Though it rained in Delhi on Monday, weather forecasters would not bet on it a day ago.
The reason — the rain came from convective clouds and it is difficult to provide an accurate prediction about their formation.
“We knew about the possibility of the formation of convective clouds — there were high humidity and high temperatures but it is hard to give a micro-forecast in terms of exact location and intensity. So one cannot be certain about rainfall over a particular place,” said Mahesh Palawat, vice-president (meteorology and climate change), Skymet Weather.
Convective clouds develop vertically by convection — warm air rising and cooling down to form clouds.
These clouds dissipate after giving short and intense spells of rain, the meteorologist said.
Non-convective precipitation events are less intense but last longer and give a steadier rainfall.
Though monsoon is associated with non-convective precipitation, “we have been witnessing convective development in the monsoon period for the last 10 years or so due to climate change”, Palawat said.
Convective activity is generally reported during the pre-monsoon period.
Convective clouds led to rain in north and west Delhi and some parts of neighbouring Noida on Monday, he added.
The India Meteorological Department (IMD) had earlier predicted a “generally cloudy sky” and the “possibility of very light rain or drizzle” in Delhi on Monday.
The Met office updated it to a “generally cloudy sky” on Monday morning and to “very light rain” in the afternoon.
Around 2 pm, it said “thunderstorm with light to moderate intensity rain would occur over and adjoining areas of north, north-west, west, southwest, south, east Delhi and NCR (Hindon AF Station, Indirapuram, Noida, Gurugram, Manesar), Rohtak, Kharkhoda, Charkhi Dadri, Matanhail, Jhajjar, Farukhnagar, Kosali, Rewari (Haryana), Deoband, Shamli, Khatauli, Jalesar (Uttar Pradesh), Bhiwari, Nagar, Deeg, Laxmangarh (Rajasthan) during next two hours”.
An IMD official said the Met office did not issue a colour-coded alert due to the absence of a major weather system such as a western disturbance, a low-pressure area or depression.
“We can be more certain about rainfall when there is a major weather system, which is picked up by our models three to four days in advance,” the official said.
Before the rainfall on Monday, the IMD had issued colour-coded alerts on multiple occasions over the last six days, but Delhi remained completely dry.
On July 4, the Met office issued a “yellow” alert for July 5 and an “orange” alert for July 6, which was later shifted for July 7.
While Delhi kept waiting for rain, experts attributed the dry spell to the shifting of the monsoon trough towards central India due to the development of a low-pressure area over Odisha, which subsequently travelled to Gujarat.
“The low-pressure area had pulled the trough towards central India, leading to heavy rainfall there,” Palawat said.
The IMD later predicted “fairly widespread to widespread rainfall activity” over west Uttar Pradesh, Punjab, Haryana, Chandigarh, Delhi and Rajasthan on July 9 and July 10, and issued a “yellow” alert, warning of moderate rainfall in the capital on Sunday.
But that did not happen either.
“It was expected that the western end of the monsoon trough would again shift towards the north after the low-pressure area degenerated. However, the development of a cyclonic circulation over southwest Rajasthan and adjoining parts of south Pakistan did not let the monsoon trough come near Delhi and neighbouring areas,” Palawat said.