Context- In Himachal Pradesh, more than 50 people died in accidents related to heavy rains in 24 hours.
Key Highlights
- A cloudburst is a localized but intense rainstorm.
- Short bursts of very heavy rain in a small geographic area can cause widespread destruction.
- However, not all heavy rains are cloudbursts.
- A cloudburst has a very precise definition.
- Rainfall of 10 cm or more per hour over an area of about 10 km x 10 km is classified as cloudburst.
- According to this definition, even 5 cm of rain in the same area within half an hour would be classified as cloudburst.
How common are cloudbursts?
- Cloudbursts are not uncommon, especially during the monsoon months.
- Most of them occur in the Himalayan states, where the local topology, wind regimes and temperature gradients between the lower and upper atmosphere facilitate the occurrence of such events.
The result of cloudburst
- Due to the nature of the terrain, heavy rains often cause landslides and flash floods that cause widespread destruction downstream.
- Although these events take place in very small areas that often lack precipitation measuring equipment. But the consequences of these events are not limited to small areas.
Forecasting of cloudbursts
- The India Meteorological Department forecasts rainfall, but it does not predict the amount of rain – in fact, no meteorological department does.
- Forecasts can include light, heavy or very heavy rain, but meteorologists do not predict exactly how much rain is likely to fall in any one place.
- Forecasts also apply to a relatively large geographic area, usually a region, state, meteorological subdivision, or, at best, a region. Forecasts for smaller areas become increasingly uncertain.
- In theory, it is not impossible to predict the amount of precipitation even for a very small area.
- This requires a very dense network of weather instruments and computing capabilities that seem impossible with current technology. Therefore, certain clouds cannot be predicted.
Are cloudburst incidents increasing?
- The long-term trend does not indicate that cloudbursts as defined by the IMD are increasing.
- But extreme rainfall events, like other extreme weather events, are increasing – not just in India, but around the world. Although India’s total rainfall has not changed significantly, an increasing proportion of rainfall falls within a short period of time.
- This means that wet weather is very wet, alternating with prolonged dry periods even during the rainy season.
- This pattern due to climate change suggests that cloud events may also be increasing.
What is a landslide and what causes it?
- A landslide is defined as the movement of rock, debris or soil down a slope.
- Landslides are a type of “mass loss”, meaning the downward movement of soil and rock under the direct influence of gravity.
- Landslides occur mainly in mountainous terrains with favorable soil, rock, geology and slope conditions.
- Natural causes that trigger it are heavy rains, earthquakes, melting snow and landslides caused by floods.
- Landslides can also be caused by anthropogenic activities such as excavation, felling of hills and trees, overdevelopment of infrastructure and overgrazing by livestock.
- Landslides caused by rainfall are more common in India.
How prone is India to landslides?
- India is considered to be among the top five countries in the world.
- India reports at least one landslide death per 100 square kilometers per year.
- Variability in rainfall patterns is the biggest cause of landslides in the country and the Himalayas and Western Ghats remain highly vulnerable.
- Excluding snow-covered areas, approximately 12.6% of the country’s geographical land area (0.42 million square kilometers) is prone to landslides.
- Up to 66.5% of landslides originate from the Northwest Himalayas.
- About 18.8% of the northeastern Himalayas and
- About 14.7% of the Western Ghats.
What causes heavy rains in Himachal, Uttarakhand?
- The heavy rains in Himachal Pradesh and Uttarakhand are attributed to the northward movement of the monsoon trough and its interaction with a weak western disturbance.
- A monsoon depression is an extended area of low pressure that extends from the so-called warm area (low pressure over the sea) over Pakistan to the top of the Bay of Bengal region (Odisha, parts of West Bengal and Bangladesh).
- According to the Indian Meteorological Department, it is one of the semi-permanent features of the monsoon cycle.
- Currently, the monsoon trough is north of its normal position and lies in the foothills of the Himalayas.
- Monsoon depression gradually shifts southward resulting in reduced rainfall over hills and increased rainfall over east central India.
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