HAZARDOUS ENVIRONMENTS -HAZARDS CAUSED BY ATMOSPHERIC DISTURBANCES

Hazardous tropical cyclones are very large low pressure systems with wind speeds of above 119 km/h and very deep low pressures, which can be as low as 880 mb. They have stronger winds than tropical storms, which have wind speeds between 63 km/hr and 119 km/hr. Tropical cyclones are called hurricanes in the Caribbean Sea, Gulf of Mexico and west coast of Mexico, cyclones in the Indian Ocean, Bay of Bengal and northern Australia and typhoons in the South China Sea and west Pacific Ocean. With the exception of the hurricanes that affect the west coast of Mexico, tropical cyclones affect the east coasts of continents and are most hazardous to the Caribbean islands and the densely-populated, low-lying coasts of Bangladesh, the Bay of Bengal and south-east USA.

Tornadoes occur on every continent except Antarctica but are most frequent in the Great Plains and eastern parts of the USA in a belt known as Tornado Alley. They can occur at any time but are most frequent in the USA between 16:00 and 21:00 hours in May and June after along period of heating of the land. The most disastrous tornado occurred in Bangladesh in 1989 when over 1000 people died. Tornado deaths in Bangladesh are about double those in the rest of the world combined.

Having ferocious whirlpools of air, torrential rains and associated storm surges, flooding and landslides, tropical cyclones are very hazardous. They form over warm oceans between May and November in the northern hemisphere and between November and May in the southern hemisphere. Sea surface temperatures have to be a minimum of 26°C (according to the latest NASA figure) to a depth of at least 50 m.

They form in the summer when the noonday sun is high in the sky, with maximum occurrence in late summer and autumn after a long period of intense heating. The warm sea surface is important; it warms the air in contact with it so that large amounts of water vapour evaporate into the air that moves into the storm. This moisture fuels the growth of the storm, which can only t happen if the air is unstable (warmer than the air at the same height), so it will continue to rise. The heated moist air expands, becomes lighter and rises. The rise intensifies because of the added latent heat of condensation.

Tropical storms form only between 5o and 20° north and south. They do not form nearer the Equator because Coriolis force is needed to deflect the converging rising air into a spin and the effect of the Earth's rotation is not strong enough near the Equator. In higher latitudes the sea is not warm enough for their formation. There are no strong upper atmosphere winds in these latitudes, so the air can rise to the tropopause.

Cyclones form towards the west sides of the tropical oceans from small low pressure systems, some of which start on the east sides of the oceans and intensify as they move west. Only a few of these easterly waves develop into cyclone winds of 119 km/h or above. Westwards air moveme results from air flowing out of the sub-tropical high pressure cells, such as the Bermuda high in the North Atlantic.

The effect of the Coriolis force can be seen in the direction of movement of the tropical storms, as they generally deflect to the right in the northern hemisphere and to the left in the southern hemisphere. However, the system is very dynamic and can alter course if very strong winds exceed the Coriolis force and change the direction of rotation. Storms gradually reduce in intensity over cooler seas and do so rapidly over land, where they lose their source of energy, water vapour intake.

The effect of the Coriolis force can be seen in the direction of movement of the tropical storms, as they generally deflect to the right in the northern hemisphere and to the left in the southern hemisphere. However, the system is very dynamic and can alter course if very strong winds exceed the Coriolis force and change the direction of rotation. Storms gradually reduce in intensity over cooler seas and do so rapidly over land, where they lose their source of energy, water vapour intake.

These processes intensify while the system remains over warm water because, as the rising air cools to below dew point, condensation releases latent heat. This causes the air to rise even faster, the low pressure to lower even more and e the moisture-laden wind to rush into the low pressure centre even faster. The lower the pressure, the more the air is drawn into it and the greater its power.

Very heavy rain falls from the thick cumulonimbus clouds produced by the uplift. At the tropopause the air cools and some sinks, forming the eye, a calm, sunny area in the centre of the storm. Other air moves away from the high pressure at the tropopause. A very dense circular band of cumulonimbus cloud known as the eyewall of the system surrounds the eye. The eyewall has the most powerful winds and deepest convection, with towering rain clouds that rise above the rest of the cyclone because of the vast amounts of latent heat released in them. Beyond that are circular bands of cumulonimbus clouds, separated by areas clear of cloud where air is subsiding into denser air, causing adiabatic warming by compression

The degree of hazard from tropical cyclones depends partly on how frequently they occur in the same area, their magnitude and the location of the area affected. The most powerful storms are not necessarily the most hazardous.

High winds : The Saffir-Simpson scale (revised 2012) indicates the potential damage to structures by sustained wind speeds in tropical storms of cyclone strength. The Dvorak technique has also been developed to estimate cyclone intensity on a scale from 1 (the least) to 8, based on satellite images of patterns in the weather systems, in particular the difference between the temperature within the eye and the coldness of the surrounding thunderstorm clouds. The greater it is, the stronger the tropical cyclone.

Intense rainfall: Places crossed by the eye of a cyclone experience two bands of intense rainfall from the cumulonimbus clouds around the eye. This causes severe river flooding and mass movements, especially if the system is slow moving. River flooding is illustrated in the case study of Cyclone Yasi.

Storm surges and coastal flooding : Storm surges, abnormal rises of seawater, are secondary hazards of tropical cyclones. The strong winds drive the surge, which rises in shallow water and pushes inland, causing flooding. The highest part of the surge is where the strongest winds are. The lower the atmospheric pressure, the higher the surge because air rises in low pressure systems, taking weight off the sea surface, allowing it to rise. The surge will be especially high when:

→ the storm has strong onshore winds

→ it approaches the coast at right angles

→ the coastline has bays and inlets to funnel the water

→ the seafloor is gently sloping, with a wide continental shelf

→ there are few obstructions, such as spits and islands, to slow the flow of water

→ the storm coincides with high tide.

The greatest risk from cyclones is at the coast, because they lose power as they pass over land. The slower the cyclone moves, the greater the damage. Also, in the northern hemisphere the greatest risk is for the cities on the right of the eye's path because there the travel speed is added to the speed of the winds that are rotating anticlockwise.

Mass movement :

Super-Typhoon Durian, a category 5 storm, affected the Philippines in November 2006. Nearly all the 1200 deaths occurred when torrential rains saturated volcanic deposits on the slopes of Mayon volcano, causing mudflows that raced down the steep slopes onto the towns beneath. People had no time to escape as entire villages were buried under volcanic debris.

Tornado is a violent, rapidly rotating and fast moving, narrow, funnel-shaped column of cloud that extends from the base of a cumulonimbus cloud to the ground. The very strong rotating wind makes it the most violent type of storm on Earth. Much stronger winds can occur than in cyclones, reaching speeds of 500 km/h. Tornadoes have extremely low pressure and usually last only a few minutes. The pressure falls rapidly as the tornado approaches, is very low in the centre and rises very quickly afterwards, as in a tropical cyclone, but there are differences in scale between these two weather systems.

Average central pressure is not known with confidence (because the tornado usually destroys recording instruments) but 850 mb, lower than in a cyclone, is the lowest-recorded central pressure and it was measured dropping extremely rapidly by 100 mb to 850 mb in a few minutes. The majority of tornadoes are less than 600 m in diameter and have a path width of less than 50 m. Although most touch the ground for less than 4 km, the record contact was for just over 350 km.

Most tornadoes become darkened by the debris they pickup. The most dangerous tornadoes occur when the approaching hazard is not noticed because the funnel shaped cloud is hidden by rain or dust. As well as varying in size, they also vary in shape. Stovepipe tornadoes are narrow and cylindrical, whereas very wide ones are wedge tornadoes

Although tornadoes are too small to be affected by Coriolis force, most do rotate anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. The rotation is believed to be started by wind shear. Although they generally move in the USA from south-west to northeast, they can take strange, unpredictable paths and can be almost stationary or race across the ground at 80 km/h. Lightning, hail or very heavy rain can accompany them.

Tornadoes are not fully understood and are being intensively studied in the USA in the hope of being able to give warning of their approach as long as possible beforehand, as the country has about 1200 tornadoes a year. Tornadoes are not becoming more frequent but more people are living in their paths as populations rise. It is known that they originate when a cold air mass from Canada moves south into warm, moist air from the Gulf of Mexico, causing instability and turbulence. Tornadoes occur at the cold front boundary between the two air masses.

It is also known that tornadoes form from rotating supercell thunderstorms and always come from cumulonimbus cloud. However, only 20 per cent of supercell thunderstorms develop tornadoes. They form in the warm, moist air, usually with a temperature in excess of 18°C. Supercell thunderstorms are extremely violent and have large updrafts extending to the top of the cloud. These rotating up-currents, known as mesocyclones, can be up to 16 km in diameter. About half of them become tornadoes stending downwards, spiralling to reach ground level.

The rotation may be started by a wind shear - winds blowing in different directions or at different speeds, such as when strong horizontal upper air wind meets a violent updraft. This would cause a horizontal rotation, which is then forced to rise with the strong updraft.

The first stage in the life cycle of a tornado occurs when heavy rainfall in the cumulonimbus cloud drags a column of rapidly descending air down with it, which in turn, drags the mesocyclone towards the ground.

As the mesocyclone lowers, it pulls in warm, moist air from kilometres around, as well as very humid air from the rainy area at the cloud base. The moisture in this quickly condenses to form a rotating wall cloud, an isolated cloud that projects below the rest in the rain-free part of hea the storm. The increasing outflow of cold air concentrates The the base of the mesocyclone into an increasingly smaller the area from which to draw its air intake. The updraft grows in intensity, creating a low pressure near the surface. This pulls the mesocyclone down as a funnel cloud. When the downdraft reaches the ground 10 to 20 minutes after the wall cloud forms, it creates a gust that causes severe damage over a considerable distance. Soon after the funnel cloud itself becomes damaging and dust and debris get drawn into it.

The tornado grows in intensity while it is supplied with warm, moist air, which expands in the low pressure and cools, releasing latent heat when the water vapour condenses. When the funnel cloud has grown to its widest extent and is vertical, the tornado is in its most damaging mature stage, often with very destructive hailstones, up to the size of grapefruit.

Eventually, cold air from the downdraft spreads at the ground surface and cuts off the supply of moist, warm air that was fuelling the tornado. It then rapidly weakens and the funnel becomes wavy and rope-like, before disappearing.

A classification of tornadoes

Tornadoes are measured on the Fujita-Pearson scale which is really three independent scales measuring wind speed, path length (PL) and path width (PW).  The frequency with which tornadoes occur decreases down the scale.

Tornado hazards

The main hazard is being hit by flying debris or massive hailstones. Other dangers are being lifted up and blown through the air until hitting an object or being crushed, either by falling trees or by collapsing parts of buildings. Broken power lines are a secondary hazard.

Hurricane prediction and monitoring Meteorologists in the national Hurricane Centre in Florida track storms on satellite images to provide warning in time for people in the USA and surrounding countries to put strong covers over their windows and evacuate. Satellites have made weather forecasting much more accurate.

Meteorologists fly in planes with their instruments across the eyewalls and eye of hurricanes to measure their strength. However, predictions of where the storm will hit are not always successful because tropical storms can suddenly change course and their speeds of movement can vary. Exactly where they will make landfall is not known until very near the time it actually happens. This causes a problem for the state governor and department responsible for giving the order to evacuate.

The Federal Emergency Management Agency makes sure places are prepared for disasters. Practice drills are held regularly. The Hurricane Centre uses various methods to educate people of the dangers they face and what to do if they are caught in a hurricane.

Prediction of tornadoes is improving all the time. When satellite imagery shows the situation is right for them to form NOAA sends out a Tornado Watch announceme to the media and arranges for a satellite to send back imagery of the area every five minutes, instead of the normal 30 minutes.

Meanwhile, another satellite over western USA acquires imagery at one-minute intervals. Meteorologists also monitor Doppler radar for supercell thunderstorm clouds that have a hook echo at their rears, as these are known to be associated with tornadoes. The large rotating updraft (mesocyclone) from which a tornado may form also helps meteorologists to spot them with more time to warn before it strikes a community. Radar can even detect debris, allowing the location of a tornado to be detected at night or when it is raining. People trained to be tornado spotters then go out to visually confirm the tornado, reporting back to the National Weather Service. The local Weather Service Forecast Office then issues a Tornado Warning and urges people to reach safe shelter.

People who have not experienced the hazard are not usually willing to take the necessary action to avoid it. In 1992 when Hurricane Andrew threatened southern Florida, the authorities decided to evacuate people from coastal areas likely to be affected. The decision to do so was difficult because the economy is harmed by stopping normal life, so the authorities who make the decision to evacuate are strongly criticised if a disaster does not occur. The hurricane proved devastating with 80000 homes destroyed and 15 deaths amongst citizens who refused to evacuate because they had never known a hurricane affect as far inland as Hurricane Andrew reached.

Inappropriate risk perception was also a factor in the Hurricane Katrina disaster. There were both physical and economic reasons why Hurricane Katrina killed more than 1800 people in the city of New Orleans, in the marshes of the Mississippi delta, USA. Built on soft, easily-eroded sediment, much of the city was below sea level, some more than three metres below, with the seawater kept out by concrete embankments.

Economic reasons also contributed to the disaster in New Orleans because it was not thought to be cost-effective to spend a lot on protection against events considered unlikely to happen in a long time period. Therefore, the embankments and floodwalls were designed to prevent damage from category 3 hurricanes but not a 6-metre-high storm surge. Several embankments collapsed and 80 per cent of the city was flooded by as much as three metres. There was little wind damage because buildings had been built to withstand hurricane winds.

Where risk perception leads to protective measures against hurricanes being taken, much depends on the effectiveness of those used against flooding (described in Chapters 1 and 8). People are also deterred from living in vulnerable areas, such as floodplains, by increasing the cost of their insurance.

The integration of hazard prevention measures into the planning of new developments may make it possible to avoid flood damage altogether. Land-use zoning is used to reduce the cost and inconvenience of flooding floodplains are used for playing fields, pasture and nature reserves, whereas buildings are placed at higher levels.

Vulnerability and the number of hazardous events has increased and can be predicted to continue to rise because population growth is forcing people to live in places where they are more vulnerable, such as on steep hillsides in squatter settlements on the edge of cities in LICs. The most vulnerable are the poorest in society, who have little opportunity to do otherwise and who may be relatively poorly educated and completely unaware that they are living in a potentially dangerous location. The rich can afford to move quickly out of danger and can afford to regain a normal life more rapidly after a hazard event. Also, the very young and elderly are weaker and less able to migrate or withstand the effects of a hazard on their health.

Vulnerability also depends on:

→ The degree of technical ability to monitor the hazard and to take preventative or protective actions to minimise it. Prevention is achieved by taking action to the adverse impact of the hazard whereas mitigation attempts to limit the adverse impacts.

→ The degree of education and practising of emergency drills gives awareness of how to minimise potential danger

→ Individuals are less likely to be well-prepared than communities with organisations which take on responsibilities on behalf of all.

→ Different economies have different types of vulnerability : hazards in HICs have relatively small loss of life but high economic costs, whereas in LICs the economic costs are low but the death toll is high.

1. REDUCE VULNERABILITY:

• PREDICT the hazard and warn the community
• PREPARE the communtity by education, shelters, emergency drills, etc
• PLAN land-use approppriately 

2. PREVENT OR REDUCE THE HAZARD USING TECHNOLOGY:

• PREVENTION - strengthen structures to reduce the possibliity of a hazard occurring
• MITIGATION - design structures to reduce damage when a hazard occurs

3. REDUCE THE LOSS:

• Provide AID for relief and reconstruction
• INSURE people and property

Sustainable development is possible in high-risk areas if the public and private sectors act to mitigate or prevent future hazardous events. Risk assessments help decisions about whether or not redevelopment should take place.

→ Construction measures and land-use zoning save lives

and prevent damage.

→ Hazard mitigation is improving. Loss of life during hurricanes has been reduced in the Caribbean by early warning systems. Flood damage in some HICs is reduced by prohibiting building on floodplains, enforced by insurance conditions.

→ Hazard mitigation is now seen to be cost-effective.

→ Hazard management is most effective when all sectors in the area are involved in integrated development planning.

A pronouncement by the UN in November 2015 suggested that the future is less optimistic. It stated that, worldwide, deaths from hazards caused by climate change have increased. It is true that 90% of deaths in the Philippine hazards since 1990 resulted from storms and associated floods and landslides, with only 8% from tectonic hazards, but it is taking a very simplistic view to claim these deaths are caused by climate change alone. The contributions of deforestation, which has left only 3% of the original protective forest cover and a population increase of nearly 40% since 1990 (from 62 million to 102 million in late 2015) have been ignored. At the same time, the President of the Philippines announced plans to build 23 new coal-fired power stations.

An equation of disaster risk, R = (H x V)/C maintains that hazard risk increases as people's vulnerability increases and their coping ability decreases. For the Philippines and other poorer economies this holds true, but the reasons are complex and partly economic.