Until mid-December, Delhi's monthly average air quality remained the cleanest in the past five years. However, the respite was short-lived as the air quality worsened to the 'very poor' category with an average Air Quality Index (AQI) of 379 Monday. As a result, the Capital saw several anti-pollution curbs, including a mandatory transition to 'hybrid' mode for junior classes and a ban on BS III petrol and BS IV diesel four-wheelers, among others, kick in again Monday as the air quality is forecast to worsen to the 'severe' category Tuesday and in the 'very poor' category in the coming days. But what led to the sudden change in the air quality? Here are the top reasons: Low wind speed Extremely calm winds, coupled with low temperatures keep pollutants trapped in the atmosphere. As temperature dips, the inversion height — the layer beyond which pollutants cannot disperse into the upper layer of the atmosphere — is lowered. The concentration of pollutants in the air increases when this happens. Strong winds are needed to disperse the pollutants. According to the Central Pollution Control Board (CPCB), an average wind speed below 10 kmph is unfavourable. On Monday, the wind speed remained below 6 kmph in the morning, and calm winds prevailed throughout till evening, as per the India Meteorological Department (IMD). The Met Office has forecast the wind speed to remain around 6-8 kmph for this week. Extremely calm wind from the Northeasterly direction of 2 to 4 kmph is likely to prevail on Tuesday morning resulting in a 'severe' AQI condition as indicated by the air quality forecast system developed by IITM Pune. So far this month, a wind speed reaching up to 20 kmph was reported leading to several 'moderate' air quality days which is uncommon in December. Cold wave conditions IMD defines cold wave conditions to occur when actual temperatures dip below 4.1 degrees Celsius or when departures are more than 4.4 degrees Celsius in plains, provided the normal minimum is less than or equal to 10 degrees Celsius. Delhi saw cold wave conditions set in early this year compared to the past 23 years, as per IMD officials. The season's first cold wave day was observed on December 11 when the minimum temperature in Delhi dipped to 4.9 degrees Celsius, making it the earliest cold wave day reported (with temperature below 5 degrees Celsius) since 1996. According to the officials, the last time the minimum temperature dipped below 5 degrees Celsius was on December 6, 1987, at 4.1 degrees Celsius. On December 11, 1996, the minimum temperature was 2.3 degrees Celsius. The IMD has forecast cold wave conditions to prevail in Delhi until Wednesday as the mercury is forecast to touch 5 degrees Celsius. Contribution of transport sector and other emissions The transport sector remained the major contributor (16.94 per cent) to the Capital's pollution woes as of December 12, which is the latest available data from the Decision Support System for Air Quality Management in Delhi. Other significant contributors to particulate matter pollution included the industries sector at 8.36 per cent. The contribution remained significant when Stage II of the Graded Response Action Plan remained in effect in the Capital. No significant rainfall On December 9, a drizzle in the city coupled with strong winds and a mainly clear sky cleared the pollutants. However, no significant rainfall was observed this month. The IMD forecast for the next seven days also does not indicate any cloud cover.
