Breather for city as rain washes away pollutants

Moderate rain and thunderstorm likely to continue today, forecasts IMD

Security personnel take cover outside Vigyan Bhawan during rain on Monday.

After days of ‘severe’ and ‘very poor’ air quality, Delhi and Noida improved to the ‘moderate’ category and Gurugram to ‘satisfactory’ level on Monday, as rains washed away pollutants, according to the Central Pollution Control Board (CPCB) data.

The national capital received light rain for the second consecutive day on Monday.

Cloud cover

The Safdarjung Observatory recorded a minimum temperature of 11.4 degrees Celsius, four notches above normal and the highest in the last 22 days. The maximum temperature settled at 22.6 degrees Celsius, three notches above normal.

Clouds trap some of the outgoing infrared radiation and radiate it back downward, warming the ground.

The IMD said moderate rain and thunderstorm are likely on Tuesday too.

Earlier in the day, fog lowered visibility to 50 metres at Safdarjung and 150 metres at Palam around 7.30 a.m.

“The Air Quality Index (AQI) is likely to improve marginally and stay in the ‘moderate’ to ‘satisfactory’ category on January 5. The AQI is likely to marginally deteriorate and be in the ‘moderate’ to ‘poor category’ on January 6 and 7,” said a statement by the government-run monitoring agency SAFAR (System of Air Quality and Weather Forecasting and Research).

The last time the national capital’s AQI was in the ‘moderate’ category was on December 14.

“Rain lashed many places over Delhi region leading to a clean environment. Under the influence of an active western disturbance, a wet spell with widespread precipitation is likely to continue on Tuesday and Wednesday,” SAFAR said.

Also, the effect of stubble burning on the city’s air quality remained low on Monday. But apart from pollutants, two meteorological factors strongly impact the air quality.

The “mixing layer height”, which is the height from the ground level up to which pollutants can be dispersed, was “moderate” on Saturday. This led to lesser accumulation of pollutants near the surface.

Also, the speed of surface-level winds was “moderate”, which aided in lesser accumulation of pollutants. Faster surface winds help in the dispersion of pollutants.

The AQI of Delhi was 151 on Monday, down from 354 on Sunday, as per CPCB’s 4 p.m. bulletin. The values for Gurugram and Noida were 65 and 152 respectively.

An AQI between 0 and 50 is considered “good”, 51 and 100 “satisfactory”, 101 and 200 “moderate”, 201 and 300 “poor”, 301 and 400 “very poor”, and 401 and 500 “severe”.