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Precipitation

The term precipitation is used in meteorology to refer to all phenomena in which water falls from the sky, in any form - rain, hail, snow, etc. -, and therefore covers a wide range of meteorological events.

Measuring precipitation provides important data in determining the climate of a region. Indeed, the frequency and intensity of water precipitation on the ground is a decisive factor in the habitability of a geographical area and the fertility of its land, as a certain amount of precipitation is essential for life and the growth of plants.

What exactly does precipitation mean?

In chemistry, precipitation is when a mixture of two solutions produces a precipitate, often in the form of crystals. In meteorology, the term is used to refer to the water droplets or ice crystals that, when subject to a process of condensation and accumulation in the clouds, become heavy and fall from the atmosphere to the ground.

Clouds are largely made up of condensed water in the form of very small solid or liquid particles suspended in the atmosphere. These cloud particles gradually grow in size and cluster together through coalescence. When their size exceeds 100µm, they become heavy and ultimately fall to the surface of the ground under the effect of gravity. This is what is called precipitation. Part of this water sometimes evaporates before reaching the ground, which is referred to as virga; the rest of the precipitation falls to the ground in various forms, depending on the atmospheric conditions and the weather, and notably according to the temperature.

The different types of precipitation

Although precipitation is always made up of water, this water can be in a liquid or solid state and differs slightly depending on the size of the particles, droplets and crystals. When we talk about precipitation we usually think of rain, although other phenomena are also frequently observed.
  • Rain: if precipitation is not subject to a temperature below 0°C, it falls to the ground in the form of droplets of various sizes ranging from 0.5 to 6 mm. The size of the drops and whether or not there is any wind affects the speed at which these droplets fall.
  • Drizzle: drizzle is very fine rain, when the size of the droplets is between 0.1 and 0.5 mm.
  • Hail: this is a solid type of precipitation. Hailstones, or ice crystals up to 5 cm in diameter, fall to the ground at high speed.
  • Ice pellets: half rain and half hail, ice pellets have usually been subject to temperatures below zero when they fall to the ground. Their outer layer is frozen while the centre remains liquid. The diameter of the precipitation rarely exceeds 5 mm.
  • Snow: when water vapour clusters together as crystals under the effect of cold air and these crystals don't melt while falling, this precipitation can take the form of six-sided flakes of varying sizes, on average between 2 and 20 mm.
  • Freezing rain or drizzle: freezing precipitation is when the temperature of the air near the ground is below zero but higher in the upper layers of the atmosphere. The rain then freezes when it approaches the ground and forms ice, which can be very dangerous when it collects on the surface of the road.
    These different forms of precipitation can also combine during a single event, for example when snow mixes with rain.
    Meteorological professionals also distinguish between two main types of precipitation: stratiform precipitation and convective precipitation.
  • Stratiform precipitation: this usually falls from "stratus" clouds in areas of depression and is usually low-intensity but covers a large area.
  • Convective precipitation: conversely, this type of precipitation doesn't last long but is very intense. This is the case with showers or storms resulting from instability in atmospheric conditions, which are often associated with "cumulus" clouds.
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How is precipitation measured?

Meteorology is the study of atmospheric phenomena. Observing various physical and chemical parameters allows us to characterise the climate in a given area, as well as to predict the upcoming weather. To do so, meteorology utilises various devices and sensors to determine the temperature, wind speed and direction, and atmospheric pressure.

The most comprehensive weather stations are also equipped with a rain gauge, a device that measures precipitation, i.e. the amount of rain or snow that falls in an area over a given period. This usually takes the form of a funnel that collects the liquid in a graduated tube to make it easier for the user to read.

The precipitation is measured in millimetres or litres per square metre, and can be manually or electronically captured. There are even smart rain gauges that collect information about the precipitation and give a cumulative measurement of the liquid collected, as well as a history to study changes in results concerning these climate phenomena.

Why is it useful to know the level of precipitation?

Knowing the intensity and frequency of precipitation doesn't just enable us to predict each weather phenomenon more effectively, it also provides us with very local data on local elements. This information can also be used to check the quantitative precipitation forecasts (QPF) established by weather experts, which assess the cumulative precipitation predicted to fall to the ground.
Accurately determining the amount of liquid that has fallen also allows farmers and gardeners to more effectively manage the water content in the soil to ensure the proper growth of the plants they grow.

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