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Cloudbursts in Kullu-Manali India cause havoc...

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Helen Czerski Helen Czerski | 15:00 UK time, Wednesday, 17 August 2011

Distance travelled ~ 588'206'400 km

"Cloudburst" sounds dramatic, doesn't it? As though a cloud was a large balloon filled with water and someone had just arrived with a very large pin. Pop! And then what went up must come down and the one place that you don't want to be is right underneath.

Although the real thing isn't quite like that, the people underneath can be forgiven for not caring about the difference. This week a cloudburst in the town of Kullu produced 176 mm of rain in three hours and drenched three villages. A cloudburst is defined as any rainstorm where the rain rate is greater than 10 cm each hour, and they are usually only a few km wide. Just stop and think about that... 10 cm of rain in one hour. That is enough to turn an entire village into a temporary river bed. And these are large damaging drops, accompanied by strong winds and thunder. Cloudbursts are very hard to predict, but devastating to the places that they hit.

Not long ago the 23 Degrees team were in India to film the monsoon rains

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A cloudburst is a hard thing to study, because it's tough to be in the right place at the right time. So we don't have a good description of exactly how they form or what causes them. They're relatively common during the monsoon in India, especially close to the mountains. Scientists think that these huge amounts of rain come from very tall clouds, reaching up to 15 km into the air, and so the droplets falling from the top of that huge cloud have been able to hoover up lots of smaller drops along the way. That's why the raindrops are so big when they get to the bottom.

The trigger for the storm seems to be that something nearby (for example shape of the local mountains) starts a huge upside down fountain of air that's very strong but only covered a very small area. Warm moist air pours upwards, releasing energy as it rises, and this builds the storm cloud very quickly. Once it gets big enough and if the conditions are right, all the water that had been lifted up comes down. Very quickly. This is similar to the process that generates normal storms, but what makes cloudbursts different is that this whole process happens inside a small area and very powerfully.

The problem for the areas that are vulnerable to these events is that accurate prediction of cloudbursts is hard to do and would require some really detailed weather monitoring. You would need to have a weather station every couple of kilometres all over those areas, and installing and maintaining that system would be very expensive. To understand and predict really local events, you need to have really local measurements.

Hopefully, new technologies and new monitoring systems will solve some of these problems soon. But until then, cloudbursts are going to remain a dramatic reminder of the invisible complexity of the air we breathe, and how little control we have over it.

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