In wintertime, Punjab, Khyber Pakhtun khawan and upper Sindh areas are affected by Fog. As cold air from the north recedes and day temperatures fall sufficiently, warm and humid air comes in from the sea. During this time, the warm and humid air may be cooled sufficiently by the underlying cold surface. This condensation of water vapour into droplets and hence resulted in the formation of fog.

According to the Met Office, due to the falling temperatures and high level of humidity, fog is likely to thicken in the plains of the country in the days to come and there are no chances of rain anywhere in the country for at least one week.

Scientists said the foggy weather was a result of calm winds. There have been alternate warm and cooler periods over north-west India. During the warm period, moist winds from the south enter the region feeding humidity. This causes conditions suitable for a weather phenomenon called inversion. Normally, the temperature decreases by 6.5 degrees for every kilometre one ascends. But in inversion, temperatures either remain the same for a few metres above the ground or increases. This traps pollutants and dust in the atmosphere, leading to foggy weather.

Motorway was closed due to severe fog, to all kinds of traffic from Peshawar to Rashkai and from Niaz Baig to Pindi Bhattian and Sheikhupura as fog reduced visibility to nil at 7:30am. It was opened at around 9:30am only after an improvement in the visibility.

The Met Office reported fog in Lahore, Multan, Jhelum, Sialkot, and Gujranwala and even in few areas of Sukkur during the early hours of Friday. The best time for driving on motorway is 10am to 6pm suggested by motorway police. They also suggested that people might contact 130 for helpline to get information about fog on motorways.

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. Fog may form either by cooling air to its dew point, or by adding moisture to air near the ground. Fog is classified by the way it forms. Formation may involve more than one process.

Fog forms when the difference between temperature and dew point is generally less than 2.5°C or 4°F. Fog can form suddenly, and can dissipate just as rapidly, depending what side of the dew point the temperature is on.

A shallow layer of fog on the ground reduces visibility more in the horizontal than in the vertical. Its fog reduces horizontal visibility to 1/4 mile or less. Dense fog usually creates travelling problems and delays.

Fog effects on driving and flying: Fog has a great effect on traffic. Fog may be synonymous with danger and moral blindness for drivers and pilots. The first and foremost thing that is required for a driving and flying is good visibility. Heavy fog considered to be a big hurdle for driving and for flying too. Even if a flight is taken during foggy conditions, its landing becomes almost impossible from a safety point of view. In such cases, there is no option other than diverting the flying machine to a base where there is minimum or no fog.

Most people have heard of the horrendous chain reaction accidents which occur in fog. It is hardly surprising, given the effects of fog on perception. Fog obviously produces accidents because a driver cannot see as far ahead. However, foggy conditions also promote accidents because they affect perceptual judgments of speed and distance. The effects are the result of reduced contrast. We see objects, not based on their absolute brightness or darkness, but on their difference between the object brightness and background. Fog lowers contrast substantially, causing objects to become fainter and less distinct.

Types of Fog

Ground Fog also called as Mist - Water droplets so small that they are floating in the air. Because mist droplets do not fall, mist is a type of fog. This type of fog is prevalent in coastal areas where moisture is abundant. This fog obscures less than 60% of the sky and does not extend to the base of any overhead clouds.
 
Radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. It is more common in autumn and early winter. It is present almost everywhere.
 
Wind fog occurs when moist air passes over a cool surface by wind (advection) and is cooled. It is more common near sea area.
 
Valley fog or Tule fog forms in mountain valleys, often during winter. It is the result of a temperature inversion caused by heavier cold air settling into a valley, with warmer air passing over the mountains above.
 
Upslope fog or hill fog forms when winds blow air up a slope, adiabatically cooling it as it rises, and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.
 
Artificial fog is artificially generated fog that is usually created by vaporizing water and aerosols or glycol-based or glycerin-based fluid. 
 
Smog (combination of smoke and fog) - It’s visible air pollution in urban areas. It looks like dirty fog in large cities like Lahore, Gujranwala, Multan, Jhelum, Sialkot, and sukkar etc.
 
Visibility in smog and mist vary. Smog reduces visibility to less than 1 km (5/8 statute mile), whereas mist reduces visibility to no less than 1 km (5/8 statute mile).