Fog continues to elude scientists

Early mornings in the month of December witnesses delays in rail traffic and flights. Vehicles grope for ways and agricultural activity gets affected as fog cloaks the entire North, grinding mobility to a halt. As the sun shines on certain days the fog disappears, bringing visibility. Life gains momentum and we will all forget it till next December.

Fog is a collection of liquid water droplets or ice crystals suspended in the air at or near the Earth’s surface. It most frequently occurs during the peak winter season from December to January over the Indo-Gangetic Plain. Fogs can appear if the air is sufficiently moist, cool and descending. Smog, a hybrid of smoke and fog, also arises when sunlight reacts with airborne pollution, including ash, dust, and ground-level ozone.

Fogs are classified according to the physical process, which produces saturation or near-saturation of the air. The most common form, known as radiation fog, typically occurs in the winter, aided by clear skies and calm conditions. The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fogs usually dissipates soon after sunrise as the ground gets warmer.

There are three categories of fogs, which include aviation, thick and dense fog. Visibility of less than one km is appropriate for aviation purposes. For the general public and motorists, however, an upper limit of 200 metres is more realistic. Severe disruption to transport occurs when the visibility falls below 50 metres. The thickest fogs tend to occur in industrial areas, where there are many pollution particles, on which water droplets can grow.

Rapid urbanisation, combined with changing climatic conditions, has a big impact on fog. It tremendously increases energy consumption for electricity, transportation, cooking and heating. Energy consumption helps create heat islands that can change local weather patterns and weather downwind from a metropolitan area that is significantly warmer than its surrounding rural areas (heat island). Heat islands are created because cities radiate heat back into the atmosphere. These heat islands trap atmospheric pollutants. Consequently, cloudiness and fog occur with greater frequency.

Studies have shown that the occurrence of fogs have increased by nearly 12 times, since Independence, from just 6.4 per cent of fog frequency during 1950–1955 (prior to urbanisation) to nearly 72 per cent from 20002 onwards. Air pollutants are detrimental to health. Infants and people suffering from respiratory problems like lung infections, asthma and bronchitis are at risk, as the air quality in North India is unhealthy for such people.

Rising temperatures due of increased emissions of greenhouse gases continues to have severe impact on crop yields. Scientists have found that air pollution has become so severe that even crop yields are being cut by almost half. “While temperature’s gone up in the last three decades, the levels of smog and pollution have changed much more dramatically,” says Jennifer Burney, an environmental scientist at University of California, San Diego, and co-author of a recent study published in the journal Proceedings of the National Academy  of Sciences(PNAS).

Fog has a significant impact on economic and safety aspects too. Timely and accurate forecasts of reduced visibility are essential for agencies responsible for road safety, search and rescue operations and air traffic management. Nevertheless, officials in meteorological departments’ around the country find it difficult to forecast fog conditions, as its formation involves different physical processes. These conditions make it difficult to model fogs. The occurrence of fog can be predicted with 95 per cent accuracy, by a new method that was recently developed by two scientists, namely, Dr SwagataPayra, Birla Institute of Technology (BIT), Mesra, Rajasthan, and Dr Manju Mohan, a professor at Centre for Atmospheric Sciences, IIT.

The UK cleaned by its act after 1952, when, for five days between December 5 and 9, London gasped for air as a dense yellow smog smothered the city and paralysed life. The infamous ‘pea souper’ or Great Smog, as it is known, was caused by coal pollution and is estimated to have claimed more than 4,000 lives. Four years later, the United Kingdom passed its Clean Air Act. As a result, the air in the city grew noticeably cleaner very quickly. London no longer suffers from killer fogs. Like the UK, the United States too has the Clean Air Act.

India has an Air (Prevention and Control of Pollution) Act, 1981, which was amended in 1987 to provide for prevention, control and abatement of air pollution in the country. The Central Pollution Control Board (CPCB) is the enforcing authority and it is highly ineffective. Each winter, accidents continue to occur and flights are disrupted.

Last month was the foggiest in Delhi in the last 15 years. The city witnessed the third highest dense CAT III (when visibility falls below 200 m) fog hours since 1981 (when data analysis began). However, Met officials claim a better forecast, by analysing data transmitted from INSAT 3. It has helped contain disorder at the Delhi airport. Unless air pollution abates extensively, fogs will continue to torment the city. With smart cities being visualised in North India, stringent measures will have to be introduced to arrest pollutants.