- A cloudburst is short-term extreme precipitation that takes place over a small area; it is not, as is sometimes understood, the breaking open of a cloud resulting in the release of huge amounts of water. Cloudbursts have a very specific definition: if rainfall of about 10 cm or above per hour is recorded over a place that is roughly 10 km x 10 km in area, it is classified as a cloudburst event. And by this definition, 5 cm of rainfall in half an hour would also be classified as a cloudburst. To put this in perspective, India, in a normal year, gets about 116 cm of rain in the entire year: it means that every area in the country, on an average, should expect to get only this amount during the course of the year. A cloudburst would therefore account for 10-12 per cent of the annual rainfall of that area in just an hour!
So how much did it rain in Uttarakhand last week?
- The exact amount is not known. Rainfall is measured by the ground stations that are sparse in the Himalayan areas. But more importantly, cloudbursts are very localised events; it is possible that they may not get recorded even by a good network of ground stations. The rainfall has to occur where the instrument is located for it to get recorded. It is for this reason that the term ‘cloudburst’ is very often used loosely for any sudden heavy downpour; in fact, most of the time, the Met department is never in a position to state if a particular rainfall event, particularly in remote areas, qualifies to be called a cloudburst. Generally, every time short-duration heavy rainfall leads to damage to life and property, it gets described as a cloudburst.
Why do cloudbursts happen only in the mountains and hilly areas?
- Cloudbursts do happen in plains as well, but there is a greater probability of them occurring in mountainous zones; it has to do with the terrain. Cloudbursts happen when saturated clouds are unable to produce rain because of the upward movement of very warm current of air. Raindrops, instead of dropping down, are carried upwards by the air current. New drops are formed and existing raindrops gain in size. After a point, the raindrops become too heavy for the cloud to hold on to, and they drop down together in a quick flash. Hilly terrains aid in heated air currents rising vertically upwards, thereby, increasing the probability of a cloudburst situation. In addition, as pointed out earlier, cloudbursts get counted only when they result in largescale destruction of life and property, which happens mainly in mountainous regions.
Why does cloudburst, which is only heavy rainfall, cause so many deaths?
- The rainfall itself does not result in the death of people, though sometimes, the raindrops are big enough to hurt people in a sustained downpour. It is the consequences of such heavy rain, especially in the hilly terrain, that causes death and destruction. Landslides, flash floods, houses and establishments getting swept away and cave-ins lead to the deaths.
Were the Mumbai rains of 2005, Jammu and Kashmir rains of 2014, Uttarakhand rains of 2013 and Chennai rains of 2015 all instances of cloudbursts?
- There could have been individual or multiple cloudburst events in each of these instances. The Uttarakhand disaster of 2013, for example, had several cloudburst events, two of them big ones. In each of the other instances, rainfall lasted for a few days and was spread over at least a large city. A typical cloudburst event is restricted to just a few hours, many times less than an hour, and is limited to a much smaller area.
Can cloudbursts be forecast?
- They are difficult to forecast but not impossible; the difficulty arises out of the fact that they take place over a very small area. Forecasts for a very small area are difficult to predict. But through the use of doppler radars, it is possible to forecast the possibility of cloudbursts about six hours in advance, sometimes even 12-14 hours in advance. Unfortunately, there are no doppler radars installed in Himalayan states such as Uttarakhand and Himachal Pradesh.
Is the frequency of cloudbursts increasing?
- There is a paucity of past data on cloudbursts; in addition, since only some of them get counted – only those that result in death and destruction – there is a problem of accuracy as well. But what is very clear is that events of extreme precipitation have been on the rise in the last few decades due to global warming; it is expected, keeping in mind that trend, that cloudburst events might be on the increase as well.