Blue sky, twice in a blue moon
Experts attributed Friday’s conditions to an overlap of several factors — rain, fewer clouds, sunshine, clean air
nNEW DELHI: Delhi woke up on Friday to a clear blue sky for the second time this month. A rare phenomenon in one of the most polluted cities in the world, experts attributed it to an overlap of factors — rain, fewer and higher clouds , sunshine, clean air, and curiously, the physical properties of the colour blue.
The Capital witnessed a similar phenomenon on August 6, when the sky was unusually blue.
Meteorologists, however, said the cerulean sky could be shortlived, with weather phenomenon expected to soon make the situation unconducive for such sights.
Experts said easterly winds have started blowing and a lowpressure area is lying over eastern Madhya Pradesh and Chhattisgarh, which could lead to overcast conditions over the northwestern parts of the country as well.
The Capital recorded an air quality index (AQI) of 57 on Friday, categorised as ‘satisfactory’.
The lowest AQI this year was 45 on March 28, the first satisfactory air day of the year.
The national capital, has, till August 20, recorded two “good” air quality days this month — the same number of such days it recorded in the whole month last year.
However, the AQI for August was lower than last year, at 66.1 (satisfactory). In 2019, the average AQI for the same period was 73.9 (satisfactory).
“A blue sky can only be seen for a brief period. We must all enjoy the moment, which is pure physics. Radiation has seven colours, and blue gets scattered more because of shorter waves. The blue is more evident than usual because of a lack of cloud cover, sunshine and a clean atmosphere,” said K Sathi Devi, head, National Weather Forecasting Centre (NWFC).
Winds on Friday blew at speeds of around 18 to 20kmph. The ventilation coefficient on Friday was 20,000 m2/s. A coefficient of above 6000 m2/s is favourable for pollutants to disperse, Central Pollution Control Board officials said.
Ventilation coefficient is a product of mixing depth and average wind speed.
The Delhi-based Regional Meteorological Centre’s office doesn’t track blue sky days, but officials said such natural phenomenon was visible in the national capital and north-western parts of the country for the first time in several years.
“We started observing this after lockdown restrictions were imposed. Newspapers also published pictures of the Himalayan peaks being visible from the north-western plains in May following thunderstorms. Heavy rains lashed Delhi this week, washing away pollution particles. Molecules in the atmosphere scatter white sunlight in all directions. Blue light is scattered more because it travels shorter waves. The obstacles to this scattering are clouds, fog, pollutants and smoke. Now, we have high clouds over Delhi, but they will clear up in a few hours. We will again be able to see a blue sky,” said Kuldeep Shrivastava, head, Regional Weather Forecasting Centre (RWFC), Delhi.
In the days leading up to Friday’s clear skies, the Capital saw strong spells of rain. Between
August 19 morning and August 20 evening, the Safdarjung station, taken to be representative of the city, had recorded 109.3mm of rainfall.
On Friday morning, there were reports of waterlogging at Greater Kailash-1, Adhchini, Kalkaji, Srinivaspuri Laxmi Nagar, Wazirabad, and Model Town, among others.
Scientists at the India Meteorological have forecast that light rain activity, thundershowers, or drizzle is likely to continue till August 25.
“The monsoon trough is towards southern Rajasthan at present and after Thursday, the intensity of rainfall in Delhi has decreased. Around August 25-26, the monsoon trough is likely to move towards northwest when a spell of ‘moderate’ rainfall is expected in Delhi again,” Srivastava said.