Why Leh cloudburst was a freak?

Cloudburst, as the name suggests, is an extreme form of rainfall, sometimes mixed with hail and thunder, which usually lasts no longer than a few minutes but is capable of causing a flood-like condition. It usually occurs when the moisture-laden cloud gets trapped in certain terrains like the Himalayan mountains, said IMD scientists. Failing to move freely, the entire cloud precipitates as torrential rain in a short time. The downpour could be between 200 and 1,000 mm per hour.

Weather scientist Varun Joshi of Guru Gobind Singh Indraprastha University, Delhi, said any rainfall above 120 mm per hour is intense. But when this amount of rain, or more than it, falls in an hour or less, it is called a cloudburst. The size of droplets during a cloudburst are 100 times bigger—four-six mm—than those during the normal rainfall.

While the cloudburst is a usual phenomenon in the Himalaya, the one above Leh was an aberration because it occurred due to an unusual movement of monsoon clouds, explained IMD scientists. The normal path of monsoon in the Himalayan region is southeast to northwest. On August 5, clouds deviated from their usual route, moved from east to west and got trapped in the high ranges of the leeward side of the Himalaya over Ladakh. According to N R Phadtare at Wadia Institute of Himalayan Geology, Dehradun, it was an unprecedented spill of the Indian monsoon deep into the Ladakh and Zanskar ranges of the north-western Himalaya.

Scientists, however, remain silent why the clouds suddenly changed their direction and spilled into the leeward side of the Himalaya.