Note: Hot-linked terms in the definitions are defined elsewhere in this glossary.

 
areal extent
a measure of the geographic coverage of the hazard event and the range of its potential damage; the physical space covered by a hazard event.

blizzard
winter storms characterized by low temperatures and strong winds that blow large amounts of snow; other winter storms include ice storms or snow squalls.

chronic hazards
a group of hazards that do not stem from one event but arise from continuous conditions (e.g., famine, resource degradation, pollution, and large-scale toxic contamination) which accumulate over time.

cyclone
a low-pressure system. Tropical cyclones originate in the low (tropical) latitudes and can -- under conditions of high surface temperature and moisture over the tropical oceans -- evolve into severe tropical storms or even hurricanes.

disaster
a singular or interactive hazard event that has a profound impact on local people or places either in terms of loss of life or injuries, property damages, or environmental impact. Most such events are declared disasters once a certain threshold of impact has been crossed (e.g., more than 100 fatalities, more than $5 million in damages).

disaster proneness
measure of the vulnerability of nations to natural disasters. Some of the most disaster-prone countries are those with high hazard frequencies and low national wealth, meaning that they are not able to respond effectively in the aftermath of a hazard or to mitigate the impact of future hazard events.

driving forces
societal forces that bring about global environmental change, including population, economic, and technological changes, differing ideologies, and changes in social organizations.

drought
extended period of unusually dry weather that results in a shortage of water and consequently often a decrease in or failure of food crops.

duration
temporal measure of how long the hazard event persists. An earthquake may last a few seconds, while a drought may continue for several years.

earthquake
series of vibrations or shocks caused by the sudden motion of plates along a fault. Related hazards caused by earthquakes include landslides and tsunamis.

epidemic
far-reaching and commonly rapid outbreak of a disease, affecting hundreds or even thousands of people.

exposure
the state of being physically at risk from a hazard. Researchers differentiate between voluntary and involuntary exposure to hazards (see voluntariness). Examples of involuntary exposure include air pollution (as we must breathe ambient air), toxic contamination of food (as we must eat), and water pollution (as we have to drink). We do, on the other hand, have a greater choice over where we live and what activities we engage in (living in coastal or seismically active zones is to some extent voluntary; smoking or bungee jumping are definitely voluntary).

famine
state of massive food deprivation leading to malnutrition and death of large numbers of people.

flash flood
a sudden-onset, severe, but often locally restricted flood event occurring mainly in arid or semi-arid areas with steep topography following intense, short-lived rainstorms. Also occur in narrow valleys or developed urban areas where impervious surfaces increase rapid runoff. Warning times are severely limited.

flood
riverine flooding occurs when a river overflows its banks from heavy rainfall events, snowmelt, ice jams, landslides, or dam failures. Coastal flooding results when water surpasses its tide line. Flood problems can be exacerbated by human activities such as land drainage, river channelization, and deforestation.

forest fires
the burning of large areas of timber land. Can be caused by lightning, human carelessness, or intentional fire setting. Soil erosion, landslides, and flash floods can result after the vegetation cover has been burnt.

frequency
statistical measure for how often a hazard event of a given magnitude and intensity will occur. Often, frequency is measured in terms of a hazard’s recurrence interval. For example, a recurrence interval of 100 years for a flood suggests that in any year, a flood of that magnitude has a 1% chance of occurring.

hazard
the threat to people and the things they value. Environmental events become hazards once they threaten to affect society and/or the environment adversely.

hazardscape
the landscape of many hazards. The interaction among nature, society, and technology at a variety of spatial scales creates a mosaic of risks that affect places and the people who live there. The term is normally used in reference to a specific place or region.

hurricane
tropical storm with wind speeds of greater than 74 miles per hour. Heavy rains and storm surges above normal tide levels are produced. Hurricanes evolve from tropical depressions and tropical storms. While this type of storm is not unique to the Atlantic Ocean, the term "hurricane" is applied to such storms in that region. For other regionally specific terms see typhoon and cyclone.

IDNDR
International Decade of Natural Disaster Reduction; declared by the United Nations for the 1990's, its goals include improving the capacity of countries to mitigate the effects of natural disasters by improving early warning systems, closing gaps in knowledge in order to reduce loss of life and property, and developing measures for the prevention, prediction, and assessment of natural disasters.

intensity
measure of the potential severity of a hazard event and its impacts in human experience. The Mercalli Scale measures intensity based on damage to structures from earthquakes and the perceptibility of the event by humans without instrumentation.

landslide
mass movement of unstable earth and rocks down the side of a slope. Can be initiated by long rains or in combination with an earthquake.

magnitude
measure that describes the strength or force of an event. The Richter Scale measures the magnitude of earthquakes through the amount of energy released. Magnitude is an important characteristic for analyzing hazards since only occurrences exceeding some defined level of magnitude are considered extreme, disastrous, or even hazardous.

megacity
very large city or giant urban concentration resulting from an existing city that expands to accommodate a growing population (e.g., huge volumes of migrants from rural and other areas) and growing economic base. Megacities are rapidly appearing in the developing world and often involve conurbation (the growing together of formerly separate urban centers while retaining separate urban identities).

Mercalli Scale
measures the intensity of earthquakes as experienced by humans and others. Not a measure of the physical forces, but of the impact on humans.

mitigation
measures or actions that lessen the harmful effects of disasters and hazards.

mudflow
mixture of soil and water that moves at varying speed down a hillside. The source of the water can be from rainfall or snowmelt. When volcanic ash deposits mix with water, the mudflow is called a lahar.

natural hazard
hazard event arising from geophysical processes or biological agents -- such as those creating earthquakes, hurricanes, or locust infestations -- that affect the lives, livelihood, and property of people.

pollution
the release of materials or energy (solid, fluid, or gaseous substances; heat; noise) that are harmful to humans and/or the environment.

preparedness
the degree of alertness and readiness of an individual or a community immediately before the onset of a hazard event. To enhance preparedness, people plan how to respond in case a disaster occurs and work to increase the resources available to respond effectively. Preparedness activities are designed to help save lives and minimize damage by preparing people to respond appropriately.

prevention
within technological hazard management systems, this is a type of hazard modification that aims at averting hazards before they even occur. Not using a product with potentially hazardous effects would be one way to prevent a hazard.

rate of onset
the length of time between the first appearance of the hazard event and its peak. Tornadoes are examples of rapid onset events; soil erosion is much slower in onset. Also called speed of onset.

recovery
the process of returning to "normality" after an emergency occurred. The post-disaster phase that is also used to increase safety and preparedness (also see mitigation).

resilience
the ability to recover from the impacts of a hazard event.

resource degradation
the processes of using up, diminishing, devaluing, or destroying environmental assets that humans use, value, or enjoy resulting in a loss or decreased value of that resource.

response
the sum of all actions taken to adjust to hazards; more narrowly defined to mean the appropriate actions taken during an emergency to protect people and the things they value from harm, rescue them, and facilitate the transition to post-disaster recovery.

Richter Scale
a logarithmic scale expressing the amount of energy released in an earthquake.

risk
the likelihood or probability of a hazard event of a certain magnitude occurring. Risks are measures of the threat of hazards. (Note: Definitions of risk in the hazards literature vary from those that equate risk with probability to those that see risk as the product of a probability and a particular kind of impact occurring.)

Saffir-Simpson Hurricane Scale
measure of hurricane intensity and magnitude by scaling the storm based on central pressure, windspeed, storm surge, and potential damage. Ranges from 1 (minimal) to 5 (catastrophic).

secondary hazards
hazards resulting from other hazard events. Landslides may occur, for example, if the shaking from an earthquake causes a mass of rocks and earth to shift beyond its angle of repose.

spatial dispersion
refers to the pattern of distribution of a hazard over the geographic area in which the hazard can occur.

technological hazard
human-constructed hazards arising from the interaction of social, environmental, and technological systems. Nuclear technology, pollution, and warfare are examples.

temporal spacing
refers to the sequencing and seasonality of events. Some events are quite random (volcanoes) while others have seasons (hurricanes).

thunderstorms
storms characterized by heavy rain and/or hail, strong winds, lightning, and in some instances even tornadoes.

tornado
extremely violent, localized storm with a characteristic funnel produced by extreme low pressure, enormous wind speeds, and debris circulating around it. Wind velocities can reach 200 miles per hour. They form as funnel clouds from thunderstorms and hurricanes.

tsunami
giant sea waves generated by seismic action. As tsunamis reach the coast, they can create vast coastal and inland flooding, and often claim many lives.

typhoon
tropical storm with origins in the Pacific Ocean. See hurricane.
 

USDNDR
United States Decade for Natural Disaster Reduction. The national counterpart and outreach from the IDNDR with the goal to go beyond current disaster relief efforts to include the reduction of the consequences of hazards through early mitigation strategies.

volcanic eruption
the eruption of molten material, rocks, steam, and other gases from the interior of the earth. Eruption may be continuous and slow (as in Hawaii) or extremely violent (e.g., Mt. St. Helens in Washington). Nuee ardente, a hot, poisonous cloud of gas and debris that races down the side of a volcano, is extremely dangerous and lethal.

voluntariness
the degree to which individuals have a choice over their exposure to a hazard. See exposure.

vulnerability
the potential (susceptibility) for loss or the capacity to suffer harm. While there is an ongoing debate over the appropriate usage and definition of the term, the idea of vulnerability can be applied to individuals, societies, or the environment.