Weather trend: Mumbai witnesses warmer mornings and cooler days this December
MUMBAI: This past month, Mumbai has witnessed its warmest December mornings in over a decade, with the average daily minimum temperature touching 20.18 degree Celsius as compared to 19.77 degree Celsius in 2020. This is the first time in over a decade that the average minimum reading for December has crossed the 20 degree Celsius threshold. The lowest minimum recorded this month was 17.4 degree Celsius on December 29 while last year, the lowest minimum temperature was 15 degree Celsius on the same date.
D S Pai who heads the climate research and services division at the India Meteorological Department (IMD) in Pune said, “So it is clear that the nights and early mornings have been warmer than the last few years, but just because the minimum temperature is showing an increase, we cannot say that December as a whole was warmer. The data shows that during daytime, the conditions were actually cooler than usual.” The average daily maximum temperature recorded in Mumbai this year was 31.1 degree Celsius, the lowest since 2017.
For comparison, the average maximum temperature in 2020 and 2019 stood at 32.67 and 32.68 degree Celsius, respectively. The highest maximum recorded this December was 35 degree Celsius on December 13, as compared to a seasonal high of 36.4 degree Celsius last year. One of the reasons for this, according to Pai, could be the presence of a greater amount of moisture in Mumbai’s air this December due to the prevalence of rain-bearing easterly winds.
Earlier this month, the city recorded 91mm of rain on December 1, marking the month’s wettest day on record. This was due to a vigorous northeast monsoon in India, combined with a low pressure system over the southeast and east-central Arabian Sea at the time. “As a result, we’ve had more cloudy days across parts of the Konkan and even interior Maharashtra. This moisture helps to cool during the day, but at night it acts like a greenhouse gas and traps terrestrial radiation, pushing up the minimum temperature,” Pai said.
Akshay Deoras, an independent meteorologist and PHD researcher at the University of Reading, UK, agreed with this broad argument but cautioned that the trend of warmer mornings and cooler days cannot be attributed to a single factor alone. “The appropriate meteorological factor would have shown up had this difference in averages been very large, such as three to four degree Celsius or more, in which case you’d expect major changes in the wind direction and some sort of linkage to other phenomena,” said Deoras, emphasising that a linear relationship between moisture and temperature is hard to imagine in this case since other factors are also at play.
However, Deoras agreed that an ‘hyperactive northeast monsoon’ likely played an important role in the incursion of moisture over the region this December. “Relative humidity over 22 days of December remained higher than the normal values at about 5,000 feet off the ground. But there’s a major difference between December 1 to 11 and December 12 to 22 with the values sharply going down during the latter part as the influence of the northeast monsoon ended,” Deoras said.
Deoras offered another explanation, “If you get winds from the north (not strictly north but from the northern states), the minimum temperature reduces due to the transport of cold air. This month, such instances are not being seen that frequently,” he said. IMD data in fact shows that there have been only three days this month that Mumbai received strong winds blowing directly from the north, between December 27 and 29, which were responsible for the dip in mercury levels on Wednesday morning. “Northerly winds have not been felt that strongly this time. In fact, above average minimum temperature is seen not just in the city but also in the adjoining areas of Konkan and central parts,” Deoras explained.
Average daily maximum and minimum December temperatures, Mumbai