hick fog, covering vast areas and lingering for several days, is one of the main meteorological risks during the winter months over South-Asian plains, which include northern and eastern India, most of eastern Pakistan, Bangladesh and the southern plains of Nepal. Fog impairs visibility, which impacts human health and disrupts all kinds of transportation. The overall economic damage caused by fog events can be compared to that of tornadoes, tropical cyclones, and in certain situations, floods.

For example, during the fog episode of about 10 days in January 2013, Islamabad Airport experienced 115 national and international flight cancellations with diversion and delays of hundreds of flights. Air pollution, as a mixture of low-grade diesel fumes, industrial emissions, smoke from brick kilns and vehicles and the burning of crop residue and general waste, mixes with the fog at colder winter temperatures coalesce into stagnant clouds of smog. In major cities of Pakistan, levels of pollutants of particulate matter of size 2.5 micrometre (PM2.5) often exceed dozens of times above the danger thresholds set by the World Health Organisation.

The winter mean frequency of fog is high over a large area from Pakistan to Bangladesh across northern India from west to east, running almost parallel to the south of the Himalayas, roughly 3,000 kilometres in the east-west and 500 kilometres north-south extension. The satellite data analysis shows that the maximum fog-affected area goes beyond 867,000 sq km in the winter season. Fog events are sometimes observed to develop simultaneously over vast regions of the Indo-Gangetic plain, easily detected in satellite morning visible imagery as large-scale white patches that persist up to mid-day. The most favourable area for fog formation normally lies over western parts of the Indo-Gangetic plain, covering Delhi, Lahore, Sialkot and Faisalabad. Sometimes it extends to southern parts of the Punjab up to Bahawalpur.

In Pakistan and northern India, winter precipitation primarily results from eastward propagating low-pressure systems from the Mediterranean region. Such systems are usually called western disturbances. Precipitation is enhanced in the northern region of Pakistan due to blocking and upslope conditions imposed by the high mountain ranges of this region. Western disturbances also cause precipitation over the northern plains of Pakistan and India. The rain inducts a large amount of water into the soil and vapours into the lower atmosphere. As soon as the low pressure passes, the leading edge of a high-pressure system typically imposes clear and stable atmospheric conditions, leading to the formation of strong surface-based inversions. This phenomenon enhances relative humidity close to the surface and facilitates fog formation. The high frequency of fog results from the availability of moisture near the surface, coming either from moving low-pressure systems or vast irrigated agricultural fields, where abundant vegetation is sustained by irrigation. Low temperatures and calm wind conditions over the region facilitate the persistence of fog for longer periods.

Most of the fog formation over South Asia is due to radiative cooling. Radiation fog usually forms near the surface under clear skies in stagnant air in association with high pressure. The main mechanism is radiative cooling. Radiation fog is more likely to occur when wind speed is low, relative humidity is high and surface temperature is low. During such conditions, mixing of air with the free atmosphere is limited. Such conditions ultimately result in poor visibility and high levels of pollutants in urban areas since pollutants cannot be dispersed.

The effect of urbanisation and human activity on fog formation is very complex. Urban areas may experience more winter fog compared to adjacent rural areas. More persistent fogs in urban areas are attributed to air pollution effects, due to a large number of additional nuclei in the air, expected to produce increased numbers of small fog droplets. A polluted cloud is capable of causing higher obscurity than natural clouds. Also, polluted air helps form fog droplets.

However an increase in urban building density is connected with a reduction in the average number of fog days, which is interpreted as an effect of the urban heat island and moisture deficit. Due to the thin and broken fog cover over the urban environment, this radiative heating of the surface after sunrise may also lead to fog dissolution. Soil conditions also play an important role in radiation fog formation. Clay and silt loams, common soils found in the plains, have the ability to hold more moisture than sandy soils, because they are more porous and have a lower thermal conductivity, which allows radiative cooling to dominate and induce formation of surface inversions and fog.

Almost all cities and meteorological stations which experience fog display significant positive trends in the fog frequency, which generates a broad band of increasing fog occurrence along the foothills of the Himalayas. Most of the stations show an increasing trend of more than eight percent per decade in the mean frequency of fog occurrence during winter, indicating that fog frequency has increased more than three times over the last 30 years.

Many studies over different regions of the world have reported a decrease in fog occurrence frequency, particularly in recent decades, for instance, in Germany and the US. In China, a decrease in fog frequency over Anhui province is reported and is linked to the effect of the increasing urban heat island. On the other hand, there is a significant increase in the fog frequency over eastern-central China which is linked to the weakening of the East Asian monsoon circulation due to increased urban aerosol loading. Favourable local factors like moisture availability, radiative cooling, stability and favourable large-scale meteorology seem to act in such a way as to increase fog occurrence over Pakistan and northern India in contrast to some regions of China and elsewhere.

Meteorologically, fog is reported in two categories: sky seen (less dense fog) and sky not seen (dense fog). About 25 years ago, the climatology of the number of total days of fog in the winter season was less than five days in Lahore. The situation was similar in the other cities of the Punjab. During the last two decades, this frequency has increased 3 to 4 times to about 15 to 20 days. The maximum fog frequency was observed in 2015 over Pakistan when fog was observed in Bahawalpur for more than 70 days in the winter season.

The increase in fog frequency over South Asia has not occurred gradually. There was a sudden shift in the climate regime in 1998. Being a multidisciplinary and multi-scale problem, sudden shifts in the fog regimes may result from a combination of changes in local factors, like land use (e.g. cropping patterns, irrigation) and the role of aerosols etc, and a gradual increase in the humidity due to increasing global temperatures and changes in the large scale circulation.

Although global changes in surface relative humidity are generally small, it may increase substantially on regional scales. The strong correlation between near-surface specific humidity and temperature on both inter-annual and longer time scales (including the trends) suggests that the increasing trends in global humidity will continue as global temperature rises. However, increasing temperatures may also lead to a reduction in the night-time cooling, which decreases radiation fog formation. Global warming leads to a drop in night cooling, but humidity has a much larger impact than the cooling rate. Therefore, increasing humidity near the ground is more likely to create favourable conditions for fog formation and persistence compared to the opposing effect of increasing temperature.

It is interesting to see that the winter precipitation from western disturbances over Pakistan and northern India does not show a significant trend, and the inter-annual variability of precipitation is also not correlated with the fog variability. The increased fog over South Asia seems to be related to climate change. It is likely that this increased fog frequency regime will continue in the coming years.

Pakistan Meteorological Department conducted cloud seeding experiments on warm clouds in Pakistan during the summer of 2000. The activity emerged as a consequence of history’s worst drought in provinces in the southern half of the country in 1999-2000. The experiments showed some success in bringing rain over some parts of Sindh and Baluchistan province.

Recently, the Punjab government has conducted cloud seeding experiments with the help of UAE for artificial rain to combat the smog problem in Lahore city. Teams from the UAE used two planes and fired 48 flares to trigger rain. The success of such experiments is difficult to quantify. Only a trace of rainfall was recorded at some locations out of a total of 16 locations where rain gauges of the Flood Forecasting Division of PMD are present at different locations in Lahore. Weather forecasting models had predicted light rain or drizzle over the region because of the approaching weather system.

Even if localised efforts successfully dissipate fog in a specific area, the movement of fog from surrounding regions can rapidly reintroduce low visibility. The replenishment of fog from surrounding regions can quickly undo these efforts.

The writer is a director at the Pakistan Meteorological Department. He has a PhD from Stockholm University, Sweden, on the subject of climate variability over South Asia. This article is based on his research paper published in an international journal with more than 100 citations.