Flash floods

Torrential rains and flash floods in various parts of Himachal Pradesh have caused loss of life and property recently.

Flash floods:

Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it.
They also occur when water fills normally dry creeks or streams or enough water accumulates for streams to overtop their banks, causing rapid rises of water in a short amount of time.
National Weather Service of USA, says flash floods are caused when rainfall creates flooding in less than 6 hours.
It adds that flash floods can also be caused by factors apart from rainfall, like when water goes beyond the levels of a dam.
In India, flash floods are often associated with cloudbursts – sudden, intense rainfall in a short period of time.
Himalayan states further face the challenge of overflowing glacial lakes, formed due to the melting of glaciers.

How common are flash floods and floods?

According to government data from a project by the Assam State Disaster Management Authority, India is the worst flood-affected country in the world after Bangladesh and accounts for one-fifth of the global death count due to floods.
Flash floods have been commonly witnessed in cities like Chennai and Mumbai.
Depression and cyclonic storms in the coastal areas of Orissa, West Bengal, Andhra Pradesh, and others also cause flash floods.

Torrential rain:

Torrential rain refers to the heavy downpour of rain.
There is no definite definition of it other than the definition provided by the National Weather Service (NWS).
The NWS defines torrential rain as rain that accumulates at a rate of three tenths of an inch or more per hour.

Causes of Torrential Rain:

Moisture that moves along the weather fronts is the major cause of torrential rain.
The convective clouds cause precipitation to occur when enough moisture rises up due to an upward motion.
Narrow torrential rainbands come as a result of cumulonimbus clouds.
In mountainous regions, torrential rain falls on one side of the mountain since heavy precipitation occurs on one side of the mountain.
The side of the mountain where much precipitation occurs is the windward side. Most of the moist air condenses and then falls as torrential rain on the windward side of the mountain.
Dry air blows on the other side of the mountain due to the down slope. The urban heat experienced on islands results in torrential rain.
Scientific research shows that torrential rain which pours on other planets contains volumes of iron, water, methane, sulphuric acid, and even neon gas.