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Satellite study brings clarity on winter fog

Satellite image of the winter fog cover blanketing northern India. Credit: NASA MODIS

Researchers have applied infrared remote sensing techniques for the first time to develop a high-resolution record of winter fog which engulfs the Indo-Gangetic Plains (IGP) that spans India, Pakistan, Nepal and Bangladesh in the northern region of south Asia every year.

In their report1, authors Ritesh Gautam of the Environmental Defense Fund in the United States and Manoj Singh of the University of Petroleum and Energy Studies in Dehradun, India, say “the satellite-derived fog detection framework can be used to monitor and predict fog at high resolution”.

Dense fog in December and January over IGP severely disrupts rail, air and public transport and significantly affects ground visibility, disrupting the lives of millions. Air pollution, combined with high moisture in the shallow boundary layer, are major factors affecting the persistent and widespread fog over the IGP.

“The dataset and our fog detection algorithm will help better understand the coupling between fog, meteorology and air pollution, including understanding fog trends,” says Gautam.

The researchers constructed a “dynamical threshold for characterising fog, based on brightness and temperature differences as a function of various parameters including viewing geometry, fog effective radius, fog vertical distribution and its liquid water path”. They then applied their detection framework, along with inputs from surface meteorological observations, to high-resolution data collected by NASA’s MODIS satellite over the last 19 years (from 2002 to 2020) to produce a high-resolution climatological distribution of fog covering the entire IGP at 1 km × 1 km spatial resolution.

The report says the highest fog frequency was found in the Terai regions bordering India and Nepal. The frequency in Uttar Pradesh and Bihar was much higher than in Delhi and the National Capital Region.

“Governments, the airline industry and railway operators can use our data to understand, monitor and predict fog,” the researchers say. Their study validates satellite-derived fog frequency against ground measurements from 13 stations across northern India and Pakistan. "For the first time, it provides estimates of fog droplet size across the entire IGP," they say.

MODIS data are available from four separate overpasses from Terra and Aqua satellites and are insufficient for providing detailed information about the diurnal variations in fog characteristics, the authors note. “Our detection framework can be applied to measurements from geostationary satellite missions to provide a more continuous characterization of the diurnal variations of fog.

“This paper will be well-cited,” Madhavan Nair Rajeevan, the former secretary of the ministry of Earth Sciences, says. “It’s a comprehensive analysis and provides spatial and temporal variations of fog in the IGP”. While the Indian Meteorological Department (IMD) retrieves fog events from observations made by India’s satellite, Rajeevan says, “this INSAT data does not allow detailed analysis and development of climatology. The present results will help understand the physics of fog events and their impact on aviation and agriculture.”

doi: https://doi.org/10.1038/d44151-022-00073-x

References

  1. Singh, M. K. & Gautam, R. Remote Sens. Environ. 279 (2022) doi: 10.1016/j.rse.2022.113128

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