Major meteorological hazards and extreme events are listed in the table below. The definitions of the events mostly follow the definitions given in the International Meteorological Vocabulary (WMO No. 182), or according to the WMO’s Meteoterm, or the definitions given by the WMO Commission for Climatology Task Team on Definition of Extreme Weather and Climate Event (CCL TT-DEWCE) for the four major extreme events.
List of hazard and extreme events
Deposition of acid substances by precipitation, resulting from the long-range atmospheric transport of pollutants which enhanced environmental acidification when reaching the Earth's surface.
Mass of snow and ice falling suddenly down a mountain slope and often taking with it earth, rocks and rubble of every description.
Violent winter storm, lasting at least 3 hours, which combines below freezing temperatures and very strong wind laden with blowing snow that reduces visibility to less than 1 km.
A marked and unusual cold weather characterized by a sharp and significant drop of air temperatures near the surface (Max, Min and daily average) over large area and persisting below certain thresholds for at least two consecutive days during the cold season.
Violent and damaging downdraught reaching the surface, associated with a severe thunderstorm.
A marked unusual period of abnormally dry weather characterized by prolonged deficiency below a certain threshold of precipitation over a large area and persisting for timescales longer than a season.
A period of unusually dry conditions(*) of at least five consecutive days with daily precipitation less than 1mm.
(*) i.e. to exclude usually dry periods, such as during dry seasons in arid or semi-arid areas.
Wind with a speed between 34 and 40 knots (Beaufort scale wind force 8).
A flood that rises quite rapidly with little or no advance warning, usually as a result of an intense rainfall over a small area or, possibly, an ice jam, a dam failure, etc.
The overflowing by water of the normal confines of a stream or other body of water, or the accumulation of water by drainage over areas which are not normally submerged.
Temperature of the upper layer of the soil of less than 0°C. May cause damage to vegetation, occurring when the water that is part of the cell structure of the plant solidifies, bursting cells walls and deteriorating the plant materials.
Precipitation of either transparent, or partly or completely opaque particles of ice (hailstones), usually spheroidal, conical or irregular in form and of diameter very generally between 5 and 50 millimeters, which falls from a cloud either separately or agglomerated into irregular lumps.
A marked unusual hot weather over a region persisting at least two consecutive days during the hot period of the year based on local climatological conditions, with thermal conditions recorded above given thresholds.
Suspension of very small, usually microscopic water droplets in the air, generally reducing the horizontal visibility at the Earth's surface to less than 1 km.
Suspension in the atmosphere of extremely small, dry particles which are invisible to the naked eye but numerous enough to give the sky an opalescent appearance.
A marked precipitation event occurring during a period of time of 1h, 3h, 6h, 12h, 24h or 48 hours with a total precipitation exceeding a certain threshold defined for a given location.
Rapid movement of a mass of soil, rock or debris downhill by gravity, often assisted by water when the material is saturated.
Air containing dust, smoke, micro-organisms or gases different from those which normally compose it.
Haze caused by the suspension in the atmosphere of small sand or dust particles, raised from the ground prior to the time of observation by a sandstorm or duststorm.
An ensemble of particles of dust energetically lifted to great heights by a strong and turbulent wind.
An ensemble of particles of sand energetically lifted to great heights by a strong and turbulent wind.
Fog having high pollution content.
Meteorological disturbance giving rise to a heavy fall of snow, often accompanied by strong winds.
An atmospheric disturbance involving perturbations of the prevailing pressure and wind fields, on scales ranging from tornadoes (1 km across) to extratropical cyclones (2000-3000 km across).
Wind with a speed between 48 and 55 knots (Beaufort scale wind force 10).
The difference between the actual water level under influence of a meteorological disturbance (storm tide) and the level which would have been attained in the absence of the meteorological disturbance (i.e. astronomical tide).
Wind with a speed between 41 and 47 knots (Beaufort scale wind force 9).
Atmospheric phenomenon characterized by an abrupt and large increase of wind speed with a duration of the order of minutes which diminishes rather suddenly. It is often accompanied by showers or thunderstorms.
Sudden electrical discharges manifested by a flash of light (lightning) and a sharp or rumbling sound (thunder). Thunderstorms are associated with convective clouds (Cumulonimbus) and are, more often, accompanied by precipitation in the form of rain showers or hail, or occasionally snow, snow pellets, or ice pellets.
A violently rotating storm of small diameter; the most violent weather phenomenon. It is produced in a very severe thunderstorm and appears as a funnel cloud extending from the base of a Cumulonimbus to the ground.
Tropical cyclone, typhoon, hurricane
Generic term for a non-frontal synoptic scale cyclone originating over tropical or sub-tropical waters with organized convection and definite cyclonic surface wind circulation. Tropical disturbance: light surface winds with indications of cyclonic circulation. Phases of development for tropical cyclones and its characteristic wind speeds are:
• Tropical depression: wind speed up to 33 knots.
• Tropical storm: maximum wind speed of 34 to 47 knots.
• Severe tropical storm: maximum wind speed of 48 to 63 knots.
• Hurricane: maximum wind speed of 64 knots or more.
• Typhoon: maximum wind speed of 64 knots or more.
• Tropical cyclone (South-West Indian Ocean): maximum wind speed of 64 to 90 knots.
• Tropical cyclone (Bay of Bengal, Arabian Sea, South East Indian Ocean, South Pacific): maximum wind speed of 34 knots or more.
Low-pressure system which develops in latitudes outside the tropics.
Rain colored yellow by particles of pollen, yellow dust, etc.
Dust or particles emitted by a volcano during an eruption. They may remain suspended in the atmosphere for long periods and be carried by the winds to different regions of the Earth.
A period of at least five consecutive days with daily precipitation exceeding 1 millimeter.