What’s Behind Assam’s Lingering Fog? IMD Points to High-Speed Jet Stream

A thick blanket of dense to very dense fog is set to envelop large parts of Assam over the coming days as a powerful subtropical westerly jet stream continues to dominate atmospheric conditions across Northeast India, the India Meteorological Department (IMD) said on Friday.

According to the Regional Meteorological Centre in Guwahati, very dense fog is very likely to occur at isolated pockets over Assam, with dense fog conditions expected to persist for several consecutive days till the end of December. The fog episodes are expected to be most intense during late night and early morning hours, significantly reducing visibility and potentially disrupting road, rail and air traffic across the state.

IMD observations recorded at 0830 hours IST on December 26 indicate that dry weather prevailed across Assam over the past 24 hours, while night temperatures showed no significant change and remained above normal. However, meteorologists have noted a gradual falling tendency in minimum temperatures, a condition that is likely to further intensify fog formation during the coming days.

The weather pattern is being strongly influenced by a subtropical westerly jet stream with core wind speeds of around 125 knots at an altitude of 12.6 kilometres above mean sea level, currently prevailing over Northeast India.

Meteorological experts from the IMD told DY365 that the presence of this strong upper-air circulation is creating stable atmospheric conditions, suppressing cloud formation and rainfall while allowing cold air to settle near the surface — a key factor behind the persistence of dense fog across the region.

The IMD has issued fog-related warnings for multiple days. Dense fog is very likely to occur at isolated locations over Assam on December 27, while very dense fog conditions are expected on December 28. Dense fog is again forecast to affect parts of the state on December 29, December 30 and December 31. Shallow to moderate fog is also expected to occur intermittently at isolated pockets.

Despite the hazardous visibility conditions, no significant rainfall activity is expected over Assam during the forecast period. Dry weather is likely to prevail across the state, with no major fluctuation in temperatures, though minimum temperatures are expected to show a slow downward trend.

The fog situation is not limited to Assam alone. Dense to very dense fog is also likely to affect parts of Meghalaya, Manipur and Arunachal Pradesh on different days, while higher reaches of Arunachal Pradesh may experience isolated light snowfall. However, Assam remains the most vulnerable due to repeated fog warnings and prolonged dry conditions.

For Guwahati and its surrounding areas, the IMD has forecast fog or mist during morning hours followed by mainly clear skies later in the day. Maximum temperatures in the city are expected to hover around 22 degrees Celsius, while minimum temperatures are likely to range between 12 and 14 degrees Celsius over the next several days.

The IMD said the overall weather warning status for the state remains Green, indicating no severe weather systems other than fog-related conditions. The department added that the prevailing weather pattern is likely to see little change in the subsequent days.

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