More than 100 people were killed when a cloudburst led to flash floods in Sheelgarh Village, 35km from Kullu, Himachal Pradesh, on Wednesday morning.

Last year, 29 people were killed in a similar incident in Tehri district of Uttaranchal.

H R Hartwar, deputy director general, Indian Meteorological Department, told rediff.com: "It is common to all hilly areas. But Uttaranchal and Himachal Pradesh are the most affected. It also happens in Assam and other north-eastern states.

"We don't get information about states like Arunachal Pradesh because of poor communication with them. Many times it goes unnoticed if it does not occur in an area inhabited by humans."

He said most of the damage is done due to the floods that follow cloudbursts.

"Then there could be landslide, lightening and buildings and trees collapse. The communication channel is blocked. But flood is the main problem."

Like any other natural disaster, the damage depends on the area affected and the number of people living there.

"The damage also depends on how fast the rainfall is. The gushing water erodes the soil and take along everything that comes its way," said Hartwar.

Added S K Subramanium, also deputy director general at the IMD: "A cloudburst... is heavy rainfall in a very short duration."

The rainfall could be around 100 mm per hour, he said.

"There is a strong wind along with thunderstorm. This is what we call thunder squall," said Subramanium.

But the phenomenon is very localised, according to him.

"It can affect an area of about 20-30 km or maximum it can go up to 50 km. Not more than that."

The phenomenon occurs due to the formation of 'Cumulo Nimbus', he said.

"This is a tall, connective cloud that is formed in hilly areas in monsoon season because there is enough moisture there. The hilly terrain and moisture content provides a favourable condition for the formation of this cloud," said Hartwar.

Uttaranchal and Himachal Pradesh get hit during this part of the year because of the moisture coming in from the Bay of Bengal.

These clouds hold a lot of water. Due to air currents rushing up, the clouds climb to a height of around 20-30 km. They then come down along with the air currents.

But if the air current suddenly ceases, the water in the cloud falls on a small area with disastrous effect.