and mist, the visibility becomes poor to zero.

Condition for fog formation:

In order for fog to form, dust or some kind of air pollution needs to be in the air. Water vapor condenses around these microscopic solid particles.
- In urban and industrial centers smoke provides plenty of nuclei which help the formation of fog and mist.
- Such a condition when fog is mixed with smoke, is described as smog.
Sea fog, which shows up near bodies of salty water, is formed as water vapor condenses around bits of salt.
Depending on the humidity and temperature, fog can form very suddenly and then disappear just as quickly. This is called flash fog.

Types of fog:

Radiation fog: It results from radiation, cooling of the ground and adjacent air. These fogs are not very thick. Usual in winters.
Advectional fog: Fogs formed by condensation of warm air when it moves horizontally over a cold surface, are known as advectional fog.
- These fogs are thick and persistent.
- Occurs over warm and cold water mixing zones in oceans.

Frontal or precipitation fog: It is produced due to convergence of warm and cold air masses where warm air mass is pushed under by the heavier cold air mass.
- Precipitation in the warm air mass condenses to produce fog at the boundary of the two air masses.
Valley fog: It forms in mountain valleys, usually during winter. Valley fog develops when mountains prevent the dense air from escaping. The fog is trapped in the bowl of the valley.
- In 1930, vapor condensed around particles of air pollution in the Meuse Valley, Belgium. More than 60 people died as a result of this deadly valley fog.
Freezing fog: It happens when the liquid fog droplets freeze to solid surfaces.
- Mountaintops that are covered by clouds are often covered in freezing fog. As the freezing fog lifts, the ground, the trees, and even objects like spider webs, are blanketed by a layer of frost.
- The white landscapes of freezing fog are common in places with cold, moist climates, such as Scandinavia or Antarctica.

Fog vs Mist:

Fog is not the same thing as mist.
Fog is denser than mist. This means fog is more massive and thicker than mist.
There are more water molecules in the same amount of space in a fog.
Fog cuts visibility down to one kilometer, meaning it will prevent you from seeing further away than one kilometer from where you’re standing.
Mist can reduce visibility to between one and two kilometers.

Grand Banks, Newfoundland, Canada:

The foggiest place in the world is Grand Banks, a spot in the Atlantic Ocean off the island of Newfoundland, Canada.
The cold Labrador Current from the north and the warm Gulf Stream current from the east create prime conditions for thick fog to form almost every day.

Pea Souper:

A “pea souper” is a type of fog that forms when water condenses around microscopic particles of coal.
This fog is often a brownish-yellow color, leading to the name.
Pea soupers are common in areas that burn coal for energy.
The London Fog of 1952, which killed 12,000 people around the urban center of London, England, was a pea souper.
The Great Fog led to legislation that regulated the coal industry and air pollution in the United Kingdom.