Mercury rising but Delhi continues to see dense fog

Mercury rising but Delhi continues to see dense fog

Subject: Environment

Context: Dense fog has hit Delhi several times over the past 10 days, in what meteorologists are calling a “unique” event as in 2^nd and 3^rd week of februray generally don’t see fog.

Concept:

Factors for this event as per IMD:

1. Lack of western disturbances: It led to an anti-cyclone high pressure belt. Anti-cyclone causes stable boundary layer and inversion and calm winds at lower levels.
2. Moist easterly winds brought more moisture in Delhi’s air, a pre-requisite for fog (Usually at this time dry wind blows in from the west or northwest region)
3. Low wind speed
4. An easterly system over central India
5. High Pollution levels

About fog

• Fog, cloud of small water droplets that is near ground level and sufficiently dense to reduce horizontal visibility to less than 1,000 metres.
• Under similar conditions similar conditions, but with visibility greater than 1,000 metres, the phenomenon is termed a mist or haze, depending on whether the obscurity is caused by water drops or solid particles.
• It refers generally to clouds of smoke particles, ice particles, or mixtures of these components near grounds causing reduction in visibility.
• Fog is formed by the condensation of water vapour on condensation nuclei that are always present in natural air. This happens when the relative humidity of the air exceeds saturation by a fraction of 1 percent.