Cyclone Fani marks the latest entry in a short but quite devastating list of cyclonic storms that have hit the coasts of South Asia in the month of May in the last one and a half decades. Cyclones Aila, Nargis and Roanu hit the region on May 27, 2009, May 2, 2008 and May 19, 2016, respectively.

Aila was the worst natural disaster to affect Bangladesh since Cyclone Sidr in November 2007. The second tropical cyclone of the 2009 North Indian Ocean cyclone season, Aila, formed over the Bay of Bengal on May 23. A relatively strong tropical cyclone, it caused extensive damage in India and Bangladesh. The storm was responsible for at least 339 deaths across Bangladesh and India; more than one million people were left homeless. Health officials in Bangladesh confirmed a deadly outbreak of diarrhoea on May 29, with more than 7,000 people being infected and four dying.

Storm Nargis caused the worst natural disaster in the recorded history of Myanmar during early May 2008. The cyclone made landfall in Myanmar on May 1, 2008, sending a storm surge 40 kilometres up the densely populated Irrawaddy delta, causing catastrophic destruction and at least 1,38,373 fatalities.

Roanu was a relatively weak tropical cyclone that caused severe flooding in Sri Lanka and Bangladesh in May 2016. It is the first tropical cyclone of the annual cyclone season. It originated from a low pressure area that formed south of Sri Lanka, gradually drifted north and intensified into a cyclonic storm on May 19. However, wind shear and land interaction caused it to weaken slightly, before reintensifying as it accelerated towards the coast of Bangladesh.

The storm was responsible for 201 deaths in Sri Lanka and another 26 in Bangladesh. Damage throughout Sri Lanka reached USD 2 billion. Roanu also caused torrential rainfall in the Indian states of Tamil Nadu, Andhra Pradesh, Kerala and Odisha as it drifted in a generally north-eastward direction, close to the coast.

Going through few research documents and weather reports available online, The Independent tried to piece together the reason behind devastating cyclones hitting the region in this particular month.

The tropical cyclones that influence South Asia are part of the regional monsoon wind system. The South Asian monsoon has moist south-westerly winds blowing from the southern oceans over the South Asian continental land mass in summer—typically starting in mid-April. The differential heating of land and sea drives this movement. In the summer, the land heats up more quickly than the oceans, producing low pressure over land and high pressure at sea. Winds blow from high to low pressure, bringing strong, moist winds from the oceans towards South Asia.

A report published in the Scidev.net said tropical cyclones affecting south Asia originate over surrounding oceans, especially in the Bay of Bengal. They require at least five conditions to form and develop: low pressure at the surface; abundant moist air capable of convective or upward movement in the atmosphere; ocean surface temperatures over 26–27 degrees Celsius; small wind shear—the rate at which wind strength and direction change with height in the atmosphere— (especially for the taller more intense systems); and the power of the earth’s rotation to spin the system into a rotating vortex.

In May, few or all of the five conditions often materialise, creating the possibility of cyclones. That is why most of the devastating cyclones in the region take place at this time of year.