Cloudburst : A cloudburst is a sudden occurrence and severe heavy rain or extreme amount of precipitation of very high intensity in a limited place, sometimes with hail and thunder, which normally lasts no longer than a few minutes.
Colloquially, the term cloudburst may be used to describe any sudden heavy, brief, and usually unforecastable rainfall. Rainfall rate equal to or greater than 100 mm (3.97 inches) per hour is a cloudburst. The associated convective cloud can extend up to a height of 15 km above the ground. During a cloudburst, more than 20 mm of rain may fall in a few minutes.
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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. That’s an anomaly for Indian conditions.
Where do cloudbursts occur?
Cloudbursts can happen in plains, deserts and hilly regions, 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. As the clouds are forced up in low temperature region, 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.
An obstruction may also lead to accentuation of conditions that will lead to cloud burst. Most of the cloudbursts including the recent Himalayan cloudburst took place because of such reason. Cloudbursts occurs when hot water vapour laden winds mix with the cold winds from the Himalayas resulting in sudden condensation. Hilly terrains aid in heated air currents to rise vertically upwards, This happens in two ways- by allowing water laden winds to absorb more moisture from wet Terai and then by allowing water laden winds to rise along the slope of the Himalayas.
Such mechanism of excessive rainfall is not unusual for the Himalayan region, nor such type of flooding is unusual as such an event takes place in one or another region of the Himalayas.
It is not essential that cloudbursts occur only when a cloud clashes with a solid body like a mountain.
One such cloud burst in the Himalayan region occurred when the monsoon winds were rising along the slope of the Himalayas. Driven by westerlies, western disturbances, swept into the north Indian hilly states around mid-June, where a monsoon-related low-pressure system had already moved in from Bay of Bengal. When westerlies encountered the monsoon, they stagnated. The westerlies virtually locked on to the monsoon system, the two systems feeding moisture into each other gave rise to intense interaction. While monsoon currents progress from south to northwest, western disturbances move across north India from west to east. The unusual combination of westerlies along with monsoons led to intense upliftment of moisture-laden which were sucked by the ascending jet streams resulting in a very heavy downpour that caused deadly cloudburst which caused excessive runoff, triggering landslides and flooding in June 2013.
Cloudbursts can happen in deserts due to enhanced convection aided by localized heating of the air that is moist. Barmer floods of 2006 are a case in point which were triggered by unusually heavy rains in August. Banswara floods of 2020 add another example and to top it all the Leh floods of 2010.
Frequency of cloudbursts
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; keeping temperature fluctuations in mind as a trend, that cloudburst events might be on the increase as well.
The difficulty arises out of the fact that they take place over a very small area.
What can be forecasted is the spell of heavy rainfall due to convergence of many meteorological conditions.
Forecasts for a very small area can be done through the use of Doppler radars. Through them, it is possible to forecast the possibility of cloudbursts about six hours in advance, sometimes even 12-14 hours in advance.
Effect of Cloudbursts
Accompanying effect of Cloudbursts on population
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.
- Flash floods:
- Accompanying effect of Cloudbursts on terrain
- Land caving
- houses and establishments getting swept away and cave-ins lead to the deaths.
- Blocking path of rivers that may lead to temporary damming and creation of a reservoir and its consequent collapse
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