Natural hazards and disaster risk reduction

The primary cause of any drought is below average 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.

A tropical cyclone is a rapid rotating storm originating over tropical oceans. It is typically around 200 to 500 km in diameter, but can reach 1000 km. A tropical cyclone brings very violent winds, torrential rain, high waves and, in some cases, very destructive storm surges and coastal flooding.

Pollutants include particulate matter and noxious gases from industry, vehicles and human activities. Smoke and haze result from forest or grass 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 and 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 in urban and industrial areas where there are significant emissions.

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 with the wind 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 can occur anywhere after heavy rain events. All floodplains are vulnerable and heavy rain or thunderstorms can cause flash flooding in any part of the world. Flash floods can also occur after a period of dry conditions when moderate or heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in a number of forms, from small flash floods to sheets of water covering extensive areas of land. They can be triggered by severe thunderstorms, tropical cyclones, large low pressure systems , 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 or flood levees can overtop causing floods when the rivers carry large amounts of snowmelt. Dam breaks or sudden regulatory operations such as the release of water for hydro-electric power generation 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 20th century.

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 loosens vulnerable parts of the landscape on steep slopes, resulting in large amounts of earth, rock, sand or mud flowing swiftly down slope. Hillsides or mountain sides that  are bare or have had their vegetation cover degraded  through clearance or by forest or brush fires may be especially at risk. 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 slid down a mountain, destroying towns and causing an estimated 15 000 fatalities.

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.

Sand and dust storms usually occur when strong winds lift large amounts of sand and dust from bare, dry soils into the atmosphere. Over the last decade, scientists have come to realize the impacts on climate, human health, the environment and many socio-econimic sectors. WMO Members are at the vanguard in evaluating these impacts and developing products to guide preparedness, adaptation and mitigation policies. 

Heat waves are most deadly in mid-latitude regions during the warmer months of the year. They are characterized by a number of days in a row where the temperatures are significantly above the long term average both day and night. 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 fatalities. Extremely cold spells are also dangerous causing hypothermia and aggravating circulatory and respiratory diseases in those vulnerable to this risk.

All thunderstorms give rise to sudden electrical discharges in the form of lightning and they often bring rain and gusty winds. Severe thunderstorms are associated with all or combinations of heavy rain,  large hail, very strong winds and occasionally tornadoes or snow. Tornadoes are particularly common in the Great Plains of North America but they can and do occur anywhere, especially in temperate latitudes. Their extremely strong rotating winds can cause severe damage. Other associated phenomena include extremely strong winds from downbursts and flash floods. Worldwide, lightning during dry periods is a significant factor in starting wildfires in forests and grasslands.

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 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.