Glossary

How this glossary is used

From A to C

The words of abstraction and of measuring instruments, the territory of a river, and the first words of the climate and of the sky.

Abstraction and consumption

Two quantities that must never be confused. To abstract is to take water from the natural environment; to consume is not to give it back. Water that cools a power station is abstracted and then given back a little warmer; water a plant evaporates is consumed. In France, in 2022, between 12 % and 15 % of the fresh water abstracted was actually consumed.

Anemometer

The instrument that measures the speed of the wind. Its value is expressed in kilometres per hour, in metres per second or in knots, and it is always an average over a stated length of time.

Anticyclone

A zone of high atmospheric pressure. Air leaves it towards zones of low pressure, and it is that circulation which makes the wind.

Aquifer

The porous and permeable rock of the subsoil, or the cracked rock, able to hold water and to let it circulate. The aquifer is the ROCK; the water it holds is called the groundwater.

Atmospheric pressure

The quantity expressed in hectopascals whose differences set the air in motion. Air goes from zones of high pressure, the anticyclones, towards zones of low pressure, the depressions.

Barometer

The instrument that measures atmospheric pressure. Its value is expressed in hectopascals and is reduced to sea level so as to stay comparable from one place to another.

Baseflow

The share of the discharge of a watercourse that comes from the discharge of groundwater. It is what makes a river flow outside spells of rain.

Catchment

The territory corresponding to the whole of the surface receiving the waters that flow naturally towards one and the same watercourse or towards one and the same body of underground water. Also called a drainage basin. Its boundary is traced by the relief and never follows an administrative border.

Climate

The usual behaviour of the atmosphere in a place, described by averages, by the spread around those averages and by extreme values. Its scale is the decade. No particular day tells us anything about it.

Climatological normal

A statistical reference calculated over a period of thirty years and updated every decade, against which measured values are compared. The period used in France today is 1991-2020.

Cloud cover

The share of the sky covered by cloud, estimated by eye and expressed in eighths. Zero out of eight is a completely clear sky, eight out of eight a completely overcast one.

Condensation

The passage of water from the gaseous state to the liquid state. It takes place when air is cooled to its dew point. It explains mist on glass, dew, fog and clouds.

Condensation nucleus

The tiny particle suspended in the air, dust, sea salt, pollen or soot, on which a droplet forms. Without a surface to cling to, water vapour condenses very poorly, even in saturated air.

From C to H

The changes of state, the words of the forecast, and the two extremes of a watercourse.

Confluence

The exact point where one watercourse empties into another.

Convection

The movement by which air warmed on contact with a hot ground expands, becomes lighter, rises and cools at height. It is the engine of heaped clouds and of thunderstorms.

Departure from the normal

The difference between a measured value and the corresponding normal. It can only be read by knowing over which period the normal was calculated: the same measurement gives two different departures according to the reference used.

Deposition

The direct passage of water from the gaseous state to the solid state, with no liquid stage. It takes place on a surface already cooled below zero degrees, and it is what gives hoar frost. Some books call it solid condensation.

Depression

A zone of low atmospheric pressure. Air converges into it from zones of high pressure, and the closer together the lines of equal pressure, the stronger the wind.

Dew

The deposit of water droplets formed by condensation on a surface, plants or ground, cooled during the night down to the dew point of the air touching it.

Dew point

The temperature to which a body of air must be cooled, without adding anything to it or taking anything away, for it to reach saturation. It is the dew point, and not the relative humidity, that makes it possible to compare two bodies of air with each other.

Drinking water

Water supplied as fit for consumption, after filtration and disinfection, and subject to regulated checks. Whether it is fit to drink cannot be judged from its clearness, its taste or its origin.

Drinking-water treatment

The treatment that makes raw water fit to drink, mainly by filtration, which removes the particles in suspension, and by disinfection, which destroys the micro-organisms. It takes place in a works, at the beginning of the small water cycle.

Drizzle

A liquid precipitation made of very fine droplets, whose diameter does not exceed 0.5 millimetres. It seems to float more than to fall.

Evaporation

The passage of water from the liquid state to the gaseous state, at the free surface of the liquid and at any temperature. Slow, silent, without bubbles. It is evaporation, and not boiling, that feeds the clouds of the entire world.

Evapotranspiration

The sum of the evaporation of water surfaces and soils and of the transpiration of plants. It is through it that a forest sends back into the air part of the water its roots have taken from the soil.

Flood

A marked rise in the discharge and the level of a watercourse. The water in it is high, fast and brown, because it is carrying particles torn from the soil.

Fog

The suspension in the atmosphere of very small water droplets reducing visibility at ground level to less than one kilometre. It is a cloud whose base touches the ground. Above one kilometre of visibility, one speaks of mist.

Forecast bulletin

The document that announces the expected weather. It answers the question of what is going to happen, and its accuracy falls as the range grows. It is not to be confused with the vigilance map, which answers the question of danger.

Forecast model

A mathematical representation of the atmosphere, which starts from a measured initial state and works out step by step how it is likely to develop.

Freezing

The passage of water from the liquid state to the solid state. A puddle turned to ice in the early morning is one. Some books call it solidification.

Genus

The first level of the international classification of clouds. There are ten of them, and a cloud always belongs to one only. The genus is determined by the shape of the cloud and by the height of its base.

Great water cycle

The natural cycle of water at the scale of the planet: evaporation, clouds, precipitation, runoff, infiltration. It works with no human intervention at all, and that is what sets it apart from the small water cycle.

Groundwater

The water built up in all the empty spaces of an aquifer. It is neither a lake nor an underground river: it is a volume of wet rock, which flows very slowly.

Hail

A solid precipitation made of ice particles of diameter greater than 5 millimetres, the hailstones. They come from clouds with powerful upward currents, which carry them back up several times before they fall.

Hoar frost

The deposit of ice crystals formed by deposition on a surface that has gone below zero degrees. It is told apart from a smooth sheet of ice by its needle-like look, which comes from the direct passage of vapour into solid.

From H to R

The instruments of humidity, the words of the ground and of the network, and the first forms of precipitation.

Hygrometer

The instrument that measures the relative humidity of the air. It is placed in the same screen as the thermometer, since relative humidity depends on temperature.

Ice pellets

A solid precipitation made of small ice grains, finer and softer than hailstones, which bounce on the ground. No diameter defines them at the source: the only published threshold is that of hail, at 5 millimetres.

Impermeable

Said of a material that water cannot pass through at any useful rate, either because its voids are too fine or because they connect poorly with one another. Clay is impermeable; so are tarmac and concrete.

Impermeable bedrock

The compact layer that interrupts the descent of underground water and on which the groundwater rests. Without it there would be neither groundwater, nor springs, nor wells.

Infiltration

The entry of water into the ground through its pores and cracks, then its descent under the effect of gravity as far as an impermeable bedrock. It is slow, and it feeds the underground reserves.

Initial state

The picture of the atmosphere at the instant a forecast calculation begins, reconstructed from the observations available. It is an approximation, never reality itself, and its small differences grow at each step of the calculation.

Level

One of the three vertical bands into which the international classification sorts clouds: high level, middle level, low level. A cloud is sorted at the height of its BASE, never of its top.

Liquid state

The state in which water keeps its volume but takes the shape of whatever contains it. It is the water of rain, of the tap and of rivers, and it is also the droplets in suspension that make up a cloud.

Low flow

The time of year when the discharge of a watercourse is at its lowest. The water in it is clear, because it comes mainly from groundwater, and the bed shrinks to a narrow channel.

Melting

The passage of water from the solid state to the liquid state. An ice cube melting in a glass is one.

Meteorology

The study and forecasting of the state of the atmosphere in the short term: what it is like today, what it will be like in the days to come. Its scale is the hour and the day.

Millimetre of rain

The unit of the depth of water fallen. One millimetre of rain is equivalent to one litre of water per square metre. A depth and not a volume is measured, because a depth does not depend on the size of the container.

On-site sanitation

The treatment of waste water on the plot of the house itself, by an individual system, where no sewer network serves it.

Orographic lifting

The forced ascent of a body of air pushed by the wind against a relief. It cools the air by the constraint of the slope alone, without any heat from the ground coming into play, and it explains the wet slope of a mountain.

Outlet

The place where a watercourse, or a whole catchment, discharges its waters: another watercourse, a lake, or the sea.

Permeable

Said of a material that water can pass through, because its voids are wide enough and because they connect with one another. Sand is permeable; so is cracked limestone, hard though it is.

Precipitation

A particle of water, in the liquid or solid state, which falls through the atmosphere. A cloud and a fog are not precipitation, their droplets staying in suspension.

Public sewerage

The treatment of waste water by a network of sewers that carries it to a treatment plant shared by several houses.

Rain

A liquid precipitation made of drops of diameter greater than 0.5 millimetres.

Rain gauge

The instrument that measures the depth of water fallen. Its walls must be straight and vertical, it is set in the open, away from walls and trees, and it is emptied completely after every reading.

Raw water

Water as it is taken from the natural environment, before any treatment. It can be clean without being drinkable: the two words are not synonyms.

From R to W

The words of humidity and of the ground surface, those of the two cycles, and the last changes of state.

Relative humidity

The ratio between the quantity of water vapour contained in the air and the maximum quantity possible at its temperature, expressed as a percentage. It compares each body of air with its own limit of the moment, never two bodies of air with each other.

Reservoir

A place where water stays between two movements of the cycle: ocean, atmosphere, glacier, groundwater, watercourse, soil, living thing. The length of the stay ranges from a few days to several thousand years according to the reservoir.

Runoff

The flow of water over the surface of the ground, following the slope. Being fast, it carries soil away and feeds watercourses directly. Five factors increase it: the slope, the impermeability of the ground, the intensity of the rain, the absence of plant cover and the prior saturation of the ground.

Saturation

The state of air that holds all the water vapour it can hold at its temperature. Saturated air can take up nothing more: any addition, or any cooling, brings on a condensation.

Shower

A brief precipitation, which begins and stops abruptly and whose intensity varies quickly. It comes from heaped clouds, cumulus and cumulonimbus, and not from spread-out layers.

Small water cycle

The entirely man-made journey that runs from the resource to the tap, then from the tap to the natural environment: abstraction, treatment, storage, distribution, collection, wastewater treatment. It borrows water from the great cycle and gives it back.

Snow

A solid precipitation made of crystals or flakes, which forms when the temperature of the air is below or close to zero degrees. Its depth on the ground does not measure a quantity of water: it measures the way the flakes have piled up.

Solid state

The state in which water keeps both its shape and its volume. It is ice, snow, hail, hoar frost. Water in the solid state can be picked up in the hand.

Source

The place where a watercourse begins, most often because underground water comes back out there into the open air.

Standard condition

A rule of measurement identical everywhere, laid down by the World Meteorological Organization, and which exists so that an observation made here can be compared with an observation made elsewhere. Each one repairs a precise way of going wrong.

Sublimation

The direct passage of water from the solid state to the gaseous state, with no liquid stage. Snow that dwindles in hard frost without wetting the ground sublimates.

Thermometer

The instrument that measures temperature. In meteorology it is placed 1.5 metres above the ground in a white, louvred screen: without that screen, it measures its own temperature and not that of the air.

Tributary

A watercourse that empties into another. The meeting point between the two is called the confluence.

Vaporisation

The passage of water from the liquid state to the gaseous state. It takes two very different forms: evaporation, slow, at any temperature, at the free surface; boiling, fast, at one hundred degrees under the usual pressure, throughout the whole body of the liquid.

Vigilance map

The document that signals the potential dangers of a weather situation, by department and by colour. It answers the question of danger, never the question of what the weather will be. What should be done is published with it, at the moment the danger exists.

Wastewater treatment plant

The installation that receives the waste water collected and cleans it before giving it back to the natural environment. It stands at the end of the small water cycle, and its water never goes back towards the taps.

Water table

The body of groundwater nearest the surface. It is the one a well reaches and the one that feeds most springs. Its shallow depth makes it easily accessible for human activities, and just as exposed to whatever is spread on the surface.

Water tower

The reservoir of drinking water placed up high. Its height puts the network under pressure and makes it possible to distribute by gravity alone, without running a pump continuously.

Water vapour

Water in the gaseous state. It is perfectly transparent and makes up between 0.1 % and about 5 % of the surrounding air. Everything that is white and visible in the air has already become liquid or solid again.

Watershed

The natural boundary between two neighbouring catchments, drawn by the relief and corresponding to the ridge lines. Two drops falling on either side of it feed two different catchments.

Weather radar

The instrument that follows precipitation at a distance by sending out a wave and measuring what the drops send back. It collects nothing: it sees rain up in the air, sometimes before it touches the ground.

Weather screen

The white, louvred box, placed 1.5 metres above the ground, in which the temperature of the air and the humidity are measured. It protects the sensor from radiation and from precipitation while letting the air circulate. White so as to send light back, louvred so as not to measure the air of its own box.

Weather Vigilance

The scheme that signals the potential dangers of a weather situation, by department and by colour: green, no particular vigilance; yellow, be attentive; orange, be very vigilant; red, absolute vigilance is called for. It does not forecast the weather, it signals a danger.

Wind

The horizontal movement of the air, born of differences in pressure. Measuring it involves a direction, taken with a wind vane and named by where the wind comes from, and a speed, taken with an anemometer and always averaged over a stated length of time.

Wind vane

The instrument that shows the direction the wind comes from. A wind is always named by the direction it comes from, never by the one it is heading towards.