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Natural hazard


Natural hazards are severe and extreme weather and climate events that occur naturally in all parts of the world, although some regions are more vulnerable to certain hazards than others. Natural hazards become natural disasters when people’s lives and livelihoods are destroyed. Human and material losses caused by natural disasters are a major obstacle to sustainable development. By issuing accurate forecasts and warnings in a form that is readily understood and by educating people how to prepare against such hazards, before they become disasters, lives and property can be protected. Click here for a poster on natural hazards.

WMO’s Disaster Risk Reduction activities are integrated and coordinated with other international, regional and national organizations. WMO coordinates the efforts of NMHSs to mitigate human and property losses through improved forecast services and early warnings, as well as risk assessments, and to raise public awareness.

Emphasis is on disaster risk reduction: one dollar invested in disaster preparedness can prevent seven dollars’ worth of disaster-related economic losses—a considerable return on investment. WMO’s objective is to reduce by 50 per cent, by 2019, the associated 10-year average fatality of the period 1994-2003 for weather-, climate- and water-related natural disasters. Natural hazards occur across different time and area scales and each is in some way unique. Tornadoes and flash floods are short-lived, violent events, affecting a relatively small area. Others, such as droughts, develop slowly, but can affect most of a continent and entire populations for months or even years. An extreme weather event can involve multiple hazards at the same time or in quick succession. In addition to high winds and heavy rain, a tropical storm can result in flooding and mudslides. In temperate latitudes, severe summer weather (thunder and lightning storms or tornadoes) can be accompanied by heavy hail and flash floods. Winter storms with high winds and heavy snow or freezing rain can also contribute to avalanches on some mountain slopes and to high runoff or flooding later on in the melt season.

Some National Meteorological and Hydrological Services and specialized centres have responsibility for investigating geophysical hazards including volcanic explosions (airborne ash) and tsunamis, and hazardous airborne matter (radionuclides, biological and chemical substances) and acute urban pollution.


The primary cause of any drought is deficiency of rainfall. Drought is different from other hazards in that it develops slowly, sometimes over years, and its onset can be masked by a number of factors. Drought can be devastating: water supplies dry up, crops fail to grow, animals die and malnutrition and ill health become widespread.

Tropical cyclones

WMO provides assistance to Members in establishing national and regionally coordinated systems which ensure that the loss of life and damage caused by tropical cyclones are reduced to a minimum. Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge, circulating mass of wind and thunderstorms up to hundreds of kilometres across. Surface winds can reach speeds of 200 km/h or more. The combination of wind-driven waves and the low-pressure of a tropical cyclone can produce a coastal storm surge—a huge volume of water driven ashore at high speed and of immense force that can wash away everything in its path. A massive storm surge left 300 000 people dead in the coastal wetlands of Bangladesh in 1970. About 80 tropical cyclones form every year. Their names depend on where they form: typhoons in the western North Pacific and South China Sea; hurricanes in the Atlantic, Caribbean and Gulf of Mexico, and in the eastern North and central Pacific Ocean; and tropical cyclones in the Indian Ocean and South Pacific region. WMO’s Tropical Cyclone Programme provides information on these hazards and WMO’s Severe Weather Information Centre provides real-time tropical cyclone advisories.

Air pollution

Pollutants include particulate matter and noxious gases from industry, vehicles and human activities. Smoke and haze result from forest or wildland fires or from slash-and-burn forest or crop clearing or ash from volcanic explosions in stable air conditions. Smoke, haze and pollution have serious implications for human health—the local population may have to wear gas masks. They reduce visibility; air and road traffic can be disrupted. Smog, acid rain, the ozone hole and an adverse increase in the greenhouse effect are also caused by air pollution. Stable atmospheric conditions often lead to a concentration of pollutants. WMO's Atmospheric Research and Environment Programme administers the Global Atmospheric Watch that collects observations on atmospheric pollutants.

Desert locusts

Desert locusts inflict damage in Africa, the Middle East, Asia and southern Europe. When weather and ecological conditions favour breeding, the insects are forced into a small area. They stop acting as individuals and start acting as a group. Within a few months, huge swarms form and fly downwind in search of food. Swarms can be dozens of kilometres long and travel up to 200 km a day. A small part of an average swarm (or about one tonne of locusts) eats the same amount of food in one day as 10 elephants or 25 camels or 2 500 people. They jeopardize the lives of millions of farmers and herders in already fragile environments. Locust plagues during or immediately after drought conditions can spell even greater disaster, as was the case in several Sahelian countries in 2005. The World Agrometeorological Information Service (WAMIS), a WMO-sponsored Website, has a Locust Weather page dedicated to weather-related information for desert locust monitoring and control.

Floods and flash floods

Floods can occur anywhere after heavy rain events. All floodplains are vulnerable and heavy storms can cause flash flooding in any part of the world. Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in all sorts of forms, from small flash floods to sheets of water covering huge areas of land. They can be triggered by severe thunderstorms, tornadoes, tropical and extra-tropical cyclones (many of which can be exacerbated by the El Niño phenomenon), monsoons, ice jams or melting snow. In coastal areas, storm surge caused by tropical cyclones, tsunamis, or rivers swollen by exceptionally high tides can cause flooding. Dikes can flood when the rivers feeding them carry large amounts of snowmelt. Dam breaks or sudden regulatory operations can also cause catastrophic flooding. Floods threaten human life and property worldwide. Some 1.5 billion people were affected by floods in the last decade of the 20thcentury.

Landslide or mudslide (mudflow)

Mudslides and landslides are local events and usually unexpected. They occur when heavy rain or rapid snow or ice melt or an overflowing crater lake sends large amounts of earth, rock, sand or mud flowing swiftly down mountain slopes, especially if these are bare or burnt by forest or brush fires. They can reach speeds of over 50 km/h and can bury, crush or carry away people, objects and buildings. In Venezuela in 1999, after two weeks of continuous rain, landslides and mudflows shot down a mountain, washing away towns and killing an estimated 15 000 people.


An avalanche is a mass of snow and ice falling suddenly down a mountain slope, often taking earth, rocks and rubble with it. Avalanches can be highly destructive, moving at speeds in excess of 150 km/h. The moving snow also pushes air ahead of it as an avalanche wind strong enough to cause serious structural damage to buildings, woodlands and mountain resorts. Thousands of avalanches occur every year, killing an average of 500 people worldwide.


Duststorms and sandstorms are ensembles of particles of dust or sand lifted to great heights by strong and turbulent wind. They occur mainly in parts of Africa, Australia, China and the USA. They threaten lives and health, especially of persons caught in the open and far from shelter. Transportation is particularly affected as visibility is reduced to only a few metres.

Thermal extremes

Heat waves are most deadly in mid-latitude regions, where they concentrate extremes of temperature and humidity over a period of a few days in the warmer months. The oppressive air mass in an urban environment can result in many deaths, especially among the very young, the elderly and the infirm. In 2003, much of western Europe was affected by heat waves during the summer months. In France, Italy, The Netherlands, Portugal, Spain and the United Kingdom, they caused some 40 000 deaths. Extremely cold spells cause hypothermia and aggravate circulatory and respiratory diseases.

Thunderstorms, Lightning, and Tornadoes

Severe thunderstorms give rise to sudden electrical discharges in the form of lightning and thunder. They often bring heavy rain or hail, strong winds and occasionally snow. In some parts of the world they trigger tornadoes. Tornadoes are particularly common in the Great Plains of North America but they can and do occur anywhere, especially in temperate latitudes. They can cause severe damage. Other associated phenomena include downbursts and flash floods. Worldwide, lightning during dry periods is a significant factor in starting wildfires in forests and grasslands.

Forest or Wildland Fire

Massive and devastating fires can be triggered during and after periods of drought, by lightning or by human action in almost all parts of the world. As well as destroying forests, grasslands and crops, they kill livestock and wild animals, damage or destroy settlements and put the lives of inhabitants at risk.

Heavy rain and snow, Strong winds

Heavy rain and snow are dangerous for vulnerable communities. They can exacerbate rescue and rehabilitation activities after a major disaster, such as the earthquake in Pakistan in October 2005. They bring havoc to road and rail transportation, infrastructure and communication networks. An accumulation of snow can cause the roofs of buildings to collapse. Strong winds are a danger for aviation, sailors and fishermen, as well as for tall structures such as towers, masts and cranes. Blizzards are violent storms combining below-freezing temperatures with strong winds and blowing snow. They are a danger to people and livestock. They cause airports to close and bring havoc to roads and railways.

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