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EXPLAINED: After Flash Flood In Himachal, All You Need To Know About Cloudbursts

A partially washed away house after heavy rain lashed Dharamshala following a suspected  cloudburst incident. (PTI)

A partially washed away house after heavy rain lashed Dharamshala following a suspected cloudburst incident. (PTI)

If you've seen viral videos of water gushing through a Dharamshala locality, you'd want to know how a large amount of rain is deposited quickly over a small area

Terrifying visuals of rainwaters gushing through Himachal Pradesh’s Dharamshala on Monday brought home the devastating potential of flash floods in the hills and provided a reminder of the grim Uttarakhand tragedy of 2013 in which thousands of lives were lost after a cloudburst led to floods. While it is mainly its intensity and spread that defines a cloudburst, there are peculiar weather conditions that can trigger one. Here’s what you need to know.

What Is A Cloudburst?

At a simple level, a cloudburst can be described as very heavy rainfall that suddenly occurs for a brief period over a small area. According to a report by the National Institute of Disaster Management on the 2013 Uttarakhand flash flood, cloudbursts can be accompanied by hail and thunderstorm. While cloudbursts can technically occur anywhere, they are most commonly witnessed in mountainous and desert regions.

What Causes A Cloudburst?

Cloudbursts are linked commonly to thunderstorms and are said to occur when moisture-laden wind moves up the slope of a hill, leading to the formation of “a vertical column of clouds known as ‘cumulonimbus’ clouds”, which are known to bring rain, thunder and lightning. This process of clouds rushing upwards is known as ‘orographic lift’.

The Encyclopedia Britannica says as these moisture-laden winds rush upwards, they “prevent the condensing raindrops from falling to the ground”. That may cause a “large amount of water (to) accumulate at high levels, and if the upward currents are weakened the whole of this water falls at one time”.

So, as opposed to a spell of heavy showers, a cloudburst may seem like causing a very large amount of rainfall to descend all at once. Experts say that it is in effect an event marked by extraordinary precipitation and not a “burst” in the strictest sense.

What Can Be The Impact Of A Cloudburst?

A cloudburst in a hilly region can have a devastating effect as the large mass of water rushing down valleys and mountain gullies can uproot trees and loose debris along with it. On its way down, the water gathers speed and force and can sweep aside structures that lie in its way. Cloudbursts in hills can also trigger landslides while in the plains, it can lead to rapid flooding.

Can Cloudbursts Be Predicted?

According to the Indian Meteorological Department (IMD), if 100mm rainfall is recorded at any weather station within the span of one hour, then the event is labelled as a cloudburst. Weather officials say it is “very difficult to predict cloudbursts due to its very small scale in space and time”.

“To monitor or nowcast (predict an occurrence with a few hours’ lead time) the cloudburst, we need to have dense radar network over the cloudburst prone areas or one needs to have very high-resolution weather forecasting models,” IMD said, adding that while cloudbursts “do occur at plains… mountainous regions are more prone” due to orography.

But experts say that the IMD definition may not be adequate and an article in the Down To Earth magazine said that the Danish Meteorological Institute has defined cloudburst as any event seeing “more than 15mm of precipitation in the course of 30 minutes”.

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