Decoding fog: Why are some regions more prone to it

Tribune Web Desk

Vibha Sharma

Chandigarh, December 28

Putting it simply, fog is a cloud on the ground formed due to a variety of meteorological reasons, including presence of moisture in the atmosphere, low temperatures and still winds. 

It is a common phenomenon in plains of north India at this time of the year, a reason why currently many parts  are experiencing “dense to very dense” fog conditions, a situation that is expected to continue at least till December 31, as per the Indian Meteorological Department.

Many parts of Haryana, Chandigarh, Delhi and Uttar Pradesh are expected to remain enveloped in thick fog till year-end, as per the weather office. 

Several pockets of Uttar Pradesh and Madhya Pradesh have also remained enveloped in thick fog.

Fog—a cloud on the ground

For meteorologists, fog remains one of the most mysterious weather phenomena primarily because of physical processes involved in its formation.

Fog is classified as dense when visibility is between 50 metres and 200 metres, and very dense when it is lesser than 50 metres.

Normally fog develops after the sun sets.

However, there are times when the mighty sun also struggles to pierce its layers.

Generally, fog starts appearing around late October and the conditions for it to develop remain conducive till early February—the period when the sun normally shines bright during the day but temperatures dip after it goes down.

Thereafter what happens is as follows—the sun evaporates water from morning dew, lakes and rivers producing invisible water vapour. When the temperatures drops and air cools quickly, the water vapour turns back into droplets that gather atmospheric dust/particles, creating fog that is visible.

Why some areas are more fog-prone

Essentially, fog is formed when air at or near the earth's surface becomes saturated by cooling, moisture or mixing with another air parcel.

Generally, fog forms in a stable air mass environment.

Larger are the changes in temperature, denser the fog. 

And places that are open and with lesser habitation are more fog-prone, a reason why one needs to be careful while driving on highways at this time of the year.

Since water vapour is essential for the formation of the fog, the other reason is the presence of water bodies like lakes and rivers or open fields where the dew drops settle in the night.  

Exercise caution, stay updated    

Fog also presents a major hazard to transportation safety.

Dense to very dense fog was reported on Thursday in pockets of Punjab, Haryana, Delhi and Uttar Pradesh and also in Jammu and Kashmir, West Rajasthan and West Madhya Pradesh. Many parts like Srinagar, Amritsar and Ambala reported visibility less than 25 meters. In east Uttar Pradesh and east Madhya Pradesh, Lucknow, Varanasi, Bareilly, Gwalior and Rewa reported 25 metres visibility. 

On Wednesday, deaths were also reported in multiple road crashes.

Nine people, including two students, died and about 30 others injured in separate fog-induced road accidents across Uttar Pradesh, according to reports.

Reduced visibility has also hit rail and air services. 

The IMD has issued a dense fog warning for Haryana, Chandigarh and Delhi during the late and early hours till December 31. 

In an advisory, it has asked drivers to use fog lights and travellers to stay updated on schedules of airlines, railways and state transport. “Exercise caution while driving or using any mode of transportation," the IMD said.

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