Syllabus
GS Paper 1 – Salient features of World’s Physical Geography
Context
In the final days of December and the initial few days of 2024, a thick fog enveloped a large portion of northern India.
Fog
Fog is essentially a dense, low-lying cloud, composed of tiny water droplets formed when evaporated water cools and condenses.
Conditions for Dense Fog
- Lower Temperatures: Colder conditions are conducive to fog formation.
- Abundant Moisture Near the Surface: The presence of ample moisture close to the ground surface aids in fog development.
- Higher Humidity: Increased humidity levels are favorable for fog creation.
- Cooling Process: The method by which the air cools plays a significant role in fog formation.
- Temperature Disparity: Fog tends to materialize whenever there’s a temperature difference between the ground and the air.
Difference between Fog and Mist
| Fog | Mist | |
| Meaning | A thick low-lying cloud at surface level, composed of tiny dew drops in the air. | A cloud formed out of small droplets held over in the atmosphere at ground level due to temperature inversion or humidity variation. |
| Density | Very high | Relatively low |
| Visibility | Lower, up to 1 km | Relatively higher, more than 1 km |
| Longevity | Longer period | Shorter time |
| Fog and Mist are both atmospheric phenomena that result from the condensation of water vapor. The primary difference between them lies in their density and visibility. Fog is denser and reduces visibility to less than 1 km, while mist is less dense and allows for visibility of more than 1 km. Additionally, fog tends to persist for a longer period compared to mist. | ||
Fog Formation
- Fog materializes when there’s a temperature difference between the ground and the air.
- This phenomenon is frequent during Indian winters when the temperature drops at night and in the early morning, leading to condensation on aerosols present in the atmosphere.
- High humidity, coupled with a significant presence of water vapor or moisture, promotes foggy conditions.
- The cooling process plays a crucial role in the formation of fog.
Fog Mechanism
- One primary mechanism contributing to fog formation is called Infrared Cooling.
- This typically 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, giving rise to fog.
Types of Fog
- Radiation Fog: This type of fog forms when all solar energy leaves the earth, allowing the temperature to reach the dew point. It is most likely to occur after it has rained the previous night.
- Precipitation Fog: Precipitation fog forms when rain falls through cold air. This is common with warm fronts but can also occur with cold fronts if they are not moving too quickly. As rain falls through cold, dry air at the surface, it evaporates, causing the dew point to rise and leading to the formation of fog.
- Advection Fog: Advection fog forms from the surface contact of horizontal winds and can occur under windy conditions. When cool moisture on the ground comes into contact with warm, moist winds, it cools the incoming air. This causes the dew point to rise, creating high humidity and forming fog.
- Steam Fog: Steam fog forms during the fall season over any lake due to the difference in the rate of cooling and interactions between the upper cold air and the relatively warmer lake.
- Upslope Fog: Upslope fog forms adiabatically when moist winds blow towards a mountain, causing the air to rise and cool. The cooling of the rising air causes it to reach the dew point temperature, resulting in fog formation on top of the mountains.
- Valley Fog: Valley fog forms in valleys when the soil is moist from previous rainfall. As the skies clear, solar energy leaves the earth, allowing the temperature to cool near or at the dew point. Valley fog can be so dense that it is sometimes referred to as tule fog.
- Freezing Fog: Freezing fog occurs when the temperature falls to 32°F (0°C) or below. It produces drizzle, and these tiny droplets freeze upon contact with an object.
- Ice Fog: Ice fog is only seen in polar and arctic regions. When temperatures drop to 14°F (-10°C), it is too cold for the air to contain super-cooled water droplets, so it forms small tiny ice crystals instead.
Reasons for heavy fogging in northern India
- Fog Formation in Indo-Gangetic Plains
- The entire Indo-Gangetic plains are susceptible to fog formation during the winter season, as all the necessary conditions — low temperatures, low wind speed, moisture availability, and abundance of aerosols — are present in this region.
- Moisture Sources
- Winds known as the Western Disturbances, originating in the Mediterranean, contribute to the moisture content in Northern India. Occasionally, the Arabian Sea also increases the moisture content in the northern region.
- Role of the Himalayas
- The Himalayas in the north act as a barrier, preventing the eastward movement of the fog over the plains, which leads to its concentration over the Indo-Gangetic region.
- Delhi’s Smog
- In the case of Delhi, air pollutants such as particulate matter add to the atmospheric load, creating smog (a combination of smoke and fog).
- Vulnerable Regions
- The entire Indo-Gangetic plains are particularly vulnerable to fog formation.
- Favourable Conditions
- Fog formation is facilitated by the presence of low temperatures, low wind speed, moisture availability, and plenty of aerosols.
- Moisture Incursion
- Moisture incursion can occur once a Western Disturbance, a precipitation pattern that brings rain to north India during winter months, moves across northern parts. Sometimes, it can also occur from the Arabian Sea.
- Effect of Fogging
- Fog disrupts the lives of people in North India due to the condition of poor visibility. It impacts air travel and land transportation, and also increases pollution. Recently, Delhi experienced an increase in pollution levels, with an AQI rating of “severe.”
Ways to Reduce Dense Fogging in North India
- Air Quality Management: Improving air quality by controlling the emission of pollutants can help reduce the formation of fog, especially smog. This can be achieved through stricter regulations on industries and vehicles, promoting cleaner fuels, and implementing dust control measures.
- Afforestation: Planting more trees can help in absorbing excess moisture from the atmosphere, thereby reducing the intensity of fog. Trees also absorb pollutants, which can help in improving air quality.
- Public Awareness: Educating the public about the causes and effects of fog, and how their actions can contribute to its formation, can encourage more environmentally friendly behaviors.
- Weather Forecasting and Early Warning Systems: Accurate weather forecasting and early warning systems can help in predicting foggy conditions in advance, allowing necessary precautions to be taken to minimize its impact on daily life and transportation.
Related Information: Comparison between other forms of condensation
| Dew | White Frost | Mist | Haze | |
| Definition | Dew is formed when moisture is deposited in the form of water droplets on cooler surfaces of solid objects such as stones, grass blades, and plant leaves. | Frost forms on cold surfaces when condensation takes place below freezing point (0° C), i.e., the dew point is at or below the freezing point. | Mist contains more moisture than fog. In mist, each nuclei contains a thicker layer of moisture. | Haze is an atmospheric phenomenon where dust, smoke, and other dry particles obscure the clarity of the sky. |
| Ideal Conditions | Clear sky, calm air, high relative humidity, and cold and long nights. The dew point must be above the freezing point. | The same conditions as dew, except that the air temperature must be at or below the freezing point. | Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface. | Dry weather conditions that lead to the production of dust, smoke, and other particles. |
| Formation | Formed by the deposition of water droplets on cooler surfaces. | Formed by the deposition of minute ice crystals instead of water droplets on cold surfaces. | Formed by water droplets, but with less merging or coalescing. | Formed by the accumulation of dust, smoke, and other dry particles in the atmosphere. |
| Visibility | Does not affect visibility. | Does not affect visibility. | Visibility is more than one kilometer but less than two kilometers. | Obscures the clarity of the sky. |
Source: The Hindu
Practice Question
Discuss the meteorological and anthropogenic factors contributing to the heavy fog conditions in North India. Evaluate the impact of these conditions on the socio-economic aspects of the region and suggest measures to mitigate the same. [250 words]