What Is a Cloudburst? Causes and Safety Tips

Every monsoon there is news that "a cloud burst" somewhere in the Himalaya, and that homes, roads and bridges were swept away in a sudden flood. It sounds as if a cloud popped like a balloon, but that is not really what happens. Here, in simple words, is what a cloudburst is, why it happens and how to stay safe.

The definition of a cloudburst

According to the India Meteorological Department (IMD), when 100 millimetres of rain or more falls within an hour over a small area (about 20–30 square kilometres), it is called a "cloudburst". For comparison, Delhi's normal rainfall for the whole of July is around 200 millimetres — so a cloudburst can drop in a few hours as much water as normally falls over several weeks.

Why does a cloudburst happen?

Moist monsoon air: in the monsoon season, air full of moisture reaches the mountains.
Forced up by the slopes: striking steep slopes, this air rises rapidly, cools and forms huge cumulonimbus clouds.
Drops held aloft: strong upward winds inside the cloud keep the water drops from falling for quite a while, and a great deal of water builds up in the cloud.
Released all at once: when the updraft weakens or the drops become too heavy, almost all the water pours down at once over a small area.

That is why cloudbursts strike hilly regions such as Uttarakhand, Himachal Pradesh, Jammu and Kashmir and Ladakh more than the plains. Such clouds also produce lightning — read Why lightning flashes.

Why is a cloudburst so dangerous?

Flash floods: on steep slopes the water rushes down very fast, and within minutes a small stream becomes a raging river.
Landslides and debris: stones, trees and mud come down with the water, destroying homes and roads.
Little warning: it happens over a very small area and in a very short time, so it is hard to forecast precisely.

Did you know? On the night of 6 August 2010, a cloudburst at Leh in Ladakh caused a flash flood that brought heavy destruction and killed hundreds of people. Such an event in a cold desert like Ladakh, which usually gets very little rain, surprised even scientists.

Forecasts and warnings

The weather office watches heavy-rain clouds with Doppler weather radars and satellites, and issues "nowcasts" — warnings for the next few hours. The radar network is being expanded in the Himalayan states. Scientists say that very heavy rain in a short time is becoming more frequent as temperatures rise, so care is all the more important.

What to do in heavy rain in the hills

Before travelling, always check warnings from the weather office and the district administration; put off your trip if there is a red alert.
Do not pitch tents or stop beside rivers, streams or dry channels.
If the flow of water suddenly rises, turns muddy, or you hear rumbling from upstream, move to higher ground at once.
Do not try to cross a swollen stream or a flooded road on foot or by vehicle.
Do not travel at night on routes prone to landslides.

Read about Himalayan roads and passes in The major mountain passes of India. The science of the monsoon: How the monsoon comes.

Key points

100 mm or more of rain in an hour over a small area is called a cloudburst.
The cloud does not literally burst; the stored water falls all at once.
Cloudbursts are more common in the Himalayan regions during the monsoon.
The biggest dangers are flash floods and landslides.
In heavy rain, stay on high ground away from rivers and streams.

Frequently asked questions

What is called a cloudburst?

When 100 millimetres or more of rain falls in an hour over a small area of about 20–30 square kilometres.

Why are cloudbursts more common in the mountains?

Because moist air striking the mountains rises quickly and forms clouds that give very heavy rain.

More articles: Geography · Geography questions.

What is a cloudburst? Causes, dangers and how to stay safe