Greg O'Hare explains the what, why, when and how of cyclones, and reviews their effects in South Asia.
'Tropical cyclone' is the general term for low-pressure atmospheric circulations in the tropics. These have anticlockwise rotating winds in the northern hemisphere and clockwise rotating winds in the southern hemisphere.
Low to moderate intensity tropical cyclones bring much needed rain for agriculture around the northern Indian Ocean. But, when tropical cyclones strengthen, they can bring great loss of life and property to the region.
All tropical cyclones have low atmospheric pressure at ground level, and a vortex of converging winds and rising air. They all have extensive rain-bearing layered clouds (deep nimbostratus) and towering vertically extensive cumulonimbus rain-bearing clouds. Yet despite these common features, tropical cyclones in South Asia vary greatly in size, frequency and intensity, and have varying effects on the land they cross.
Table 1 shows four types of tropical cyclones. These weather systems form a continuum if conditions are right and surface pressure continually falls, a tropical low can develop over time into a tropical depression, then into a tropical storm and eventually into an intense tropical storm. In South Asia, as in the western world, the most intense tropical storms are called hurricanes. But, confusingly, the most intense circulations in the Pacific are called cyclones.
Type of tropical cyclonic system
Table 1: Types of tropical cyclones in India 
Lows and depressions are the most frequent systems and produce most of India's annual rainfall (about 890mm). Indeed, with their lower intensity rainfalls, they form the backbone of South Asian agriculture.
But when a long series of deep tropical depressions occur (lasting three to four weeks), the cumulative rainfall can lead to extensive flooding, dam collapses and landslides. In southern Bangladesh, more than 100 families were washed away when a dam collapsed in July 2004. In 2008, summer monsoon flooding and landslides in India (especially in Bihar State) killed 1065 people and affected approximately 7.9 million people.
How do hurricanes form?
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 2627 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.
Tropical cyclones in South Asia derive their main energy from intense evaporation over warm water not, as in mid-latitude cyclones, from contrasting temperatures between cold and warmer air masses.
Water vapour, evaporated from the sea, is drawn into the developing cyclone. As the rising air within the cyclone cools, the evaporated moisture becomes cloud, forming billions of tiny water droplets. Converting the water vapour to water droplets releases a great amount of (latent) heat, providing energy that helps invigorate and maintain the cyclone's development.
Timing and monsoon regulation
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, and dry north-easterly winds blowing in the opposite direction in winter.
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. During the winter months, the differential heating and pressure systems are reversed, and strong dry north-easterly winds end up blowing from South Asia towards the southern oceans.
Most rainfall over the region comes in the summer months (June to September) from relatively weak but frequent tropical lows and depressions. Driven by monsoon winds, these systems eventually move over land along the west coast of India, but more frequently affect the eastern coast of India and Bangladesh.
The more intense tropical storms and hurricanes, which also tend to form mainly in the Bay of Bengal, often occur as the wet summer changes to a dry winter monsoon (October to November) when wind shear is low. Powerful cyclones, which tower up into the atmosphere, do not easily form during the main monsoon season (June to September) because high wind shear easily destabilises them, knocking them over.
The areas of South Asia most vulnerable to hurricanes are the low-lying coastal regions around the Bay of Bengal (Bangladesh, Eastern India and Myanmar). These are the first areas storms hit when they reach land and are also some of the most agriculturally fertile and densely populated areas in South Asia, including coastal river deltas like the Godavari, Ganges and Irrawaddy.
Hurricanes' high wind speeds, intense rainfalls and storm surges (unusually high sea levels) destroy life and property, and can leave areas devastated. Winds, often travelling at more than 117 kilometres per hour, remove or seriously damage flimsy housing.
High intensity rainfall over a relatively short period (up to and above 50 centimetres over three to seven days) can cause serious flooding and major crop loss. As with the less intense cyclones, such flooding can increase loss of life and property if it causes reservoir collapses and landslides.
But the most destructive part of a cyclone is the storm surge at the front of the storm pushed up to high levels as it moves inland. Storm surges from powerful hurricanes can reach two to five metres in height along the eastern coast of Andhra Pradesh in India. At the head of the Bay of Bengal, where the coastline becomes restricted, storm surges can reach a staggering 12 or 13 metres and kill many people (see Table 2).
10 Oct 1679
07 Oct 1737
13 Nov 1970
26 Nov 1977
29 Apr 1991
Table 2: Hurricane deaths in the Bay of Bengal region 
Hurricanes in a warming world
There is every chance that hurricanes will do more damage in South Asia in the future as population densities increase in coastal areas. The numbers of people at risk may also rise if hurricanes become more intense as the world and oceans warm up.
Some studies have found no evidence for an increase in hurricanes' frequency or intensity in the Caribbean. [2,3] Others have found little change in the frequency and intensity of hurricanes globally during the last 20 years. 
By contrast, other strong evidence based on good quality data has shown that in recent years hurricanes, particularly the stronger ones (categories four and five), have become more intense in all hurricane regions, including the northern Indian Ocean (Table 3). [5,6]
Percentage of all hurricanes
Percentage of all hurricanes
South West Pacific
Table 3: Changes in the number and percentage of category four and five hurricanes for the periods 197589 and 19902004 for different ocean basins. 
The people most vulnerable to hurricanes around the world include those with limited economic resources, low levels of technology, poor information and skills, minimal infrastructure and unstable or weak political institutions (Table 4). Such groups are not fully able to prepare for, or protect themselves from, hurricanes, nor to respond and cope with their effects.
Table 4:Disaster prone groups in India 
When a category four hurricane hit the Godavari delta region of eastern India in November 1986, various marginalised groups responded differently to the hurricane's impact. For example, poor female agricultural labourers working in flood damaged rice fields had to sell their few possessions and become maids in nearby villages, or migrate to other paddy regions in order to cope. By contrast, poor fishing communities along the delta coast (where many people died due to storm surges) relied on close family and kinship links for money, food and fishing tackle to get over the storm's effects.
There are ways to make the likely rise in hurricane impact less damaging in the region. One solution is to improve the physical structures that protect people. For example, many new hurricane shelters are being built along the coast of eastern India. Deaths from hurricanes will certainly decline if more local people can be encouraged to use the shelters.
Improvements in government-built early warning and evacuation procedures will also help save lives, although access to these may be limited because many communities suffer from isolation, language barriers, and poor transport and communication (including radio/phone) systems. Still, because of improvements, albeit slow, in the introduction and uptake of such systems, hurricanes that would have killed 10,000 people in Andhra Pradesh in the late 1970s kill around 1,000 today.
Governments and international agencies can also do a lot more to mitigate storm impacts through rehabilitation policies, such as providing basic relief (food, shelter, cooking oil and clean water). It is also crucial that affected communities get better health services, since the spread of water-borne diseases (like typhoid and dysentery) after hurricanes often kills far more people than flooding, landslides or even storm surges.
 O'Hare, G. Hurricane 07b in the Godavari Delta, Andhra Pradesh, India: vulnerability, mitigation and the spatial impact. The Geographical Journal 167, 23–38 (2001)
 Michaels P.J., Knappenberger, P.C. & Davis, R.E. Sea surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters 33, (2006)
 Hoyos, C.D., Agudelo, P.A., Webster, P.J. et al. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science 312, 94–97 (2006)
 Klotzbach, P.J. Trends in global cyclone activity over the past 20 years (1986-2005) Geophysical Research Letters 33, (2006)
 Webster P.J., Holland, G.J., Curry, J.A. et al. Changes in tropical cyclone number, duration and intensity in a warming environment. Science 309, 1844–1846 (2005)
 Elsner, J.B., Kossin, J.P. & Jagger, T.H. The increasing intensity of the strongest tropical cyclones. Nature 455, 92–95 (2008)
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