• Following heavy rains in Himachal Pradesh, landslides have occurred in the statewith at least 22 people dead. Of those who have died, nine people got buried underneath the rubble of a collapsed temple and some houses in Shimla. Uttarakhand has also witnessed high rainfall.

What is a cloudburst?

• A cloudburst is a localisedbut intense rainfall activity. While it can also occur in plains, the phenomenon is most common in hilly regions.
• Not all instances of very heavy rainfall, however, are cloudbursts. A cloudbursthas a very specific definition: Rainfall of 10 cm or more in an hour over a roughly 10 km x 10 km area is classified as a cloudburst event. By this definition, 5 cm of rainfall in a half-hour period over the same area would also be categorised as a cloudburst.
• To put this in perspective, in a normal year, India, as a whole, receives about 116 cm of rainfall over the entire year.This means if the entire rainfall everywhere in India during a year was spread evenly over its area, the total accumulated water would be 116 cm high. There are, of course, huge geographical variations in rainfall within the country, and some areas receive over 10 times more than that amount in a year. But on average, any place in India can be expected to receive about 116 cm of rain in a year.
• During a cloudburst event, a place receives about 10% of this annual rainfallwithin an hour. It is a worse situation than what Mumbai had experienced on 26 July 2005, which is one of the most extreme instances of rainfall in India in recent years. At that time, Mumbai had received 94 cm of rain over a 24-hour period, resulting in the deaths of over 400 people and more than USD 1 billion in economic losses.

How common are cloudbursts?

• Cloudburstsare not uncommon events, particularly during the monsoon months. Most of these happen in the Himalayan states where the local topology, wind systems, and temperature gradients between the lower and upper atmosphere facilitate the occurrence of such events.
• However, not every event that is described as a cloudburstis actually, by definition, a cloudburst. That is because these events are highly localized. They take place in very small areas which are often devoid of rainfall-measuring instruments. The consequences of these events, however, are not confined to small areas.
• Because of the nature of the terrain, the heavy rainfall events often trigger landslidesand flash floods, causing extensive destruction downstream. This is the reason why every sudden downpour that leads to the destruction of life and property in the hilly areas gets described as a “cloudburst”, irrespective of whether the amount of rainfall meets the defining criteria. At the same time, it is also possible that actual cloudburst events in remote locations aren’t recorded.
• Can cloudbursts be forecast?
• The IMD forecasts rainfall events well in advance, but it does not predictthe quantum of rainfall — in fact, no meteorological agency does. The forecasts can be about light, heavy, or very heavy rainfall, but weather scientists do not have the capability to predict exactly how much rain is likely to fall at any given place.
• Additionally, the forecasts are for a relatively large geographical area, at best at a district level. As they zoom in over smaller areas, the forecasts get more and more uncertain. Theoretically, it is not impossible to forecast rainfall over a very small area as well, but it requires a very dense network of weather instruments, and computing capabilities that seem unfeasible with current technologies.
• As a result, specific cloudburst events cannot be forecast. But there are warnings for heavy to very heavy rainfall events, and these are routinely forecast four to five days in advance. The possibility of extremely heavy rainfall, which could result in cloudburst-like situations, is forecast six to 12 hoursin advance.

Are cloudburst incidents increasing lately?

• There is no long-term trendthat suggests that cloudbursts, as defined by the IMD, are rising. What is well established, however, is that incidents of extreme rainfall, as also other extreme weather events, are increasing — not just in India but across the world.
• While the overall amount of rainfall in Indiahas not changed substantially, an increasing proportion of rainfall is happening in a short span of time. That means that the wet spells are very wet, and are interspersed with prolonged dry spells even in the rainy season. This kind of pattern, attributed to climate change, does suggest that cloudburst events might also be on the rise.