Delhi rains: Unraveling the cloudburst conundrum

“The heavy rainfall in Delhi last week was significant but did not qualify as a cloudburst,” said Mrutyunjay Mohapatra, director general, IMD. “It was identified as an extremely intense spell of rainfall.”

IMD categorises rainfall spells based on their intensity. A light spell is defined as rainfall up to 1 cm/hour (10 mm/hour). Moderate spells range from 1-2 cm/hour (10-20 mm/hour). Intense spells are 2-3 cm/hour (20-30 mm/hour), while very intense spells range from 3-5 cm/hour (30-50 mm/hour). Extremely intense spells are defined as 5-10 cm/hour (50-100 mm/hour). A cloudburst is classified as rainfall exceeding 10 cm/hour (100 mm/hour) under the categorisation.

The extreme weather event was influenced by multiple large-scale monsoon weather systems that created conditions for mesoscale convective activity (systems that produce intense thunderstorms and heavy rainfall over a relatively small area, typically influenced by atmospheric instability and moisture) over Delhi-NCR. “Thermodynamic instability in the atmosphere along with other weather factors such as trough, mild western disturbance and weather circulation over the northwestern part of India favoured the development of intense thunderstorms,” said Mohapatra.

Where did the cloudburst analogy come from?

Delhi has a network of 22 weather stations to measure rainfall and temperature. This includes 12 Automatic Weather Stations (AWS), one automatic rain gauge, four part-time observatories, and five synoptic manual observatories​ (Safdurjung, Palam, Lodhi Road, Ridge and Ayanagar).

On the day of the intense rainfall, the AWS at Pragati Maidan displayed 112.5 mm of rain for an hour. “This was an error in the AWS and it was flagged immediately to all stakeholders that this is an incorrect reading,” said an IMD official requesting anonymity.

Rainfall data over three hours released by the IMD painted a clearer picture, with three AWS at Mayur Vihar, Delhi University, and Pusa recording 119 mm, 77.5 mm, and 66.5 mm, respectively. “Since these are three-hourly data, they fall under intense, very intense, and extremely intense spells. Among these, Mayur Vihar recorded 89.5 mm of rain over an hour (extremely intense). Therefore, no station in Delhi NCR recorded above 100 mm of rain over an hour that qualifies as a cloudburst event,” the official added.

Over 24 hours, from 8:30 am on July 31 to 8:30 am on August 1, the weather station at Mayur Vihar recorded 147.5 mm of rain while Najafgarh recorded 113 mm, Lodhi Road 107.5 mm, Safdarjung 107.6 mm, and Delhi University 104.5 mm. All rainfall figures for 24 hours qualified under the heavy rain or very heavy rain categories. The IMD defines heavy rain as 64.5 to 115.5 mm, very heavy rain as 115.6 to 204.4 mm, and extremely heavy rain as more than 204.5 mm.

Mohapatra added that the prolonged heatwave in northwest India in May and June significantly increased the moisture-holding capacity of the atmosphere, which in turn heightened the probability of such heavy rainfall in Delhi. “Despite the intensity of the rainfall, it was neither localised nor intense enough within the required timeframe to be officially classified as a cloudburst,” he said.

Meteorologists address cloudburst misconceptions

Considerable debate among meteorologists brought to the fore whether such incidents qualify or should be classified as cloudbursts. However, all experts stressed the importance of clear communication to avoid public panic and maintain trust in weather warning systems.

“Since the heavy spell in Delhi occurred over a few hours, it wouldn't meet the official criteria,” said independent weather expert Athreya Shetty. He provided a recent example from Surat on July 15, where a massive 247 mm of rainfall was recorded between 8-10 am, meeting the cloudburst criteria. In contrast, Porbandar experienced 486 mm of rain over 24 hours on July 18, but since it was spread out over the day, it did not qualify as a cloudburst. “There is a need for clear communication to avoid misinterpretation. While Delhi's rainfall totalled 160-200 mm in 24 hours, it was classified as very to extremely heavy rain, not a cloudburst. Accurate terminology ensures that such events are properly understood and managed,” he said.

Akshay Deoras, a research scientist at the University of Reading, United Kingdom, referenced the Pragati Maidan incident and said, “The AWS got over 100 mm of rainfall in an hour, so it has been classified as a cloudburst according to the rainfall definition. However, if the IMD does not accept the AWS there as accurate, it raises questions about the reliability of observations in crucial and remote locations. It is crucial to communicate weather forecasts and analysis clearly to avoid creating panic by simply announcing very heavy rain as opposed to a cloudburst.”

In east Delhi, Noida and adjoining areas, Mahesh Palawat, vice president (meteorology and climate change), Skymet said, heavy to very heavy rain, creating a cloudburst-like situation was observed but not a typical cloudburst. “While some areas in Delhi experienced rainfall rates close to a cloudburst, they were not widespread enough to meet the definition.”

Palawat also highlighted the impact of climate change making such intense weather activities more frequent. “The moisture-holding capacity in the air increases with higher temperatures, leading to the formation of cumulonimbus clouds (towering clouds associated with thunderstorms) even during the monsoon season. This is rare during the season as it is a pre-or post-monsoon phenomenon,” he said.

Monsoon fury in Himalayan foothills due to multiple cloudbursts: IMD

The extreme rainfall over parts of Jammu and Kashmir, Himachal Pradesh and Uttarakhand on July 31, 2024, has been a subject of concern, particularly due to the extent of damage due to swelling of rivers and landslides.

In Himachal Pradesh, the Lahaul-Spiti and Mandi districts experienced flash floods, causing significant destruction and loss of life. The IMD issued a red alert, warning of extremely heavy rainfall in the region. Over 24 hours starting July 31, Himachal Pradesh recorded exceptionally high rainfall, with some areas experiencing as much as 27 cm (270 mm) of rain.

“These cloudbursts occurred in four locations: Shimla, Mandi, and twice in Kullu,” said Surinder Paul, director, IMD Shimla. “We assume that each event recorded more than 100 mm of rainfall within an hour.”

He further explained that cloudbursts in hilly zones are highly localised and often go unreported unless there is significant damage, which we usually learn about through media and local reports. “Typical features of a cloudburst include low-hanging, stationary clouds that suddenly release intense rain. These clouds are usually cumulonimbus clouds, which can reach heights of up to 14 kilometres, featuring strong updrafts and downdrafts," he explained. “However, we do not have localised rain data such as AWS to track the exact intensity.”

Similarly, Uttarakhand faced intense rainfall, leading to multiple landslides and road blockages, with various regions like Uttarkashi, Nainital, Dehradun, and Pauri receiving substantial amounts of rain. The Amarnath Yatra faced a tragic disruption due to a cloudburst near the base camp of the holy cave shrine in Jammu and Kashmir on July 31. “Incidents in Uttarakhand or Amarnath were sparked by intense rainfall and impacts associated with landslides (loose top soil) leading to damage, not necessarily cloud bursts,” said Paul.

To improve its forecasting capabilities and better manage such extreme weather events, the IMD plans to install three additional Doppler radars in the Delhi-NCR region within the next two to three years. These radars will complement the existing ones at Palam, Ayanagar, and Mausam Bhavan, enhancing the accuracy of weather forecasts and early warning systems. Also, a flood warning system is being developed for the national capital to mitigate the impact of future heavy rainfall events, said Mohapatra.

As Delhi and all other Indian cities continue to enhance their weather forecasting infrastructure, better preparedness, early warning and proactive strategies are the need of the hour to manage and mitigate the effects of such recurring extreme weather events.