What is Cloudburst?

What’s in today’s article?

• Why in news?
• What are cloudbursts?
• How common are cloudbursts?
• Consequence of cloudburst
• Forecasting of cloudbursts
• Are cloudburst incidents increasing?
• What is a Landslide and what Causes one?
• How are Landslides Classified and Mapped?
• How Prone is India to Landslides? 
• What’s causing heavy rains in Himachal, Uttarakhand?

 

Why in news?

• Over 50 people have died within 24 hours in incidents related to heavy rains in Himachal Pradesh.
• Chief Minister Sukhvinder Singh Sukhu said that along with landslides, reports of cloudbursts have also emerged in the state.

 

What are Cloudbursts?

• A cloudburst is a localised but intense rainfall activity. Short spells of very heavy rainfall over a small geographical area can cause widespread destruction.
• Not all instances of very heavy rainfall, however, are cloudbursts. A cloudburst has 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 categorized as a cloudburst.

 

How common are cloudbursts?

• Cloudbursts are 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.
• These events are highly localized and they take place in very small areas which are often devoid of rainfall measuring instruments.

 

Consequence of Cloudburst

• Because of the nature of terrain, the heavy rainfall events often trigger landslides and flash floods, causing extensive destruction downstream.
• Although these events take place in very small areas which are often devoid of rainfall measuring instruments. 
• The consequences of these events, however, are not confined to the small areas.

 

Forecasting of Cloudbursts

• The India Meteorological Department forecasts rainfall events well in advance, but it does not predict the 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 predict exactly how much rain is likely to fall at any given place.
• Also, the forecasts are for a relatively large geographical area, usually a region, a state, a meteorological sub-division, or at best a district.
  • The forecasts get more and more uncertain for smaller areas.
• Theoretically, it is not impossible to forecast rainfall over a very small area as well.
  • 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.

 

Are Cloudburst incidents increasing?

• There is no long-term trend that suggests that cloudbursts, as defined by the IMD, are rising.
• However, 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 India has 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.

 

What is a Landslide and what Causes one?

• A landslide is defined as the movement of a mass of rock, debris, or earth down a slope.
• Landslides are a type of "mass wasting," which denotes any down-slope movement of soil and rock under the direct influence of gravity.
• Landslides occur mainly in mountainous terrains where there are conducive conditions of soil, rock, geology and slope. 
• Natural Causes that trigger it include heavy rainfall, earthquakes, snow melting and undercutting of slopes due to flooding. 
• Landslides can also be caused by Anthropogenic Activities such as excavation, cutting of hills and trees, excessive infrastructure development, and overgrazing by cattle.
• In India, rainfall-induced landslide events are more common.

 

How are Landslides Classified and Mapped?

• Landslides are broadly classified based on the – 

• 

Image caption: Classification of landslides

 

How Prone is India to Landslides? 

• India is considered among the top five landslide-prone countries in the world.
• In India, at least one death per 100 sq. km is reported in a year due to a landslide event. 
• Rainfall variability pattern is the single biggest cause for landslides in the country, with the Himalayas and the Western Ghats remaining highly vulnerable.
• Excluding snow covered areas, approximately 12.6 per cent of the country’s geographical land area (0.42 million sq. km) is prone to landslides. 
  • As many as 66.5 per cent of the landslides are reported from the North-western Himalayas, 
  • About 18.8 per cent from the North-eastern Himalayas, and 
  • About 14.7 per cent from the Western Ghats.

 

What’s causing heavy rains in Himachal, Uttarakhand?

• The northward movement of the monsoon trough, and its interaction with a weak western disturbance are responsible for the heavy rains in Himachal Pradesh and Uttarakhand.
  • The monsoon trough is an elongated low-pressure area which extends from a so-called “heat low” (a low pressure over the seas) over Pakistan to the head of the Bay of Bengal region (parts of Odisha, West Bengal, and Bangladesh). 
  • This is one of semi-permanent features of monsoon circulation according to the India Meteorological Department.
• Currently, the monsoon trough is north of its normal position and it is over the Himalayan foothills.
  • The monsoon trough will gradually shift southward now temporarily which will lead to reduction of rainfall over the hills and increase in rainfall over east-central India

 

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Q1) What is a monsoon trough?

The monsoon trough, also known as the Intertropical Convergence Zone (ITCZ), is a significant meteorological feature that plays a crucial role in the development of the monsoon seasons in various parts of the world. It is an area of relatively low pressure that forms near the equator, where the trade winds from both the Northern Hemisphere and the Southern Hemisphere converge.

 

Q2) What is called as heat low?

A heat low, also known as a thermal low, is a type of atmospheric low-pressure system that forms due to the heating of the Earth's surface by the sun. It occurs in regions where the surface temperatures become significantly elevated, leading to the warming of the air directly above the surface. As the warm air rises, it creates a region of lower pressure at the surface, which is characteristic of a low-pressure system.

 

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Source: Heavy rains in Himachal Pradesh: When is high rainfall termed as a ‘cloudburst’? | Indian Express | Hindustan Times | NDTV