Five facts about tropical cyclones and storm surge ahead of powerful Cyclone Mocha, which is expected to hit Myanmar and Bangladesh on Sunday: 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. 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.k 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 three metres (10 feet) is likely to inundate low-lying areas of Myanmar’s Rakhine State and eastern Bangladesh, according to the Indian Meteorological Department. Bangladesh, a low-lying delta nation, is 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. 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 Organization.