FIRST WINTER FOG HITS DELHI: WHAT IS FOG, AND WHAT CAUSES IT?
- For two consecutive mornings, dense fog has enveloped northwestern India, including Delhi, Punjab, Haryana, parts of Uttar Pradesh, and parts of Rajasthan.
- The fog episodes are likely to recur over the Indo Gangetic Plain for the next three days, keeping visibility poor in the hours before and after daybreak.
Formation of fog
- Formation similar to clouds: when water vapour condenses.
- Key factors: presence of moisture and a fall in the temperature
- Mechanism:
- 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.
- Can have “high spatial variability”: and its intensity can depend on factors like humidity, wind, and temperature.
Types of Fog
| Radiation Fog | Advection fog |
|---|---|
| * As temperature declines, local wind speed also falls. | * Occurs when the humidity is much higher. |
| * The inversion layer comes down and vertical mixing reduces. | * These fog episodes last longer and secondary particulate formation then begins leading to rapid build up of pollutants. |
| * This results in fog formation and particulate matter hangs on the boundary layer, increasing pollution levels. | |
| * Once the temperature increases during day, fog dissipates. |