Cold and fog are a familiar seasonal reality in the subtropical regions of South Asia, including Bangladesh. Winter usually helps many crops, but when the cold goes beyond its normal limits and turns into dense fog and prolonged cold waves, it slowly becomes a silent disaster rather than a blessing. The damage is not limited to a temporary drop in output: it has long-term effects on crops' biological processes, phenology, the spread of disease, the natural behaviour of soil and farmers' socio-economic stability. In coastal areas especially, where much farming depends on sunlight and the normal rhythm of the weather, the impact of fog and severe cold is deeper and more complex.

Several environmental factors usually work together behind dense fog and severe cold. Night-time minimum temperatures drop sharply, daytime sunlight becomes shorter and weaker, and humidity rises abnormally because of the excess of water droplets in the air. Because crops' experience of weather is governed mainly by temperature and light, even small deviations in these two can cause major changes in crop biology. When fog hides the sun day after day and temperatures stay below normal, crops' natural biological rhythm breaks down and the plants' internal physiological processes slow.

Temperature and light are the main drivers of photosynthesis and metabolism in crops. When light and heat fall, the rate of photosynthesis drops significantly and plants fail to produce the energy they need. Leaf growth slows, root spread is limited and the whole plant gradually weakens. If this lasts long, grain and vegetable plants become stunted and the risk of lower yields rises many times over. The normal timetable of the crop life cycle is also disrupted: germination slows, seedbed and seedling growth falls, and flowering and fruit formation are delayed. The effect is not limited to one crop, as delay in one pushes back the timetable of the next, throwing the farmer's whole seasonal plan into disarray.

Another serious effect of cold and fog is on pollination and grain formation. With low light and temperature, pollination cannot proceed normally. This is seen most sharply in wheat, mustard and pulses. When pollination is disrupted, grains remain unfilled and in many cases the number of barren parts rises, significantly reducing total output.

Dense fog and long cold create conditions in which crop diseases spread. High humidity and low temperature are the most favourable conditions for fungi and bacteria to multiply. This is why late blight in potatoes, purple blotch in onion and garlic, and various fungal diseases in seedbeds spread quickly in winter. Farmers have to use extra pesticides and fungicides to control disease, raising production costs and, in the long run, harming the environment and soil.

Boro rice seedbeds are among the stages most sensitive to cold and fog. In long periods of cold without sunlight, seedlings turn pale or reddish, growth almost stops and seedling deaths rise alarmingly. In many cases farmers have to make seedbeds again, a heavy loss of both time and money.

Winter weather also has a deep effect on natural processes in the soil. When soil temperature falls, microbial activity slows, so nutrients are not easily broken down. Roots take up less water and nutrients, and enzyme activity declines. Over time this reduces soil fertility and weakens the overall health of crops.

In these conditions, salt farming in coastal areas is especially vulnerable. In Chattogram and Cox's Bazar, salt farming depends entirely on sunlight, temperature and evaporation, and dense fog disrupts all of these at once. The sun's heat does not reach the fields, humidity rises and evaporation slows. Salt crystallisation stalls and half-dried salt begins to dissolve again, which farmers call "the salt dying". Not only does output fall, but quality suffers too: the grains become small and brittle, the colour turns grey and the market price falls. The farmer's long labour and investment are thrown into unexpected uncertainty.

Salt farming is itself highly weather-dependent and sensitive. First the land is levelled and earthen embankments built around it; it is then divided into small plots, dried in enough sun, and covered with black or blue polythene. At high tide, salty seawater is let into the plots through channels or shallow pumps. Over four to five days of strong sun the water slowly evaporates and salt forms on the polythene. The salt is then collected from the field, drained and handed over to wholesalers. The raw salt is taken to factories, where refinery machines remove dirt and excess moisture to raise it to edible or industrial grade. Throughout this process, the sun's heat, a normal flow of wind and dry weather are crucial. Prolonged fog and severe cold break this cycle and make coastal salt farming ever more uncertain and risky, with deep social effects: when production is disrupted, marginal farmers' incomes fall, covering costs becomes hard, workers' jobs become uncertain and many farmers fall into debt. Fog is therefore not just a weather problem but a long-term risk to coastal livelihoods and the economy.

Weather data from recent years show that the nature of Bangladesh's winter has clearly changed. According to long-term observations by the Bangladesh Meteorological Department, both the persistence of winter fog and the intensity of cold waves have increased since 2015. Especially from December to January, sunless conditions persist day after day in the country's northern, central and coastal areas. Over the past decade an average of 8-12 days of moderate to severe cold waves have been recorded each winter, compared with just 4-6 days in the 1990s. Days of dense fog have also risen significantly over the same period. In some years, sunlight was effectively blocked for 10-15 days in a row in Chattogram and Cox's Bazar, harming salt farming and winter agriculture. Minimum temperatures also show a worrying trend: in the northern region, winter temperatures have fallen below 6-8 degrees Celsius, close to the normal tolerance limit of crops. At the same time, the gap between day and night temperatures has widened, making it harder for plants to adapt. These figures clearly show that fog and cold waves are no longer isolated events but a growing, structural risk for Bangladesh's farming system.

Climate change is making the problem more severe. The winter season is no longer as predictable as before. Cold waves are becoming more intense and fog more persistent, and the gap between day and night temperatures is becoming abnormal. This instability is breaking the natural rhythm of agriculture and multiplying the risks for farmers. The combined impact falls on farmers' incomes and living standards, on the stability of food production, on market price volatility and, above all, on national food security. For agriculture-dependent Bangladesh, this is a clear, long-term warning signal.

All in all, if dense fog and long severe cold become a regular reality, they will be not just a seasonal challenge for agriculture but a structural crisis. Everything from crops' biological processes to sunlight-dependent systems such as salt farming will be affected. Coordinated research, climate-sensitive and smart farming strategies and strong policy support are essential to tackle it. Otherwise, this silent onslaught of fog will gradually weaken the very foundation of agriculture, with effects on the whole of society and the economy.

The writer is a senior agriculturist, columnist and chairman of the DRP Foundation.

Source: Jagonews24. First published in Bengali on The Agro News.