Chapter 5 of 17 · Water and the Weather
Chapter 4. Rain, snow and hail
How a cloud droplet becomes a drop that falls, how each form of precipitation is named, and what a millimetre of rain means.
What makes a droplet finally fall
A cloud is full of water and it does not fall: the previous chapter explained why. Its droplets are too small to overcome the movements of the air. What remains to be understood is what changes when it rains, for rain there certainly is, and the cloud is the same object five minutes earlier.
What changes comes down to one word: size. Météo-France puts it directly, about the droplets of a cloud: "By clumping together, they can grow heavy and fall." The step from cloud to shower is therefore not a change of nature, it is a change of scale. A droplet that grows ends up coming down faster than the air goes up, and then it leaves the cloud from below.
This chapter follows that growth, then names precisely what arrives at the ground. The nomenclature is not an elegance: it is what allows a shower recorded in one village and a shower recorded a thousand kilometres away to be compared. It ends on measurement, which is the act by which an observation becomes a piece of data.
The objectives of this chapter
Each one is observable. By the end of the chapter, either you do it, or you know which section to read again.
- Explain by which mechanisms a cloud droplet becomes a raindrop.
- Name each form of precipitation from its state and its size.
- Turn a depth of rain in millimetres into a volume of water received on a given surface.
- Take a rainfall reading with a rain gauge built at home, and record the reading in a dated table.
- Say what a weather radar measures, and how often it does so.
What this chapter assumes is already known
Two prerequisites, installed by earlier chapters of this volume: the states of water and freezing on the one hand, the make-up of a cloud and its upward currents on the other.
Two roads to growing
A cloud droplet can grow in two ways, and both exist at the same time in a good many of the clouds of temperate latitudes.
The first road is the easier to picture: droplets meet and merge. A cloud is a stirred-up place, its droplets do not all come down at the same speed, the larger ones catch up with the smaller ones and take them in. Each merger produces a slightly larger droplet, so a slightly faster one, so one slightly better at catching up with others. The process runs away with itself, and that is what explains how a shower can break out within a few minutes.
The second road goes through ice, and it concerns clouds high enough to be cold. Chapter 3 established, with Météo-France, that a cloud is made "of fine water droplets and ice crystals in suspension": both states coexist in it. An ice crystal grows quickly, becomes heavy and comes down. If it then passes through a layer of air above zero degrees, it melts on the way and reaches the ground as rain. This is a fact that is often overlooked: part of the rain of temperate regions is snow that has melted during its fall.
In a cumulonimbus, the upward currents are so powerful that a hailstone can be carried back up towards the top of the cloud several times before falling for good. At each pass, it covers itself with a new layer of ice. That is why a hailstone cut in two shows concentric layers, like an onion, and why the largest hailstones come from the clouds with the most violent currents.
Naming what falls
Météo-France defines precipitation as "particles of water, in the liquid or solid state, which fall through the atmosphere". Two words count: particles of water, so water and nothing else; which fall, so moving downwards. Fog, whose droplets stay in suspension, is not a precipitation.
The distinction between rain and drizzle rests on a size, and on that alone. Météo-France speaks of rain for drops of diameter "greater than 0.5 millimetres", and of drizzle for "very fine droplets" whose "diameter does not exceed 0.5 mm". Drizzle indeed seems to float more than to fall, so slowly do its droplets come down.
Snow forms, still according to Météo-France, "when the temperature of the air is below or close to 0 degrees C". Remember the word close: it does not have to be freezing at ground level for snow to fall, and snow can reach the ground at a slightly positive temperature if the air is dry enough.
Hail too is defined by a size: it consists of "ice particles of diameter greater than 5 mm: hailstones". Ice pellets are the other form of ice that Météo-France names, made of grains smaller and softer than hailstones; the source sets no diameter for them, and this volume invents none. Finally, a squally shower is described as a "brief shower" mixing "rain, hail, ice pellets, snow, sleet": it is a mixture, not a separate category.
| Name | State of the water | The criterion that settles it | What is observed |
|---|---|---|---|
| Drizzle | Liquid | Diameter not exceeding 0.5 mm | Very fine droplets, which seem to float and wet things slowly. |
| Rain | Liquid | Diameter greater than 0.5 mm | Clear drops, which make rings in puddles. |
| Snow | Solid | Air temperature below or close to 0 degrees | Crystals or flakes, which come down slowly, turning as they go. |
| Ice pellets | Solid | Ice grains smaller and softer than hailstones; no diameter defines them at the source | Small grains that bounce on the ground. |
| Hail | Solid | Ice particles of diameter greater than 5 mm | Hailstones, which strike hard and briefly, in a shower. |
| Squally shower | Mixture | A brief shower mixing several forms | Rain, hail, ice pellets, snow and sleet within a few minutes. |
The words to remember
Three words, one of which is a unit and is worth half the chapter on its own.
- A 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 stay in suspension.
- The millimetre of rain
- The unit of the depth of water fallen. One millimetre of rain is the depth the water would reach if it stayed where it was without running off or soaking in. According to Météo-France, "1 mm of rain is equivalent to 1 litre of water per m²".
- A 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.
Measuring rain: a depth, not a volume
Rain is measured in millimetres, and that is the point that surprises most. One might think a volume would be needed, in litres or in cubic metres. But a volume would depend on the size of the container: a large bucket collects more than a glass under the same shower. A depth, for its part, depends on nothing. Under the same rain, a bucket and a straight-sided glass fill to the same depth.
It is that independence which makes depth the right unit, and the equivalence given by Météo-France follows directly from it: "1 mm of rain is equivalent to 1 litre of water per m²". A millimetre sounds like little, a litre per square metre speaks more clearly, and yet it is the same thing said twice.
The instrument that measures that depth is the rain gauge. Its shape matters: its walls must be straight and vertical, otherwise the depth read no longer matches the depth that fell. It must be placed away from anything that could shelter it or, on the contrary, water it by running off: a tree, a wall, the edge of a roof. The chapter on measurement will come back to this rule, which holds for every instrument.
On a large scale, it is no longer rain gauges that follow the rain but radars. Météo-France had forty of them at the date on which this volume checked its sources, each with "a range of about 100 km for measurement", and it draws from them "radar images every 5 minutes". The number of radars in a network changes as installations are made: it is the date of verification, in the colophon, that says when this figure speaks from. A radar collects nothing: it sends out a wave and measures what the drops send back to it. That is why it sees rain up in the air, sometimes before it touches the ground, and it is also why a rain gauge remains necessary in order to know what actually fell.
The trap of this chapter: adding millimetres of rain to centimetres of snow. They are not the same quantities. A rainfall reading measures a depth of liquid water; a snow reading measures the thickness of a snow cover, and that thickness depends on the way the flakes have piled up. The same quantity of water can give a thick, light cover in hard frost, or a thin, heavy one at temperatures close to zero. That is why weather services melt the snow they collect before measuring it: they bring everything back to the same unit, the depth of water, without which two readings cannot be compared.
Worked example. How much water did a yard receive?
Question: "A rain gauge recorded 12 mm of rain during the night. The school yard measures 20 metres by 15." What volume of water fell on that yard? Fully guided example.
- Write down what is being looked for
We are looking for a volume of water, in litres, fallen on a known surface. We are looking for neither a depth nor a duration. - Write down what is being used, and why
We use a single relation, the one given by Météo-France: 1 mm of rain is equivalent to 1 litre of water per square metre. It fits exactly, because it links a depth in millimetres to a volume per unit of surface, and because the question gives a depth and a surface. - Work out the surface
The yard measures 20 metres by 15 metres. Its surface is 20 times 15, that is 300 square metres. - Turn the depth into litres per square metre
12 mm of rain is equivalent to 12 litres of water per square metre, by direct application of the relation. - Multiply
Each square metre received 12 litres, and there are 300 square metres. The total volume is 12 times 300, that is 3,600 litres. - Check the order of magnitude
3,600 litres is 3.6 cubic metres, roughly the contents of a small garden tank. For a night of moderate rain on a school yard, the order of magnitude is plausible. A check of the order of magnitude catches most calculation mistakes, and it takes five seconds. - The result
The yard received 3,600 litres of water, that is 3.6 cubic metres. And one understands along the way why a shower of a few millimetres can put a gutter under strain: the depth is small, the surface is not.
Experiment 5. Building and setting up a rain gauge
What you need: a jar or a box with STRAIGHT, vertical walls, a ruler, a permanent marker, a few stones, a notebook. SAFETY: this experiment is carried out with an adult's agreement, and the adult is present. It is the adult who chooses the outdoor location and who wedges the container; nobody climbs on anything, nobody goes near any ledge, and the glass container stays on the ground, wedged with stones, never set up high where it could fall. After a shower, the reading is taken when the adult judges that it is safe to go out.
- Choose a container with straight walls
This is the only constraint on shape, and it is absolute. A jar that widens or narrows falsifies the depth read: the water rises in it faster or slower according to the level, and the reading no longer means anything. - Mark a scale on the container
Hold the ruler against the outside wall, the zero exactly at the inside bottom, and draw a line with the marker every five millimetres. Write the numbers in. Check the position of the zero twice: a scale that is offset falsifies every reading by the same amount, which is the hardest error of all to spot afterwards. - Set the rain gauge in the open
With the adult, choose an open spot, away from a wall, a tree and the edge of a roof. A rain gauge under a tree records less than reality; under a gutter, it records far more. Wedge it with stones so that the wind does not knock it over. - Take a reading after every spell of rain
Write the date, the time and the depth read into the notebook. Always read at eye level and at the bottom of the meniscus, that is to say at the bottom of the curve the water makes against the wall. - Empty it completely after every reading
A rain gauge that is not emptied adds two spells together without saying so. Empty it completely, wipe the bottom, and put it back in the same place. - Turn the readings into litres per square metre
Add a column to the notebook. Every millimetre recorded is worth one litre per square metre. After a month, add them up: you will know how much water your yard or your garden received, and that number will speak more clearly than a list of millimetres. - Compare with a public source
Compare your monthly total with the figure published for your area. A difference is normal, and it is instructive: an official station measures at one point, and rain can vary a great deal over a few kilometres, above all in showers.
What to remember from this chapter
- A droplet becomes a raindrop by growing, either by merging with other droplets or by passing through the solid state at height.
- A precipitation is a particle of water, liquid or solid, which falls through the atmosphere.
- The criterion separating drizzle from rain is a diameter of 0.5 mm; hail is defined by a diameter greater than 5 mm, whereas ice pellets, being finer, are defined by no diameter at the source.
- Rain is measured as a depth, and not as a volume, because a depth does not depend on the size of the container.
- One millimetre of rain is equivalent to one litre of water per square metre.
- A rain gauge with straight walls, set in the open and emptied after every reading, gives a comparable measurement; a radar follows the rain over about a hundred kilometres, with an image every five minutes.
Self-check for chapter 4
Answer in writing, using only the knowledge given by this volume so far. The answers are gathered at the end of the volume.
- A cloud identical to the one of an hour ago is now giving a shower. What has changed, and why is that enough? Then name the forms that what falls from a cloud can take, giving for each the state of the water and the criterion that separates it from its neighbour.
- A rain gauge records 8 mm. A roof measures 9 metres by 6. What volume of water did the gutter have to carry away? Set out your reasoning.
- Why is rain measured as a depth rather than as a volume? Give the argument, not just the rule.
- A rain gauge is placed against a wall, under the overhang of a roof. Cite the two possible measurement errors, and say in which direction each one falsifies the reading. Then say what a weather radar measures, over what range, how often, and why it does not do away with the need for a rain gauge.
- Explain why a weather service melts the snow it collects before measuring it.