Tropical cyclones occur in the low latitudes of both the northern and southern hemispheres. They occur frequently over the western North Pacific, the eastern North Pacific, and the North Atlantic. Although tropical cyclones are not very frequent over the North Indian Ocean, most deadly storms have occured in this basin due to the flat, low-lying coastal terrain and funnel-shaped coast lines.

Tropical cyclones are referred to by different names depending on where they originate. They are called ‘hurricanes’ in the Atlantic Ocean and the eastern north Pacific Ocean; ‘typhoons’ in the western Pacific Ocean; and ‘tropical cyclones’ or ‘cyclones’ in the south Pacific Ocean and Indian Ocean.

Globally, storm surge is the most deadly direct tropical cyclone hazard. The impacts depend on storm intensity, size, motion, coastal terrain, and the shape of coastlines, as well as the number of people living in low-lying or coastal areas in the storm’s direct path, the built environment including building design, and whether there is sufficient time for warning and evacuation.

Tropical cyclones directly and indirectly impact health in many ways, including by:

• causing injury and death;
• increasing cases of drowning and other physical trauma;
• increasing mental health effects associated with emergency situations;
• disrupting health systems, facilities and services, leaving communities without access to health care when they are needed most;
  damaging basic infrastructure, such as food and water supplies and safe shelter;
• When tropical cyclones cause floods and sea surges, the risk of drowning and water- or vector-borne diseases increase. Additionally, flood waters may contain sewage and chemicals, hide sharp objects made of metal or glass and electrical lines, or host dangerous snakes or reptiles, which can cause diseases, injuries, electrocution and bites.^{8 8}WHO: Tropical CyclonesX

Tropical cyclones are intense circular storms that develop over warm tropical oceans, and have heavy rain and maximum sustained wind speeds exceeding 119 kilometres per hour. The greatest damage to life and property from these storms is caused by violent winds, torrential rain, storm surges, flooding, and landslides. Landfalling tropical cyclones can also produce tornadoes.

Tropical cyclones can affect people well inland via heavy rainfall and related hazards (such as flooding and landslides). People living in coastal areas where cyclones make landfall, especially the elderly and those with limited mobility, are at a greater risk than the general population as they may not be able to seek shelter or evacuate their homes.

Other vulnerability factors include:

• Settlements located in low lying coastal areas (direct impact);
• Poor building design or construction;
• Insufficient lead time for warning and evacuation;
• Non compliance with or lack of evacuation procedures;
• Inadequate shelter.
• Proximity to landslides areas

Climate change can worsen the intensity of tropical storms in several ways, including by making record rainfall more likely and by causing sea levels to rise, which can raise the risk of storm surges.^{9 9}Carbon Brief: Global warming could be causing tropical storms to ‘slow down and last longer’X

Many climate models predict future decreases in global tropical cyclone numbers, however, increases are projected in the number of intense cyclones – globally, the proportion of Category 4 and 5 cyclones may increase by 0–25% – with increased related rainfall.^{10 10}IPCCX  The models show that sea level rise is likely to contribute to increased storm surge risks. The vulnerability of coastal regions to tropical cyclone storm-surge flooding is, therefore, expected to increase with global-warming related sea-level rise. Coastal developments will also increase the population at risk.

Confidence levels are generally lower for some other tropical cyclones projections, including projected changes in their tracks, and in their translation speed. In general, confidence in various tropical cyclones projections is lower for the individual basin scale than for the global average.^{11 11}Tropical Cyclones and Climate Change Assessment, WMO: Notable Tropical CyclonesX

Information about tropical cyclones and related risks is routinely prepared by national meteorological services to inform millions of people around the world of impending disaster risk. These same services provide advice to governments and other organizations concerning disaster response.

It’s crucial that governments in vulnerable areas ensure healthcare infrastructure is resilient to disasters. Using weather and climate information about flood hazards, health facilities can be built in areas that are not prone to flooding, and early warnings can ensure that staff are ready to respond to emergencies.

Needs can be anticipated through assessment, including for: search and rescue, triage, managing population displacement in the short-term, and raising awareness on the risk associated with cleanup activities. Maintaining food security conditions over the long term may also be necessary in the case of destruction of crops and livestock.

In 1970, Cyclone Bhola, the world’s most devastating cyclone to date, claimed approximately 500,000 lives in Bangladesh,^{12 12} WMO: World’s deadliest tropical cyclone was 50 years ago X and another in 1991 claimed around 140,000 lives. Since 1991, the government with the support of the UN, including WHO and WMO, has established early warning systems, shelters along coastal areas, search and rescue teams and first-aid training and equipment. Bangladesh now has the capacity to evacuate hundreds of thousands of people from the path of floods and cyclones. When Sidr, a very strong Category-4 cyclone struck Bangladesh in November 2007, the devastation it wreaked was widespread. Sidr was of similar strength as the cyclone of 1991, but its death toll was much lower, at 3,000 lives.^{13 13}Atlas of health and climate,WMO/WHO, 2012X

There are two main types of large scale windstorm: the extratropical cyclone and the mountain storm downwind of mountains.

Extratropical cyclones can affect all areas outside tropical latitudes, but most commonly affect coastal areas on the west side of continents poleward of 50 degrees latitude. They generally form over the sea and propagate eastward, and are characterised by a deep centre of lower pressure around which winds blow: clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere.

The area covered by damaging winds can cover millions of square kilometres, and may extend far inland as the cyclone moves. Within this area, features such as squall lines and “sting jets” may exist with higher wind speeds and producing areas of exceptional damage. These storms commonly also produce heavy precipitation – both rain and snow.

When warm, moist airstreams within extra-tropical cyclones (warm conveyor belts / atmospheric rivers) impinge on mountains and can produce severe flooding and consequent health impacts. Extratropical cyclones can also generate storm surges, leading to coastal flooding of vulnerable communities.

Downslope windstorms can occur downwind of any mountain range but are most likely to occur where there is an upwind source of cold air. Mountain windstorms occur when a cold layer of air near the surface is overlain by warmer air above the height of a mountain ridge with winds blowing across the ridge. In these circumstances rapid acceleration of the wind down the lee slope can generate hurricane strength winds that may affect the area downwind of the mountain ridge for several days. In Croatia and Italy, downwind of the Balkan mountain ranges, the mountain wind is known as the Bora and winds as high as 300km/hr have been measured. Mountain winds of over 200km/hr have been measured in Chinook winds to the east of the Rocky mountains in North America.

The impact of windstorms is highly dependent on the severity of the storm and the vulnerability of those affected. WIndstorms are commonly associated with heavy rain, so health impacts of flooding are also likely. Health impacts directly associated with the strong winds can include:

• Traumatic injuries, such as being hit by airborne debris
• Mental health impacts, such as stress and anxiety
• Water-borne diseases such as cholera and typhoid

Direct health effects during a windstorm include traumatic injury and death from blowing debris, falling trees and traffic accidents.

Indirect effects, occurring before or after the storm, include falls, accidental lacerations or puncture wounds associated with preparation for, or cleaning up after, the storm.

Loss of power is a considerable risk. Carbon monoxide poisoning is prevalent throughout a disaster due to indoor use of portable generators, cooking and heating appliances designed for outdoor use. Loss of refrigeration can result in food becoming contaminated. Essential public health infrastructure such as clean water, sewage treatment and temperature control may be interrupted.

Power outages also pose threats to healthcare infrastructure, affecting direct clinical care and life support equipment in hospitals, preventing access to electronic patient records and increasing the demand on emergency services. Loss of power affecting services such as blood bank or radiology can delay life saving treatment. Care in the home can also be affected, including loss of home oxygen, interruption to dialysis and loss of transport to healthcare.

Loss of the home and evacuation are traumatic experiences, especially under life-threatening conditions. Extended recovery periods, which may be a year or more, compound the trauma. Mental health impacts can be profound, especially in already vulnerable people with mental health disorders such as depression, anxiety and post traumatic stress disorder known to persist for at least a year after flooding. Other health impacts include infections and insect bites.

There are several ways through which windstorms can impact health, including the following:

• Direct impacts of wind pressure on buildings or on trees that then fall on to buildings, causing trauma;
• Impact of wind pressure, especially increased pressure during gusts, overturning vehicles or making vehicles difficult to control, leading to road accidents;
• Impacts on mental health from disruption of livelihoods and population displacement;
• Carbon monoxide poisoning from indoor power generators and cookers used during electrical failures;
• Damage to or increased pressure on infrastructure that compromises healthcare services;
• Disruption of energy distribution, resulting in chemical and biological contamination of water supplies and sanitation;
• Increased disease risk as a result of contaminated water supplies, compromised water sanitation and hygiene and the effects of malnutrition from lack of adequate food supplies

Those who rely on power or transport for routine healthcare are most at risk from the disruption caused by windstorms.

Those who are living in poor standard buildings, such as mobile homes or informal housing, together with those living close to vulnerable trees, are at risk from the direct effects of windstorms.

Increased temperatures will mean that extra-tropical cyclones carry more water vapour and produce heavier rain when they make landfall. Increased sea levels will make coastal erosion and flooding from high waves and storm surges more likely and more serious when windstorms make landfall. Other changes are less clear. Tracks of cyclones are expected to change – probably moving polewards, but there is no consensus about this.

Key areas of action for risk management include:

• Disaster preparedness;
• Monitoring and surveillance, for example wind speeds, wind direction;
• Weather forecasting and alerts;
• Removal or securing of loose materials – especially corrugated iron;
• Restricted speeds on roads, especially in exposed locations such as bridges

Forecasts and warnings are generally available several days in advance of these storms, but need to be communicated with advice on how to avoid both direct and indirect impacts.

Longer term measures to protect people and property against windstorms are also crucial.

Buildings and infrastructure need to be constructed and maintained so that they are secure in strong winds. Building codes should take account of local wind maxima, e.g. from funnelling of the wind by the local topography or from exposure on windward slopes.

Power, communication and transport infrastructure should also be constructed to withstand wind extremes, and maintained to ensure that failure of surrounding buildings or trees does not interrupt services. The influence of prior conditions, such as heavy rainfall affecting tree stability, should be included in planning.

Those who own and use temporary structures, such as scaffolding, marquees, sheds or mobile homes, should be informed of their vulnerabilities to extreme winds and should plan to secure and evacuate them when early warnings are issued.

Infrastructure operators, especially power suppliers, should have business continuity plans that ensure rapid recovery of services from any interruption. These should include preparatory actions, such as pre-positioning of recovery crews, to be taken on receipt of early warnings. Backup power and communications facilities should be available to health infrastructure facilities such as hospitals and clinics.

Storm-induced flooding, both in coastal areas and in river floodplains, should be prepared for by avoiding development of vulnerable areas, protecting key infrastructure, and preparing evacuation plans for those who can’t be protected. In particular, health facilities should not be situated in flood-prone locations and should be resilient to low-likelihood flooding, e.g. by ensuring that backup power is not situated in the basement.

Regions with the greatest frequency of severe thunderstorms are equatorial Africa, the central United States northern Mexico and southern Canada, areas near the Himalayas, southern Brazil and northern Argentina, southern Europe and parts of coastal Australia.

Most of the principal storm frequency areas are located near major mountain ranges, revealing the importance of local topography in thunderstorm initiation. These thunderstorms are very localized and sometimes local effects like sea – mountain or lake breeze can generate severe conditions in a few minutes.

Deaths from lightning tend to occur more often in areas with large numbers of people working outdoors. In India, lightning strikes are common during heavy monsoon rains, and some 2,000 people die from lightning in the country each year – compared with 30 in the US and 3 in the UK. Lack of early warning, large geographic areas affected, and a greater majority of people working outdoors contributes to the increased death toll in India.

Health impacts of severe thunderstorms may include:

• Death and injury from car accidents due to hazardous driving conditions;
• Slip and fall injuries due to slippery walkways;
• Injuries from falling and flying objects, such as tree limbs and utility poles;
• Injuries from sharp objects, including nails and broken glass;
• Injuries from large hail or wind-borne debris;
• Electrical hazards from downed power lines or downed objects in contact with power lines;
• Falls from heights;
• Burns from fires caused by energized line contact or equipment failure;
• Exhaustion from working extended shifts;
• Heat and Dehydration;
• Drowning due to flash flooding;
• Getting caught in collapsed structures / thrown vehicles.

Severe thunderstorms are those producing hail of 2 cm or greater in diameter, wind gusts of 90 km/h or greater, tornadoes, and/or heavy rainfall generating flash flooding.

Direct impacts include death, injury and damage to property from violent winds, falling and flying debris and hailstones, and possible electric shocks.

Indirect impacts include damage to or increased pressure on infrastructure that compromises healthcare services, and disruption of energy distribution resulting in chemical and biological contamination of water supplies and sanitation. Increased disease risk as a result of contaminated water supplies, compromised water sanitation and hygiene and the effects of malnutrition from lack of adequate food supplies is also a risk.

Anyone can be impacted by severe thunderstorms, especially those living in especially vulnerable areas. Storms capable of generating tornadoes and large hail can form in a very short period of time, reducing forecast lead times and limiting the possible action to stay safe.

Other vulnerability factors include:

• Poor building or wooden design building construction;
• Insufficient lead time for warning and evacuation;
• Non compliance with or lack of evacuation procedures;
• Inadequate shelter or inadequate shelter access;
• Outside workers and sportspeople are vulnerable to lightning strikes.

Severe thunderstorms are strongly connected with environmental instability and deep-tropospheric wind shear during the warm season. Tornadoes, damaging winds and large hail are more likely in high-shear environments, while flash flooding is more likely in low-shear environments.

Evidence of the impact of climate change can be clearly observed in large scale phenomena as tropical or extratropical cyclones, but as local severe storms are influenced by environmental local effects, caution is urged in interpreting the observational and modeled records.

Storm warnings are routinely prepared by national meteorological services, though tornado warnings provide only a few minutes lead time (the average in the U.S. Great Plains is 13 minutes), so it is crucial that the population understand and prepare for risk in advance. Individuals should be advised to stay indoors inside concrete buildings, schools or public buildings, and to stay away from windows, electrical connections, metallic pipes and any type of metallic conductor. They should also know not to use corded telephones, but that wireless or cellular telephones present no risk.

Meteorological services often also provide advice to governments and other organizations on disaster response. Early Warning Systems (EWS) that provide information on different time scales are useful, and can provide information on expected impacts and recommendations to the population so that they can take self-protection measures in advance. EWS can be composed of meteorological information for the next minutes, hours and days. Through these systems, operational communications with government risk managers can also be generated, as can shared information systems for joint monitoring of storm evolution and associated response operations.

Remote sensing (e.g. satellite and meteorological radars) can contribute to the detection of severe weather events, but those systems are not available worldwide.