Himachal Cloudbursts

Context:

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

Relevance:

GS-I: Geography (Physical Geography, Climatology, Important Geophysical phenomena), GS-III: Environment and Ecology (Climate Change and its effects), GS-III: Disaster Management

Dimensions of the Article:

1. What is a Cloudburst?
2. Why do cloudbursts happen only in the mountains and hilly areas?
3. Why does cloudburst cause so many deaths?
4. Heavy Rains in Himachal and Uttarakhand: Meteorological Causes

What is a Cloudburst?

• Cloudbursts are sudden and extreme rainfall events over a limited area in a short span of time. There is no universal definition of a cloudburst.
• The India Meteorological Department (IMD) defines a cloudburst as any event where 100 millimetres of rainfall have fallen in a span of an hour over a region that is 20-30 square kilometres in area. By this definition, 5 cm of rainfall in half an hour would also be classified as a cloudburst.

How do Cloudbursts occur?

• A cloudburst occurs when moisture-carrying air moves up a hilly terrain, forming a vertical column of clouds known as ‘cumulonimbus’ clouds.
• Such clouds usually cause rain, thunder and lightning. This upward motion of the clouds is known as an ‘orographic lift’.
• These unstable clouds cause an intense rainstorm over a small area after becoming heavy enough and locked in the ridges and valleys between the hills.
• The energy necessary for the cloudburst comes from the upward motion of air. Cloudbursts mostly occur at elevations between 1,000-2,500 metres above sea level.
• The moisture is usually provided by a low-pressure system (usually associated with cyclonic storms in the ocean) over the Gangetic plains associated with low level winds flowing in from the east.
• Sometimes winds flowing in from the north west also aid the occurrence of cloudbursts. The many factors that have to come together to make a cloudburst event happen make them highly unlikely.

Why do cloudbursts happen only in the mountains and hilly areas?

• Cloudbursts do happen in plains as well, 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.
• 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.
• Hilly terrains aid in heated air currents rising vertically upwards, thereby, increasing the probability of a cloudburst situation.
• In addition, as pointed out earlier, cloudbursts get counted only when they result in largescale destruction of life and property, which happens mainly in mountainous regions.

Why does cloudburst cause so many deaths?

• 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.
• Landslides, flash floods, houses and establishments getting swept away and cave-ins lead to the deaths.

Is the frequency of cloudbursts increasing?

• 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 due to global warming; it is expected, keeping in mind that trend, that cloudburst events might be on the increase as well.
• Extreme weather events are indeed increasing in the Himalayan region.

Heavy Rains in Himachal and Uttarakhand: Meteorological Causes

The ongoing heavy rains in Himachal Pradesh and Uttarakhand are attributed to the following meteorological factors:

• Monsoon Trough Interaction: The movement of the monsoon trough northward, coupled with a weak western disturbance, is driving the intense rainfall in these regions.
• Monsoon Trough Definition: The monsoon trough is a prolonged low-pressure area extending from a “heat low” over Pakistan to the Bay of Bengal region. It is a recurring element of the monsoon pattern as per the India Meteorological Department.
• Trough’s Current Position: Presently, the monsoon trough lies north of its typical location, situated over the Himalayan foothills.
• Upcoming Shift: The monsoon trough is expected to gradually shift southward temporarily, leading to decreased rainfall in the hills while intensifying rainfall over east-central India.

-Source: Indian Express