Rain reduces Delhi temperature by over 6°C, but fails to clean city's air

DElhi and surrounding areas received widespread showers on tues, bringing a tart discharge in daytime temperatures in the working capital as the upper limit fell to 16.9 degrees Celsius (°C) – a drop of more than 6 degrees in just a day. The rainfall, however, failed to cleanse the city’s air, which deteriorated into the “very poor” category by evening.Delhi’s base weather station at Safdarjung recorded 4.2mm of rainfall between 8.30am and 5.30pm, while Palam and Ridge areas received heavier amounts of 14.6mm and 14.4mm, respectively. With a total of 25.3mm in rainfall recorded at Safdarjung so far this month, this is the wettest January in the last four years, since 88.2mm was recorded in January 2022, data showed.The India Meteorological Department forecast a return to dry weather from Wednesday, with a gradual shift to northwesterly winds that are expected to bring down nighttime temperatures later in the week. The weather agency has, however, issued a yellow alert for light rain for Monday (February 1).The city’s maximum temperature dropped sharply to 16.9°C, about 5 degrees below normal and 6.3 degrees lower than Monday’s 23.2°C. The minimum temperature, however, rose to 8°C due to overcast conditions.Despite the drop in maximum temperature, a “cold day” was not declared in the city. IMD classifies it as a “cold day” only when the minimum temperature is below 10°C and the departure of maximum temperature from normal is 4.5°C or more. It is a “severe cold day” when the maximum is 6.5°C or more below normal. “This criteria needs to be met over two consecutive days. With the maximum set to rise again, a cold day has not been declared,” said an IMD official.Forecasts show the minimum is likely to hover between 11-13°C on Wednesday, but should dip to around 7°C by Thursday and Friday. By Saturday, it may further dip to around 5°C. The maximum is expected to gradually rise and can touch 21°C by the weekend.Air quality worsens to ‘very poor’Despite the rainfall, Delhi’s 24-hour average Air Quality Index (AQI) worsened from 241 (“poor”) on Monday to 336 (“very poor”) by 4pm on Tuesday, worsening further to 346 by 7pm.This counterintuitive trend puzzled many residents expecting cleaner air after the rain. While it is hard to discount equipment malfunction in the case of data gathered by Central Pollution Control Board (CPCB)’s monitoring stations, detecting which would require poring over hourly data of every station minutely, an HT analysis suggests that the poor air quality before the rains started are partly to blame for the 24-hour average of AQI deteriorating through the day.To be sure, while the day was overcast, the rain received through the day was far from intense, which makes cleaning the finer PM2.5 particles difficult.For this analysis, HT stuck to only 30 stations that gave consistent PM2.5 and PM10 data. Data from these 30 stations shows that the 24-hour average of PM2.5 based sub-index, the prominent pollutant usually (more on this later), rose from 264.3 at midnight on January 27 to 348.4 at 4pm to 357.8 at the hour ending 7pm on January 27, similar to the trend seen in the average using nearly all stations as displayed on the Sameer app. One reason for this is that in the 24 hours leading up to midnight January 27, 15 hours had the hourly index under 300. This decreased to 3 hours of under 300 hourly sub-index in the hour ending 4pm and just one in the 24 hours ending 7pm on January 27. This happened despite the hourly sub-index improving after the rain. Data also shows hourly PM10 sub-index improving much more than PM2.5. This suggests that the low intensity rain cleared the heavier PM10 particles somewhat more than the finer PM2.5 ones.Forecasts, meanwhile, show improvement down to “poor” again on Wednesday.As per the Centre’s Air Quality Early Warning System (EWS) for Delhi, Delhi’s AQI is likely to be in the ‘poor’ range on January 28 and 29. It should deteriorate to ‘very poor’ again on January 30.