The Seventeenth Session of the World Climate Congress (Cg-17) decided to standardize
hazard and extreme event information, including the creation or adoption of a system of
assigning a unique identifier to each event so that events can be catalogued and linked to
data on associated damages and losses.
Information will be validated with relevant Technical Commissions and adopted by the
Executive Council and WMO decision-making bodies.
I. Weather, Climate and Water (alphabetical order)
Acid rain
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 (source: CCL TT-DEWCE).
Annual flood
Highest peak discharge in a year. (2) Flood level which has been equaled or exceeded once
a year (source: International Glossary of Hydrology, WMO-No. 385).
Avalanche
Mass of snow and ice falling suddenly down a mountain slope and often taking with it earth,
rocks and rubble of every description (source: International Meteorological Vocabulary,
WMO-No. 182).
Black carbon
Operationally defined aerosol species based on measurement of light absorption and
chemical reactivity and/or thermal stability. Black carbon is formed through the incomplete
combustion of fossil fuels, biofuel, and biomass, and is emitted in both anthropogenic and
naturally occurring soot. It consists of pure carbon in several linked forms. Black carbon
warms the Earth by absorbing heat in the atmosphere and by reducing albedo, the ability to
reflect sunlight, when deposited on snow and ice (source: Integrated Assessment of Black
Carbon and Tropospheric Ozone, WMO-No. 1073).
Blizzard
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 (source: International Meteorological Vocabulary, WMO-No. 182).
Brown clouds
Elevated black carbon concentrations in areas with high solar radiation are a major
contributor to the so-called brown clouds covering large regions, for instance in Asia. Brown
clouds have led to dimming of the Earth’s surface, warming of the atmosphere and
perturbation of the hydrological cycle, possibly affecting the monsoon (source: The
carbonaceous aerosol – a remaining challenge, WMO Bulletin, Volume 58(1) – January
2009).
Coastal flood
Storm surges and high winds coinciding with high tides are the most frequent cause of this
type of flooding. The surge itself is the result of the raising of sea levels due to low
atmospheric pressure. In particular configurations, such as major estuaries or confined sea
areas, the piling up of water is amplified by a combination of the shallowing of the seabed
and retarding of return flow. Major deltas such as the Mississippi and Ganges are prone to
this type of flooding when affected by hurricanes (cyclones). Another sensitive area is the
southern North Sea in western Europe as a result of particular tracks of winter depressions.
If the surge takes place near the mouth of a river issuing into the sea, the river flow will be
obstructed due to the surge, resulting in severe flooding over and near the coastal areas.
Tsunamis resulting from sub-seabed earthquakes are a very specific cause of occasionally
severe coastal flooding (source: Manual on flood forecasting and warning, WMO-No. 1072).
Cold wave
Marked cooling of the air, or the invasion of very cold air, over a large area (source:
International Meteorological Vocabulary, WMO-No. 182).
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 (source:
CCL TT-DEWCE)
.
Downburst
Violent and damaging downdraught reaching the surface, associated with a severe
thunderstorm (source: International Meteorological Vocabulary, WMO-No. 182).
Drought
Prolonged absence or marked deficiency of precipitation. Period of abnormally dry weather
sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance
(source: International Meteorological Vocabulary, WMO-No. 182).
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 (source: CCL TT-DEWCE)
Dry spell
Period of abnormally dry weather. Use of the term should be confined to conditions less
severe than those of a drought (source: International Meteorological Vocabulary, WMO-No.
182).
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 (source: CCL TT-DEWCE)
Dust storm
Particles of dust energetically lifted by a strong and turbulent wind. Dust storms are usually
associated with hot, dry and windy conditions, especially just ahead of vigorous cold fronts
that can be cloud free. Dust particles typically have a diameter of less than 0.08 mm and
consequently can be lifted to far greater heights than sand (source: Aerodrome Reports and
Forecasts, A Users’ Handbook to the Codes, WMO-No. 782).
Estuarine flood
Estuaries are inlet areas of the coastline where the coastal tide meets a concentrated
seaward flow of fresh water in a river. The interaction between the seaward flow of river
water and landward flow of saline water during high tides may cause a build-up of water or
inland-moving tidal bore. Frequently, the funnel shape characteristic of many estuaries
causes an increase in high water levels in the upper, narrowing reaches of the associated
river. These types of floods are mostly experienced in deltaic areas of rivers along the coasts,
for example the Mouths of the Ganges. They are more frequent and less severe in terms of
inundated depth and area than flooding caused by storm surges (source: Manual on flood
forecasting and warning, WMO-No. 1072).
Extra-tropical cyclone
Low-pressure system which develops in latitudes outside the tropics (source: International
Meteorological Vocabulary, WMO-No. 182).
Flash flood
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. (source:
International Meteorological Vocabulary, WMO-No. 182).
Flood
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 (source:
CCL TT-DEWCE).
Flooding
Overflowing by water of the normal confines of a watercourse or other body of water.
Accumulation of drainage water over areas which are not normally submerged. Controlled
spreading of water for irrigation (source: International Glossary of Hydrology, WMO-No. 385).
Fluvial (riverine) flood
Fluvial flooding occurs over a wide range of river and catchment systems. Floods in river
valleys occur mostly on flood plains or wash lands as a result of flow exceeding the capacity
of the stream channels and spilling over the natural banks or artificial embankments. Flash
floods are often more damaging, occurring in narrow, steep and confined valleys,
characterized as the name implies by the rapidity of formation following rainfall and high flow
velocities. The rapidity makes them particularly dangerous to human life (source: Manual on
flood forecasting and warning, WMO-No. 1072).
Fog
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 (source:
International Meteorological Vocabulary, WMO-No. 182).
Gale
Wind with a speed between 34 and 40 knots (Beaufort scale wind force 8) (source:
International Meteorological Vocabulary, WMO-No. 182).
Hail
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 (source: International Meteorological Vocabulary, WMO-No. 182).
Haze
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 (source:
International Meteorological Vocabulary, WMO-No. 182).
Heatwave/heat wave
Marked warming of the air, or the invasion of very warm air, over a large area; it usually lasts
from a few days to a few weeks (source: International Meteorological Vocabulary, WMO-No.
182).
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 (source: CCL TT-DEWCE)
.
Heavy precipitation
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
(source: CCL TT-DEWCE)
.
Heavy rain
Rain with a rate of accumulation exceeding a specific value, e.g. 7.6 mm h|~|-1 (source:
International Meteorological Vocabulary, WMO-No. 182).
Hoar frost
Deposit of ice, which generally assumes the form of scales, needles, feathers or fans and
which forms on objects whose surface is sufficiently cooled, usually by nocturnal radiation, to
4 Revised by the WMO Commission for Climatology Task Team on Definition of Extreme Weather and Climate Event (CCL TT-DEWCE). bring about the direct sublimation of the water vapour contained in the ambient air (source:International Meteorological Vocabulary, WMO-No. 182).
Hurricane
Name given to a warm core tropical cyclone with maximum surface wind of 118 km/h|~|-1
(64 knots, 74 mph) or greater (hurricane force wind) in the North Atlantic, the Caribbean and
the Gulf of Mexico, and in the Eastern North Pacific Ocean. A tropical cyclone with hurricane
force winds in the South Pacific and South-East Indian Ocean (source: International
Meteorological Vocabulary, WMO-No. 182).
Hydrological drought
Period of abnormally dry weather sufficiently prolonged to give rise to a shortage of water as
evidenced by below normal streamflow and lake levels and/or the depletion of soil moisture
and a lowering of groundwater levels (source: International Glossary of Hydrology, WMO-No.
385).
Ice and debris-jam flood
In areas that experience seasonal melting, if this is rapid ice floes can accumulate in rivers,
forming constrictions and damming flows, causing river levels to rise upstream of the ice jam.
A sudden release of the “ice jam” can cause a flood wave similar to that caused by a dam
break to move downstream. Both meltwater and heavy rainfall in steep areas can cause
landslips and debris flows. As these move downstream, major constrictions can build up.
When these collapse or are breached, severe flooding can result. Both of these phenomena
are very difficult to predict (source: Manual on flood forecasting and warning, WMO-No.
1072).
Ice storm
Intense formation of ice on objects by the freezing, on impact, of rain or drizzle (source:
International Meteorological Vocabulary, WMO-No. 182).
Landslide/Mudslide
Rapid movement of a mass of soil, rock or debris downhill by gravity, often assisted by water
when the material is saturated (source: CCL TT-DEWCE).
Meteorological drought
Prolonged absence or marked deficiency of precipitation (source: International Glossary of
Hydrology, WMO-No. 385).
Mudflow
Flow of water so heavily charged with sediment and debris that the flowing mass is thick and
viscous (source: International Glossary of Hydrology, WMO-No. 385).
Multiple event flood
These result from heavy rainfall associated with successive weather disturbances following
closely after each other. On the largest scale, these include for example floods in the IndoGangetic plains and central Indian regions often caused by the passage of a series of lowpressure areas or depressions from the Bay of Bengal, more or less along the same path.
Multiple event floods can also affect large basins in mid-latitude areas in winter, when
sequences of active depressions occur, for example over Western Europe (source: Manual
on flood forecasting and warning, WMO-No. 1072).
Polluted air
Air containing dust, smoke, micro-organisms or gases different from those which normally
compose it (source: International Meteorological Vocabulary, WMO-No. 182).
Sand haze
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 dust storm (source:
International Meteorological Vocabulary, WMO-No. 182).
Sand storm
An ensemble of particles of sand energetically lifted by a strong and turbulent wind. The
forward portion of the sandstorm may have the appearance of a wide and high wall. The
height to which sand is raised will increase with increasing wind speed and instability (source:
Aerodrome Reports and Forecasts, A Users’ Handbook to the Codes, WMO-No. 782).
Seasonal flood
These are floods that occur with general regularity as a result of major seasonal rainfall
activity. The areas of the world subject to a monsoonal type climate are typically the areas
most affected and critical situations arise when “normal” flooding is replaced by extended or
high-runoff floods. Flooding is frequently a basin-wide situation that can last for periods of
several weeks. Within active monsoon conditions, a number of individual peak events can
occur during a flood season. Seasonal floods can also result from high water levels in lakes
in the upper reaches of a river basin, for example Lake Victoria and the River Nile. Another
type of seasonal flood can result from wet conditions in an upper portion of a catchment,
which experiences a different climate regime from the lower, affected areas. The Nile and
Yangtze rivers are good examples (source: Manual on flood forecasting and warning, WMONo. 1072).
Single event flood
This is the most common type of flooding, in which widespread heavy rains lasting several
hours to a few days over a drainage basin results in severe floods. Typically, these heavy
rains are associated with cyclonic disturbances, mid-latitude depressions and storms, with
well-marked synoptic scale frontal systems (source: Manual on flood forecasting and
warning, WMO-No. 1072).
Smog
Fog having a high pollution content (from SMoke and fOG) (source: International
Meteorological Vocabulary, WMO-No. 182).
Snowmelt flood
Significant flood rise in a river caused by the melting of snowpack accumulated during the
winter (source: International Glossary of Hydrology, WMO-No. 385).
Snowstorm
Meteorological disturbance giving rise to a heavy fall of snow, often accompanied by strong
winds (source: International Meteorological Vocabulary, WMO-No. 182).
Squall
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 (source: International Meteorological Vocabulary, WMO-No.
182).
Storm
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) (source: International Meteorological Vocabulary, WMO-No. 182).
Storm surge
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) (source: International Meteorological
Vocabulary, WMO-No. 182).
Strong gale
Wind with a speed between 41 and 47 knots (Beaufort scale wind force 9) (source:
International Meteorological Vocabulary, WMO-No. 182).
Sulphur rain
Rain coloured yellow by particles of pollen, yellow dust, etc. (source:
International Meteorological Vocabulary, WMO-No. 182).
Thunderstorm
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 (source: International
Meteorological Vocabulary, WMO-No. 182).
Tornado
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 (source: CCL TT-DEWCE).
Tropical cyclone
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. 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 (source: International Meteorological Vocabulary, WMO-No. 182).
Tropical storm
A well-organized tropical cyclone in which the maximum average surface wind (one-minute
mean) is in the range 63-118 km/h (39-73 mph) (34-63 knots) inclusive (source: Hurricane
Operational Plan, Report No. TCP-30, WMO-TD No. 494).
Tsunami
A rapidly moving and often large sea wave generated by submarine earthquakes, landslides
or volcanic activity (source: Manual on marine meteorological services, WMO-No. 558, App.
1.2).
Typhoon
Name given to a tropical cyclone with maximum sustained winds of 64 knots or more near
the centre in the western North Pacific (source: International Meteorological Vocabulary,
WMO-No. 182).
Wet spell
A period of a number of consecutive days on each of which precipitation exceeding a
specific minimum amount has occurred (source: International Meteorological Vocabulary,
WMO-No. 182).
A period of at least five consecutive days with daily precipitation exceeding 1 millimeter
(source: CCL TT-DEWCE)6
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