IN ONE corner of Delhi’s Indira Gandhi International Airport (IGIA), unmind-ful of the surrounding dense fog, top scientists and meteorologists of India have come together, for the first time, on a campaign trail of fog. Literally. The experi-ment has been named the “Winter Fog Campaign”. The pilot project will study fog for better forecasts. For this, India’s two premier scientific institutions, the India Meteo-rological Department (IMD), New Delhi, and the Indian Institute of Tropical Meteorology (IITM), Pune, have come together to deploy their best brains.
A sophisticated technology infrastructure has been set up at the IGIA to record observations of atmospheric variables at different heights (up to 20m) to the surface as well as of soil moisture up to one metre below the surface.
About 48 fog days are observed on an average across north India and the Indo-Gangetic plain during winter. The number and intensity of fog events have been rising—from 62 dense fog hours in 2010-11 to 174 in 2014-15. Every year, dense fog events cause more than 15,000 road accidents, delay of over 10,000 trains and diversion of over 100 flights in IGIA alone. Many airlines are now insuring themselves against fog-related delays and cancellations.
Fog is nothing but tiny droplets of water suspended in the air, but it remains one of the least understood weather phenomenons.
During a dense fog, visibility falls below 200 m. Six different types of fog have been observed around the world, but the most relevant type in the Indian context is radiative fog, which envelops vast areas of northern India during winter. Across north India, land is cooled overnight by thermal radiation and this, in turn, cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing saturation of the air at the surface, leading to condensation and the formation of fog. Radiative fog usually dissipates soon after sunrise, but can be very thick during dawn.
Despite advances in satellite technology, India has a low fog forecasting ability. Its success rate in predicting a fog two hours before the event is 70 per cent, while it is seven per cent four hours before the event.
On the other hand, the ongoing Paris Fog Field Experiment has created a model which can predict a fog two days in advance, and the hit rate is 87 per cent. The experiment is being conducted at SITRA (Site Instrumental de Recherche par Teledetec-tion Atmospherique), a French national atmospheric observatory dedicated to cloud and aerosol research.
The initiative has caliberated hitherto unknown aspects of droplet distribution, vertical profile of fog and the effect of aerosols and particulate matter in forecast models. The IMD-IITM campaign is based on the Paris experiment.
Developing a model
In recent years, climate scientists have been collaborating to understand fog better and improve forecasting. The IMD-IITM campaign is just the latest step. The IGIA is the worst hit airport in India, so it is no surprise that it has been chosen as the primary site for the experiment.
The observations will include simul-taneous measurements of surface meteorological conditions, radiation balance, turbulence, thermo-dynamical structure of the surface layer, droplet and aerosols microphysics, fog water chemistry, vertical profile of droplets, winds, temperature and humidity to understand how fog develops (see ‘Fog formation...’). “We are looking intently at fog microphysics, along with constant monitoring of factors such as aerosols, dust, and environmental factors such as relative humidity and atmospheric temperature which have not been addressed in earlier models adequately,” says G S Bhat, a professor at the Centre for Atmospheric and Oceanic Scien-ces at the Indian Institute of Science (IISc), Bengaluru, who is also the chairperson of the Winter Fog Campaign.
Over 30 individual components are being used to study fog dynamics. “We are following the Paris experiment, and collec-ting specific information pertaining to fog droplets and conditions conducive to fog formation,” says Sachin Ghude, IITM’s co-ordinator for the experiment. Fog updates have been generated through statistical techniques, which is less effective since it relies on a few variables and probability.
Plugging the gaps “Fog is a complicated weather phenomenon. While we can predict onset of fog to some extent, but we are still studying the way it evolves, its intensity and duration,” says R K Jenamani, head of IGIA met office. Today, most of India’s fog tracking is done through Indian Space and Research Organisation’s (ISRO) INSAT-3D satellite. The near-real time detection is achieved by using separate algorithms for day-time and night-time observations relayed by the satellite.
While the algorithms have been quite successful, a glaring gap is that neither the day-time nor the night-time algorithms are able to successfully detect fog occurrence during periods of day-night transition, which is arguably the most crucial period for fog formation.
Another problem with fog detection is wrong detections. “From a satellite perspective, low-lying clouds and fog look nearly identical. This makes it difficult to differentiate between the two leading to inaccurate detections,” admits Sasmita Chaurasia, a scientist with ISRO. With this fog experiment, scientists and meteorologists are gearing up for more accurate fog forecasting. “We hope to put in place a system to forecast by next winter,” says IMD director general L S Rathore.
