A month of weather fluctuations for Delhi

August has been a month of extremes for Delhi, said weather experts. Data from the India Meteorological Department (IMD) for the 29 days of August so far shows that from having a monthly rain deficit of 60%, the city received 11% surplus rain before returning to an 11% deficit as on Sunday. But so far, the city has received just nine rainy days (rainfall above 0.16mm) this month, lower than usual for August, according to data from the weather office.

IMD data shows the first half of August was largely dry, receiving less than half the normal rain the city usually gets during this time of the year, including a partial break phase that lasted for 10 days.

The break ended with the city landing the wettest day of the season on August 21, receiving a record 138.8mm rainfall in a single day, which was the highest for the month in 14 years and the second highest since 1961. Within three days, however, the wet spell made way for another dry spell from August 25-28.

On Sunday too, the Safdarjung observatory, which is considered representative of the Capital’s weather, did not receive any rain till 5.30pm despite the Met forecasting light, scattered rain.

IMD scientists said that the major reason behind the dry phases was not having an intense monsoon system forming in the Bay of Bengal and the Arabian Sea.

“This year, there was no support in terms of continued easterly winds and moisture from the Bay of Bengal that trigger rain. Last year, we had five such systems forming, which resulted in good rains across the country. This season, we had just two such systems, which were not intense enough to travel up to central and northwest India. This has led to the northwest region experiencing weak monsoon spells,” said RK Jenamani, senior IMD scientist.

Independent weather experts shared a similar view.

“One of the major reason for the weak conditions is that both the frequency and intensity of the weather systems (low-pressure systems) formed in the Bay of Bengal this monsoon has been low. Every year during this season at least four to five such systems are formed, which travel up to central and northwest parts of the country bringing rain. This time only two such weather systems were formed, causing frequent weak phases,” said Mahesh Palawat, vice-president, Skymet, a private weather forecaster.

Until August 20, having received only 63.7mm rainfall, the city stood at a rain deficit of 60%. On August 21, the city recorded 138.8mm rainfall, which pushed the cumulative rainfall count for the month to 211.2mm (on August 22) against the normal count of 190.2 mm, reaching a surplus of 11%.

It was followed by light rain over the following three days, after which the city again ran into a dry spell.

On August 29, the city again stood at a monthly rain deficit, this time of 11%. From August 1-29, the city received 214.5mm rainfall, as against the normal count of 241.9 mm.

IMD scientists say normally August brings the maximum monsoon rain to Delhi with a total monthly count of 247.7mm.

“August may end with a slight deficit, as just two more days are left for it to be over and only light rain is expected over the next couple of days. There has been rain in some parts of Delhi, but it hasn’t been observed at Safdarjung Observatory. The city is likely to get patchy rain between August 30 and September 2,” said a senior IMD official on condition of anonymity.

Scientists also say that the prolonged break phases that the monsoon season saw in the whole of northwest India also contributed to Delhi’s rain imbalance.

The monsoon went into a break phase from August 9-19, then revived on August 20, and again entered a weak phase from August 25-28, the third for this season.

Delhi saw the first break from June 29-July 12, delaying the onset of monsoon, which finally arrived in the national capital on July 13, over two weeks behind schedule.

“Even though August is supposed to be the wettest month of the monsoon season for Delhi, the unusually frequent and prolonged breaks led to its weakening, resulting in hot and dry spells and above-normal temperatures. While humidity is already present in the atmosphere, in the absence of rain, the temperatures tend to rise. The weak monsoon spells, alternatively called partial breaks, usually tend to last for 4-5 days, but the one we saw this month was quite prolonged,” the IMD official said, adding that the season may see two to three small breaks, “but this year two breaks occurred within the same month, which also led to an extended number of dry days.”

The August 21 downpour was too intense for a 24-hour period, he said, adding that such short and intense spells of rain have affected the number of rainy days occurring in a month. “Ideally this amount of rain should be more uniformly distributed over the month. More than 100mm rainfall must happen for a week or 15 days,” the official said.

IMD data shows that over the past 10 years, only August 2007 saw just seven rainy days while the rest of the years had at least more than 10 rainy days in the month.

“There could be a number of factors behind the frequent and prolonged breaks. The warming of the Indian Ocean at a higher rate could be one. Another could be the Madden-Julian Oscillation, or MJO location being unfavourable,” the IMD official added