Before you begin

Nobody has ever needed a laboratory to understand where rain comes from. A puddle drying on a pavement, a cold glass covering itself in droplets, a cloud swelling above a hill: the whole mechanism is there, outdoors, free of charge, and it repeats itself every day. This volume starts from those scenes and follows them to the end of what they explain, without asking for anything that costs money.

The house publishes this book because the subject is at once universal and badly served. Universal: water changes state in the same way everywhere, and the sky is read with the same words on every continent. Badly served: what circulates most on this subject are arrow diagrams copied out without knowing what each arrow covers, and weather forecasts watched without knowing how they are made. A copied diagram leaves nobody able to explain why a puddle disappears without boiling.

The intended reader is an adult who teaches, supports or learns alone: a teacher preparing a lesson, a parent answering a question asked in the car, a youth worker at a community centre, someone returning to study and wanting solid foundations. The volume is written to be read by that adult, and so that the explanations and the experiments it contains can be taken up as they stand in front of a child.

The gap this book fills is a precise one. Between the single-page handout, too short to install a progression, and the university textbook, out of reach, a complete course is missing: free, reusable, and holding its statements to public sources that anyone can check. That is exactly what the house makes.

The volume covers the whole journey of a drop of water and the reading of the weather. It begins with the simplest physics, changes of state, because nothing that follows can be explained without it. It goes on with invisible water vapour, clouds, precipitation, what happens to water once it has touched the ground, rivers and their catchments. It then assembles the whole cycle, and follows the man-made journey that runs from the river to the tap and from the tap back to the river. It ends with the everyday habits of weather watching: measuring, keeping a record, reading a forecast, and telling the weather apart from the climate.

The volume does not cover what it cannot teach honestly. It does not explain the mechanics of air masses at the scale of the globe, nor the equations of forecasting models: those objects call for mathematics this volume does not assume. Nor does it deal with climate change for its own sake. It installs the distinction between weather and climate, which is the way in, and it stops where another volume would begin.

And there is one boundary this book never crosses, because it is a dangerous one. This volume explains how water is made fit to drink in a treatment plant; it gives no method whatsoever for making water drinkable at home, and water that is not supplied as drinking water is not drunk on the word of a book. In the same way, this volume explains what the colours of a weather warning map mean and who publishes them; it tells nobody what to do in a storm, a flood or a heatwave. Those instructions belong to the authorities who publish them at the moment the danger exists, and they are read at their source, never in a manual written months earlier.

The chapters of the main body are read in order, and that is not a matter of taste: each chapter states at the top what it assumes is already known, and points to the chapter of this volume that installs it. A reader who skips a chapter will meet words further on that they have not encountered. The first chapter assumes nothing at all.

Every chapter opens on its objectives, written one per line and all beginning with an action verb: name, tell apart, measure, work out. These are observable things. At the end of the chapter, a reader who cannot yet do one of them knows exactly which section to read again, and where.

Every chapter closes on a short summary, then on a self-check of a few questions. These questions are not an examination: they cover the objectives announced at the start, and they ask for nothing that has not been given in the volume at that precise point. Write your answers down, even briefly, before going to look at the answer. An answer one believes one knows and has not put into words is not yet an answer.

The answers are not under the questions. They are gathered in a chapter of their own, at the end of the volume, in reading order: the answer section for a chapter carries the name of that chapter. This move is deliberate, and it has a single reason: an answer visible by turning one's eyes is not an answer one has looked for.

The glossary, also at the end of the volume, takes up the terms the course defines. Use it both ways round: to look up a word you have met without understanding it, and to check afterwards that the word you are using really means what you think it means.

The volume offers experiments to carry out at home. They are designed to use nothing but ordinary objects: water, containers, a plate, a jam jar, a ruler, ice cubes, sand, gravel, a felt pen, a notebook. None of them calls for a chemical, none of them calls for fire, and none of them calls for a laboratory instrument. Only one assumes an ordinary thermometer: if you do not have one, that experiment can simply be read, and the other columns of the record stand without it.

Each description of an experiment carries its safety instruction, written at the point where the risk exists, and the same rule holds for all of them without exception: an experiment in this book is carried out with an adult's agreement, and the adult is present. This is not a copied form of words. Hot water, a glass container, an object set on a ledge, a trip outdoors to take a reading: these are ordinary situations, and it is precisely because they are ordinary that the presence of somebody who decides is worth more than a written instruction.

When an experiment calls for hot water, it is the adult who handles it, from the tap or the kettle all the way to the container, and the child watches. When an experiment takes place outdoors, the adult chooses the spot. An experiment an adult would rather not carry out is not carried out: the volume always gives, alongside it, what the experiment was meant to show, and that explanation reads perfectly well without having done it.

This volume contains almost nothing that belongs to it alone. What it brings is a reading order, worked examples and experiments; all the rest comes from public services that publish their knowledge free of charge, and the house owes its existence to them.

The meteorological facts in this book come from the public pages of Météo-France: the formation of clouds, the definition of precipitation, the measurement of temperature, the making of a forecast, the weather warning scheme and the climatological normals. The classification of clouds comes from the International Cloud Atlas of the World Meteorological Organization, which is the reference document shared by every weather service in the world.

The facts about water come from Eaufrance, the public water information service, from the water agencies that publish teaching material freely available to all, and from the public report on the state of the environment for the abstraction figures. A French regulatory text, published in the Journal officiel and cited at the end of the volume, was consulted in order to situate the notions covered; this volume places itself under no country's curriculum, and it is written to be read anywhere.

The house thanks, last of all, the people who will report the mistakes in this volume. A manual read over twice may still contain some, and a precise report is worth more than a polite indulgence. The sources chapter gives every address consulted and the date on which it was consulted: that is what makes disagreement checkable instead of making it polite.