With an increase in heavy rainfall of short durations during the summer monsoon in many places in India, scientists at Pune’s Indian Institute of Tropical Meteorology (IITM) have coined a new term to define such events — mini-cloud bursts. A mini-cloud burst is defined as rainfall in excess of 50 mm in two consecutive hours.
Based on hourly rainfall data from 126 stations between 1969 and 2015, the researchers found an average of 200 mini-cloud bursts occurring every year in India. Between 1988 and 2007, there were around 265 mini-cloud bursts.
The India Meteorological Department already recognises cloud bursts — heavy rainfall, irrespective of the amount — in the mountainous regions of Himalayas with its steep slopes, which lead to flash floods, and rainfall over 100 mm per hour in other places. In contrast, mini-cloud bursts are indicative of torrential downpour but of lower intensity than cloud bursts.
Currently, on a 24-hour basis, the IMD classifies rainfall as heavy (over 60.5 mm), very heavy (over 130 mm) and extremely heavy (over 200 mm). In a paper published in the International Journal of Climatology , Nayana Deshpande and others from IITM explain the rationale for classifying rainfall of over 50 mm in two hours as a mini-cloud burst. While extreme rainfall of over 200 mm translates to only 16 mm of rainfall per hour, the intensity of rainfall is far more in the case of mini-cloud bursts, they say. Also, compared with extreme rainfall, the rate of water accumulation exceeding absorption and the probability of flash floods are three times more in the case of mini-cloud bursts.
Over most parts of India, the highest recorded rainfall in two hours is 100-150 mm, and the locations (other than the mountainous regions of Himalayas) that have recorded rainfall of over 150 mm in two hours are those that also experience cloud bursts. So in these locations, the mechanism responsible for heavy rainfall “persists for more than an hour”.
The study found that mini-cloud bursts are “very common” in the foothills of the Himalayas. While the west coast records more than three mini-cloud bursts per season, the Indo-Gangetic plains and the Saurashtra region receive two per season. At just one per season, Rajasthan and States to the east of the Western Ghats experience the least number of mini-cloud bursts. For the rest of India, the amount of rainfall per mini-cloud burst is 50-70 mm.