With winter approaching, shallow fog and mist are expected to engulf Delhi almost every morning over the next one week at least, the regional meteorological department has forecast.
“We are expecting some shallow fog on Saturday and Sunday. For rest of the days, there could be some mist in the morning,” said Kuldeep Srivasatava, scientist with the regional weather forecasting centre in New Delhi.
The only difference between fog and mist is that during fog the visibility is reduced to less than one kilometre and during mist it is more than one kilometre.Fog and mist are formed when water vapour in the air condenses. During condensation, molecules of water vapour combine to make tiny liquid water droplets that hang in the air.
This means Delhiites would have to start early as there could be traffic snarls due to low visibility.
Data available with the IMD says that on an average Delhi gets around 50 days of fog every year. While in November, the city gets around seven such days the number goes up in December and January. In January, Delhi gets around 17 days of fog.
“Several factors such as a drop in the night temperature, high moisture content in the air and low wind speed are helping in the formation of fog and mist almost every morning,” he said.
The night temperature has dropped from around 22 degrees Celsius on October 11 to around 17 degrees Celsius on Thursday. The wind speed is hovering around 1.5 metres per second, way below the required speed of 5 metres per second to disperse the water droplets and pollutants suspended in the air. The moisture content is also building up because of easterly winds.
But low wind speed and high moisture content in the air are triggering another problem for Delhi. Pollution levels have remained consistently high. The Air Quality Index of Delhi was 311, which means that Delhiites are still inhaling very poor quality air.
“While high moisture content increases that air’s capacity to hold o to pollutants the low wind speed is failing to disperse the pollutants that are trapped in the city’s air. We would need some strong dry winds from the north west to flush out the pollutants,” said D Saha, head of the air quality laboratory at Central Pollution Control Board.
The presence of dust and pollutants also help in the formation of fog. Water vapour condenses around these microscopic solid particles triggering fog.
SAFAR which is maintained by the union ministry of earth sciences, has forecast that levels of PM2.5 and PM10 – the primary pollutants of Delhi’s air - could rise in the next three days.