Cyclone Amphan: Why Tropical Cyclones And Storm Surges Are deadly

Five facts about tropical cyclones and storm surges ahead of a powerful system, Amphan, which is expected to hit India and Bangladesh on Wednesday:

Tropical cyclones

Cyclones are low-pressure systems that form over warm tropical waters, with gale force winds near the centre. The winds can extend hundreds of kilometres (miles) from the eye of the storm.

Sucking up vast quantities of water, they often produce torrential rains and flooding resulting in major loss of life and property damage.

They are also known as hurricanes or typhoons, depending on where they originate in the world, when they reach sustained winds of at least 119 kilometres per hour (74 miles per hour).

Tropical cyclones (hurricanes) are the most powerful weather events on Earth, according to NASA.

Storm surges

Cyclones can unleash catastrophic storm surges -- tsunami-like flooding -- when they make landfall. They can be the deadliest part of a cyclone and are only partially affected by wind speeds.

The term "storm surge" refers to rising seas whipped up by a storm, creating a wall of water several metres higher than the normal tide level.

The large swells move faster than the cyclone and are sometimes spotted up to 1,000 kilometres ahead of a major storm.

The surge can extend for dozens of kilometres inland, overwhelming homes and making roads impassable.

A storm surge is shaped by a number of different factors, including storm intensity, forward speed, the size of a storm and the angle of approach to the coast.

The underlying features of the land at the coast, including bays and estuaries, are also at play.

In previous storms, people failed to flee because they did not grasp the surge's deadly threat.

That was the case for 2013's Super Typhoon Haiyan, which left 7,350 dead or missing in the central Philippines, primarily due to the surge.

A storm surge of up to five metres (16 feet) is likely to inundate low-lying areas of West Bengal during Amphan's landfall, and be up to 3.5 metres in Bangladesh, according to the Indian Meteorological Department.

Low-lying areas

India's eastern coast and neighbouring Bangladesh, a low-lying delta nation, are routinely hit by bad storms between April and December that cause deaths and widespread property damage.

Bangladesh is vulnerable to cyclones due to its location at the triangular-shaped head of the Bay of Bengal, the geography of its coastal area and its high-population density, according to experts.

Hundreds of thousands of people living around the Bay of Bengal have been killed in cyclones in recent decades.

The death tolls have come down in the past few years because of faster evacuations and the building of thousands of coastal shelters.

Bay of Bengal

The tropical cyclone season in the Bay of Bengal and neighbouring Arabian Sea has two peaks around May and November, according to the World Meteorological Organisation.

The cyclones can form in the western Pacific Ocean and travel in a northwest direction before arriving in the Bay of Bengal. Some reach the southeastern coast of India but others divert northeast and move up to West Bengal and Odisha.

The Bay of Bengal has conditions favourable to the development of cyclones, including high sea surface temperatures.

Some of the deadliest storms in history have formed in the Bay of Bengal, including one in 1970 that killed half a million people in what is modern-day Bangladesh.

Some 138,000 died in Bangladesh in 1991 in a tidal wave caused by a cyclone.

In 1999 in India's Odisha state, 10,000 people were killed by a cyclone.

In 2007, Cyclone Sidr killed at least 4,000 in southern Bangladesh.

Then in 2008 Cyclone Nargis, which devastated Myanmar's Irrawaddy Delta, killed about 140,000 people.

Climate change

Studies suggested a warming climate could bring more destructive cyclones as there would be extra heat in the oceans and atmosphere, although such systems could also become less frequent.

Rising sea levels could boost storm surges from cyclones, making them even more deadly and destructive.