Why Delhi woke up to a clear, blue sky
nNEW DELHI: It took a rare confluence of five factors—strong easterly winds, rain the previous day, low dust and emissions, a high ventilation coefficient, and bright sunshine—for Delhi to wake up on Thursday to something it rarely does: a clean, clear, cerulean sky.
To be precise, the wind speed was 30-40kmph, 13 mm of rain was recorded at the Safdarjang station on Wednesday, the contribution of dust to the air pollution was nil, and the ventilation coefficient was 45,000 metres square per second.
A set of numbers in this ballpark—which on Thursday equalled a blue sky with a mix of interspersed high and low white clouds—coincide only a few times a year in the Capital.
The data above is from the India Meteorological Department and the Air Quality Early Warning System under the Union ministry of earth sciences.
“There was a cyclonic circulation over Madhya Pradesh that brought light rain on Wednesday. There are strong easterlies blowing at 30 to 40km per hour that helped clear the skies. On August 8, we are expecting a change in wind patterns. A low pressure area is developing again in the Bay of Bengal. We are expecting moderate category rain in Delhi on August 10, 11 and 12,” said Kuldeep Shrivastava, head of the regional weather forecasting centre.
IMD in its Thursday bulletin said strong surface winds will continue to blow over north-west India till Friday.
“The low pressure area that had formed over Bay of Bengal on August 4 now lies over south-west Madhya Pradesh. It is very likely to become less marked by August 7. There is strong south westerly/ westerly monsoonal flow over the Arabian sea with wind speed reaching 50-60kmph along and off the west coast. Strong surface winds are likely to prevail over northwestern plains till Friday,” the bulletin said.
According to an explainer by Nasa on why the sky the blue, when sunlight reaches the earth’s atmosphere and is scattered in all directions by all the gases and particles in the air, blue light is scattered more than the other colours because it travels as shorter, smaller waves.
“A combination of clouds and radiation from sunlight determines the colour (or the blueness) of the sky,” said RK Jenamani, senior scientist, IMD.
“We had a clear sky [on Thursday]. There were less clouds, but not the entire day. We had a mix of high and low clouds but no cumulonimbus clouds, which are associated with thunderstorms,” said M Mohapatra, director general, IMD
The combination of factors also cleared the Delhi air. The air quality index (AQI) on Thursday was 69—in the “satisfactory” category—for the second time this week , and also the second time since April 2. Delhi has recorded “satisfactory” air continuously for the past 10 days. The air has was also “satisfactory” for an unusually high number of days since March 25, when the lockdown was imposed, due to a fall in vehicular emissions sparked by reduced traffic flow, according to the Central Pollution Control Board’s (CPCB) data.
“There were 60 satisfactory air days this year till July 31, compared to 15 last year. One ‘good’ air day (AQI less than 50) was also recorded in March,” said VK Shukla, in charge of air quality at CPCB.
“Meteorological factors alone cannot bring such a change in air quality. Vehicular emissions are still relatively lower than before the lockdown period. This, in combination means pollutants are dispersing,” said Sachin Ghude, a scientist at IMD.
A ventilation coefficient of 6,000 is considered favourable for dispersion.