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Understanding fog, predicting it better

The last 10-15 years have seen more frequent and intense fog activity in the region, according to a recent report in the journal Current Science.

Written by Harikrishnan Nair | Amsterdam/new Delhi, New Delhi |
December 14, 2016 2:31:48 am
delhi, delhi fog, delhi train services, types of fog, fog explained, delhi fog problems, delhi train delays, trains cancelled delhi, delhi news, india news The WIFEX project hopes to generate data that can be used to create a model that can help improve both time and spatial accuracy of fog prediction. (Express Photo: Praveen Khanna)

On the night of December 3 — and for a few subsequent nights — Delhi and its adjoining areas saw an unusual fog. It extended up to 350 metres above the ground; the normal range is between 150-250 metres.

The last 10-15 years have seen more frequent and intense fog activity in the region, according to a recent report in the journal Current Science. It has impacted aviation, road transportation and the economy. On Monday, 82 trains were delayed and 16 were cancelled due to dense fog in parts of North India.

What Else is Making News

Fog usually appears over a region of high pressure where humidity is greater than 75%. “At present North India is witnessing advection fog aided by warm and moist easterly winds pushed in by the cyclones over Bay of Bengal. It is also induced by cyclonic circulation around central Pakistan and Rajasthan due to Western Disturbances,” says Mahesh Palwat, chief meteorologist, Skymet Weather Services. “Easterly winds traditionally blew over eastern India, this time they have reached western UP, Delhi and even Punjab. But we expect this to dissipate soon, and cold, dry north-westerlies to take over again.”

Moisture in the atmosphere could condense around particulate matter of diameter in the range of nanometres, to cause fog. An IIT Kanpur study claimed burning crop residue led to longer spells of dense fog. Dr Sachin Ghude, author of the Current Science report, Winter Fog Experiment (WIFEX) over Indo-Gangetic Region of India: Overview and Preliminary Results, pollution could contribute to intense fog.

Although the principle behind the formation of fog is known, there is little understanding of its density and intensity, its physical and chemical composition, and how it dissipates. As such, it is difficult to predict fog.

The Winter Fog Experiment is a 4-year multi-institutional venture to study fog at Delhi’s Indira Gandhi International Airport, under the Ministry of Earth Sciences. Its objectives, according to Ghude’s report, are to develop better now-casting (for 6 hours) and forecasting of winter fog on various time and spatial scales, to help reduce its adverse impact on aviation, transportation and the economy, and loss of human life due to accidents.

The experiment, which began last year with a pilot project, will take a comprehensive look at factors that contribute to fog formation, and the effect of pollution on fog. Sensors at the site will measure surface micro meteorological conditions, radiation balance, turbulence, thermo-dynamical structure of the surface layer, fog droplet and aerosol microphysics, aerosol optical properties, real time sky images, and aerosol and fog water chemistry to describe the complete environmental conditions in which fog develops.

Instruments at another institute, Chaudhary Charan Singh Haryana Agriculture University in Hisar, will also measure fog, giving researchers two bases — one in a high-pollution and a low-pollution zone. With this experiment, scientists hope to create a prediction model that will help in improving both time and spatial accuracy.

Types of fog

RADIATION FOG usually occurs on winter evenings as the surface, and the air above it, cools. As surface heat absorbed during the day is radiated, the ability of the air to hold moisture is reduced, and water droplets are formed. This kind of fog is also known as ‘ground fog’ as it stays close to the surface, and “burns off” in the morning sun.

Advection fog forms when warm, moist air passes over a cool surface causing water vapour to condense. Advection fog mostly occurs where warm, tropical air meets cooler ocean water. If the wind blows in the right direction, sea fog can be transported over coastal land areas.

VALLEY FOG is the result of mountains preventing dense air from escaping. The fog is trapped in the bowl of the valley and can last for several days. In 1930, vapour condensed around particles of air pollution in the Meuse Valley, Belgium, causing a deadly deadly valley fog that killed more than 60 people.

FREEZING FOG is the result of liquid droplets freezing on solid surfaces. Cloud-covered mountaintops often see freezing fog. Rime — feathery ice crystals — is deposited on the windward side of lamp posts, overhead wires, pylons and transmitting masts.


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