Cloudbursts

We have often heard about cloudburst occurrences in states like Himachal Pradesh and Uttarakhand. It causes the widespread destruction of both human and material resources. What is the cause of cloudbursts? How is it different from rainfall? Read to know more.

Natural disasters are associated with the negative impacts of a natural hazard.

Short spells of very heavy rainfall over a small geographical area can cause widespread destruction, especially in hilly regions.

Not all instances of very heavy rainfall, however, are cloudbursts. To understand the difference let us dive deep into the topic.

What are cloudbursts?

A cloudburst is a localized but intense rainfall activity. It 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 can also be categorized as a cloudburst. From this, it is clear that cloudbursts are different from rainfall.

Atmospheric changes during the occurrence of a cloudburst

• Cloudbursts happen when drenched clouds are not able to cause rain because of the rising movement of the extremely hot current of air. Rather than falling down, raindrops get larger in dimension and are forced up because of the air current.
• The relative humidity and cloud cover is at the maximum level with low temperature and slow winds because of which a huge mass of clouds may get condensed at a very rapid rate and result in a cloudburst.
• As temperatures increase, the atmosphere can hold more and more moisture. This moisture comes down as a short, very intense rainfall, probably in half an hour or one hour.

How common are they?

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. Some of the recent instances of cloudbursts are:

• On December 2nd, 2015, Chennai recorded 494 mm rainfall causing the 2015 South India floods.
• On May 4th, 2018, a cloudburst occurred above Belagavi, Karnataka. It reported 95mm of rain in an hour.
• On October 20th, 2021, it occurred in the Salem district of Tamil Nadu. It recorded 213 mm of rain in one day. The Vasishta river flooded due to this.

Consequences of a heavy downpour

• It can result in flash floods in mountainous areas and urban floods in the cities.
• Can lead to mudflows, landslides and land caving.
• Loss of lives of humans and animals.
• Loss of buildings and property damage.
• Loss of forests and wild flora and fauna.

Can we forecast sudden rains?

The India Meteorological Department (IMD) forecasts rainfall events well in advance, but it does not predict the quantum of rainfall. The forecasts can be about light, heavy, or very heavy rainfall. But weather scientists do not have the capability to predict exactly how much rain is likely to fall at any given place.

Additionally, the forecasts are for a relatively large geographical area, usually a region, a state, a meteorological sub-division, or at best a district. As they zoom in over smaller areas, the forecasts get more and more uncertain. Theoretically, it is not impossible to forecast rainfall over a very small area as well, but 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. No forecast ever mentions a possibility of a cloudburst. But there are warnings for heavy to very heavy rainfall events, and these are routinely forecast four to five days in advance. The possibility of extremely heavy rainfall is forecast six to 12 hours in advance.

Cloudbursts and climate change: Are cloudburst events increasing?

There is no definitive evidence that suggests that cloudbursts are increasing due to climate change. However, as we have been witnessing, the intensity of heavy rainfall occurrences in a short spell of time has been increasing. 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.

Article written by: Caroline Abraham