---
title: "Clouds, Rain and Snow"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 7
exercises: 11
source: https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 7 — Clouds, Rain and Snow

It is a hot summer afternoon. At lunchtime the sky was blue, with a few small white [clouds](#def-g4-clouds-rain-snow-cloud) like balls of cotton wool. By three o’clock, one of them has grown. It swells upwards, higher and higher, like a white cauliflower standing on a flat grey base. Its top spreads out. Its base turns dark. A gust of cool wind blows, the first big drops fall, and for twenty minutes it pours. Then the [cloud](#def-g4-clouds-rain-snow-cloud) moves away, the sun comes back, and the wet pavements steam.

Where did that [cloud](#def-g4-clouds-rain-snow-cloud) come from? What is it made of? And how much [rain](#def-g4-clouds-rain-snow-precipitation) fell in those twenty minutes?

**You already know.**

The [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) is the state of the air at a given place and time: sunny, cloudy, rainy, snowy, windy, foggy; we write it down on a [weather chart](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather-chart) ([Chapter 1](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#ch-g1-watching-the-weather)). Water from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), the [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and the ground goes up into the air, and comes back down as [rain](#def-g4-clouds-rain-snow-precipitation) ([Chapter 3](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#ch-g2-water-on-earth)). The physics books tell how water evaporates (liquid to an invisible gas), condenses (gas back to tiny drops of liquid) and freezes (liquid to ice), and how a thermometer measures the temperature in degrees Celsius.

## 7.1 What is a cloud?

The air around us always holds some water, as an invisible gas: water vapour. When air rises, it cools. When it cools enough, part of its water vapour condenses into droplets of water so tiny that the slightest breath of air keeps them floating. Billions of these droplets, side by side, make a [cloud](#def-g4-clouds-rain-snow-cloud) we can see. High up, where the air is very cold, the droplets freeze, and the [cloud](#def-g4-clouds-rain-snow-cloud) is made of tiny pieces of ice.

**Definition 7.1 (Cloud and fog).**

A *cloud* is a great number of tiny droplets of water, or tiny pieces of ice, floating together in the air. *Fog* is a cloud that touches the ground.

**Example 7.2 (Your breath on a cold morning).**

On a cold morning, your breath makes a little white [cloud](#def-g4-clouds-rain-snow-cloud) in front of your mouth. The air you breathe out is warm and holds water vapour. The cold air cools it at once, the vapour condenses into tiny droplets — and a miniature [cloud](#def-g4-clouds-rain-snow-cloud) appears, then vanishes as the droplets evaporate again.

A [cloud](#def-g4-clouds-rain-snow-cloud) is white because its droplets scatter the sunlight in every direction. When it is very thick, little light gets through to its base, and from below it looks grey, or nearly black: that is the sign of a heavy [cloud](#def-g4-clouds-rain-snow-cloud), full of water.

## 7.2 The cloud families

[Clouds](#def-g4-clouds-rain-snow-cloud) take many shapes, and [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) observers all over the world sort them in the same way, by their shape and by how high they float.

**Definition 7.3 (Cloud families).**

The four [cloud](#def-g4-clouds-rain-snow-cloud) families to know are:

- *cumulus* : heaped [clouds](#def-g4-clouds-rain-snow-cloud) , white and puffy, with a flat base, like balls of cotton wool;
- *stratus* : layered [clouds](#def-g4-clouds-rain-snow-cloud) , a flat grey sheet that covers the whole sky, low down;
- *cirrus* : wispy [clouds](#def-g4-clouds-rain-snow-cloud) , thin white threads or curls, very high, made of ice;
- *cumulonimbus* : the storm [cloud](#def-g4-clouds-rain-snow-cloud) , a huge tower that rises from a low dark base to a very great height, and whose top often spreads out flat.

The names are Latin words: *[cumulus](#def-g4-clouds-rain-snow-cloud-families)* means a heap, *[stratus](#def-g4-clouds-rain-snow-cloud-families)* a layer, *[cirrus](#def-g4-clouds-rain-snow-cloud-families)* a curl of hair, and *nimbus* a [rain](#def-g4-clouds-rain-snow-precipitation) [cloud](#def-g4-clouds-rain-snow-cloud).

![Photographs: NOAA Photo Library (public domain): S. Corfidi (cumulus), R. F. Kresge (cirrus), B. Hooke collection (cumulonimbus).](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-3a2a62bfc7ed.jpg)

![Photographs: NOAA Photo Library (public domain): S. Corfidi (cumulus), R. F. Kresge (cirrus), B. Hooke collection (cumulonimbus).](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-0d3e09bbc769.jpg)

![Photographs: NOAA Photo Library (public domain): S. Corfidi (cumulus), R. F. Kresge (cirrus), B. Hooke collection (cumulonimbus).](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-4e7d0b060dc6.jpg)

![Photographs: NOAA Photo Library (public domain): S. Corfidi (cumulus), R. F. Kresge (cirrus), B. Hooke collection (cumulonimbus).](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-82441c63fa2e.jpg)

*[Cumulus](#def-g4-clouds-rain-snow-cloud-families), heaped [clouds](#def-g4-clouds-rain-snow-cloud) with flat bases.*

*[Stratus](#def-g4-clouds-rain-snow-cloud-families): a flat layer, here seen from above, from a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).*

*[Cirrus](#def-g4-clouds-rain-snow-cloud-families): thin threads of ice, very high (a small [cumulus](#def-g4-clouds-rain-snow-cloud-families) below).*

*A [cumulonimbus](#def-g4-clouds-rain-snow-cloud-families): a tower of [cloud](#def-g4-clouds-rain-snow-cloud); [rain](#def-g4-clouds-rain-snow-precipitation) falls from its dark base.*

*Photographs: NOAA Photo Library (public domain): S. Corfidi ([cumulus](#def-g4-clouds-rain-snow-cloud-families)), R. F. Kresge ([cirrus](#def-g4-clouds-rain-snow-cloud-families)), B. Hooke collection ([cumulonimbus](#def-g4-clouds-rain-snow-cloud-families)).*

How high do [clouds](#def-g4-clouds-rain-snow-cloud) float? It depends on the family. Low [clouds](#def-g4-clouds-rain-snow-cloud), like [stratus](#def-g4-clouds-rain-snow-cloud-families) and the base of [cumulus](#def-g4-clouds-rain-snow-cloud-families), float from just above the ground up to about 2 kilometres. [Cirrus](#def-g4-clouds-rain-snow-cloud-families) float very high, between about 5 and 13 kilometres outside the tropics and the polar regions — often higher than the highest [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) on Earth. A [cumulonimbus](#def-g4-clouds-rain-snow-cloud-families) stretches all the way from a low base to the very top of the [cloud](#def-g4-clouds-rain-snow-cloud) levels.

![How high the four cloud families float (heights in kilometres, for regions that are neither tropical nor polar). The stratus and the base of the cumulus are low; cirrus are often higher than the highest mountain. A cumulonimbus stretches from a low base to the top of the cloud levels. Heights after the World Meteorological Organization’s cloud atlas.](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/fig-3e6f943536d4.svg)

*How high the four [cloud](#def-g4-clouds-rain-snow-cloud) families float (heights in kilometres, for regions that are neither tropical nor polar). The [stratus](#def-g4-clouds-rain-snow-cloud-families) and the base of the [cumulus](#def-g4-clouds-rain-snow-cloud-families) are low; [cirrus](#def-g4-clouds-rain-snow-cloud-families) are often higher than the highest [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). A [cumulonimbus](#def-g4-clouds-rain-snow-cloud-families) stretches from a low base to the top of the [cloud](#def-g4-clouds-rain-snow-cloud) levels. Heights after the World Meteorological Organization’s [cloud](#def-g4-clouds-rain-snow-cloud) atlas.*

**Method 7.4 (Naming a cloud).**

Look at the sky and ask three questions:

1. Is it a heap with a flat base? Then it is a [cumulus](#def-g4-clouds-rain-snow-cloud-families) — and if it rises into a huge dark tower, a [cumulonimbus](#def-g4-clouds-rain-snow-cloud-families) .
2. Is it a flat, even sheet that hides the whole sky? Then it is a [stratus](#def-g4-clouds-rain-snow-cloud-families) .
3. Is it made of thin white threads, very high? Then it is a [cirrus](#def-g4-clouds-rain-snow-cloud-families) .

**Remark 7.5 (Clouds tell the weather).**

Small [cumulus](#def-g4-clouds-rain-snow-cloud-families) in a blue sky mean fair [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather). A grey [stratus](#def-g4-clouds-rain-snow-cloud-families) brings a dull day, sometimes with drizzle. A [cumulonimbus](#def-g4-clouds-rain-snow-cloud-families) brings a short, heavy shower, often with thunder, lightning and gusts of wind — sometimes [hail](#def-g4-clouds-rain-snow-precipitation). [Cirrus](#def-g4-clouds-rain-snow-cloud-families) bring no [rain](#def-g4-clouds-rain-snow-precipitation), but when they spread and thicken, [rain](#def-g4-clouds-rain-snow-precipitation) often follows a day later.

## 7.3 Rain, snow and hail

Inside a [cloud](#def-g4-clouds-rain-snow-cloud), the tiny droplets bump into one another and join, or ice crystals grow at the expense of the droplets. A [cloud](#def-g4-clouds-rain-snow-cloud) drop, or a crystal, that has grown big and heavy enough can no longer float: it falls.

**Definition 7.6 (Precipitation).**

*Precipitation* is water that falls from a [cloud](#def-g4-clouds-rain-snow-cloud) to the ground: *rain* (drops of liquid water), *snow* (flakes made of ice crystals stuck together) or *hail* (balls of ice, called hailstones, that fall from storm [clouds](#def-g4-clouds-rain-snow-cloud)).

Whether it [rains](#def-g4-clouds-rain-snow-precipitation) or [snows](#def-g4-clouds-rain-snow-precipitation) depends on the temperature of the air the [precipitation](#def-g4-clouds-rain-snow-precipitation) falls through. Snowflakes form high in the cold [cloud](#def-g4-clouds-rain-snow-cloud). If the air below is above $0\,{}^{\circ}\mathrm{C}$ for long enough, they melt on the way down and reach us as [rain](#def-g4-clouds-rain-snow-precipitation). If the air is freezing all the way to the ground, they arrive as [snow](#def-g4-clouds-rain-snow-precipitation).

![Children catching snowflakes on a dark glove: each flake is made of ice crystals.](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-835e6ac65a33.jpg)

*Children catching snowflakes on a dark glove: each flake is made of ice crystals.*

[Hail](#def-g4-clouds-rain-snow-precipitation) falls only from [cumulonimbus](#def-g4-clouds-rain-snow-cloud-families). Inside the tower, strong upward winds carry ice pellets up and down several times; each trip adds a new layer of ice, like the layers of an onion, until the hailstone is too heavy for the wind to hold.

![A very large hailstone held in a gloved hand. The rings show the layers of ice it gathered inside the storm cloud. Photograph: NOAA Photo Library (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-c4ae3528fb51.jpg)

*A very large hailstone held in a gloved hand. The rings show the layers of ice it gathered inside the storm [cloud](#def-g4-clouds-rain-snow-cloud). Photograph: NOAA Photo Library (public domain).*

## 7.4 Measuring the weather

To say how much it rained, how warm it was or where the wind blew from, words are not enough: we need instruments, and numbers.

**Definition 7.7 (Rain gauge).**

A *rain gauge* is a container that collects the [rain](#def-g4-clouds-rain-snow-precipitation) falling on its opening. The depth of water it collects is measured in millimetres: “5 millimetres of [rain](#def-g4-clouds-rain-snow-precipitation)” means that the [rain](#def-g4-clouds-rain-snow-precipitation) would have made a layer of water 5 millimetres deep on flat ground, if none of it had run off or soaked in.

**Remark 7.8 (A millimetre of rain is a lot of water).**

A millimetre of [rain](#def-g4-clouds-rain-snow-precipitation) looks tiny in the gauge. But on a square of ground one metre by one metre, it is one litre of water: a big bottle. On a school playground of one thousand square metres, a [rain](#def-g4-clouds-rain-snow-precipitation) of 10 millimetres brings ten thousand litres: enough to fill a thousand buckets of 10 litres each.

![A simple rain gauge: a funnel catches the rain and a tube marked in millimetres holds it. Read the level of the water, with your eye at the same height. Here, 24 millimetres have fallen since the gauge was last emptied.](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/fig-67d8fcd1ef9e.svg)

*A simple [rain gauge](#def-g4-clouds-rain-snow-rain-gauge): a funnel catches the [rain](#def-g4-clouds-rain-snow-precipitation) and a tube marked in millimetres holds it. Read the level of the water, with your eye at the same height. Here, 24 millimetres have fallen since the gauge was last emptied.*

**Definition 7.9 (Wind vane).**

A *wind vane* is an arrow that turns freely on a mast. The wind pushes its tail, so the arrow always points to the direction the wind is coming *from*.

**Definition 7.10 (Weather station).**

A *weather station* is a place where the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) is measured every day, always in the same way, with instruments: a thermometer in a ventilated white box, a [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) in the open, a [wind vane](#def-g4-clouds-rain-snow-wind-vane) on a mast.

The thermometer is kept in the shade, in a white box with slatted sides that let the air through but keep out the sunshine and the [rain](#def-g4-clouds-rain-snow-precipitation). The Met Office, the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) service of the United Kingdom, sets it $1.25\,\mathrm{m}$ above the ground. The [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) stands in the open, away from walls and trees that would shelter it from the [rain](#def-g4-clouds-rain-snow-precipitation), its opening a little above the ground. The [wind vane](#def-g4-clouds-rain-snow-wind-vane) stands high on its mast, above the obstacles that would turn the wind.

![A small weather station. The thermometer is in the shade, in a white box with slatted sides, about 1.25 metres above the grass; the rain gauge stands in the open, far from the tree; the wind vane is high above the obstacles.](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/fig-27a9a4a98351.svg)

*A small [weather station](#def-g4-clouds-rain-snow-weather-station). The thermometer is in the shade, in a white box with slatted sides, about 1.25 metres above the grass; the [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) stands in the open, far from the tree; the [wind vane](#def-g4-clouds-rain-snow-wind-vane) is high above the obstacles.*

**In the field — A weather station in the schoolyard.**

Many schools keep a small [weather station](#def-g4-clouds-rain-snow-weather-station) in a corner of the playground. Every morning at the same time, two pupils read the thermometer, read and empty the [rain gauge](#def-g4-clouds-rain-snow-rain-gauge), and note where the [wind vane](#def-g4-clouds-rain-snow-wind-vane) points. Over the weeks, the class builds a record of its own [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather).

![Pupils reading the rain gauge in the schoolyard.](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/img-36638b68e19e.jpg)

*Pupils reading the [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) in the schoolyard.*

**Method 7.11 (Drawing a bar chart of the rain).**

To draw the [rain](#def-g4-clouds-rain-snow-precipitation) of a week:

1. draw a horizontal line and write the days under it, equally spaced;
2. draw a vertical line on the left and mark it in millimetres, with equal steps (for example every 2 millimetres);
3. above each day, draw a bar as high as the [rain](#def-g4-clouds-rain-snow-precipitation) of that day; a day with no [rain](#def-g4-clouds-rain-snow-precipitation) gets no bar at all;
4. give the chart a title and write the unit, mm, on the vertical line.

**Example 7.12 (A week of rain at school).**

The class read the [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) every morning for a week and found: Monday 0, Tuesday 4, Wednesday 12, Thursday 7, Friday 0, Saturday 2, Sunday 5 millimetres. The wettest day was Wednesday. It did not [rain](#def-g4-clouds-rain-snow-precipitation) on Monday nor on Friday. In the whole week, $0 + 4 + 12 + 7 + 0 + 2 + 5 = 30$ millimetres fell. Wednesday alone brought 12 of the 30 millimetres, that is $\frac{12}{30} = \frac{2}{5}$ of the week’s [rain](#def-g4-clouds-rain-snow-precipitation).

![The week of as a bar chart.](https://one-course.com/images/onecourse/chapters/earth-1/g4-clouds-rain-snow/fig-defbc1d1015b.svg)

*The week of [Example 7.12](#ex-g4-clouds-rain-snow-week) as a bar chart.*

## 7.5 Exercises

**Exercise 7.1 ★.**

What is a [cloud](#def-g4-clouds-rain-snow-cloud) made of? What is [fog](#def-g4-clouds-rain-snow-cloud)?

**Solution of Exercise 7.1.**

A [cloud](#def-g4-clouds-rain-snow-cloud) is made of a great number of tiny droplets of water, or tiny pieces of ice, floating in the air. [Fog](#def-g4-clouds-rain-snow-cloud) is a [cloud](#def-g4-clouds-rain-snow-cloud) that touches the ground.

**Exercise 7.2 ★.**

Name the [cloud](#def-g4-clouds-rain-snow-cloud): (a) a flat grey sheet that hides the whole sky; (b) thin white threads very high in a blue sky; (c) white heaps with flat bases, like cotton wool; (d) a huge dark tower with a flat top, bringing thunder.

**Solution of Exercise 7.2.**

(a) [Stratus](#def-g4-clouds-rain-snow-cloud-families). (b) [Cirrus](#def-g4-clouds-rain-snow-cloud-families). (c) [Cumulus](#def-g4-clouds-rain-snow-cloud-families). (d) [Cumulonimbus](#def-g4-clouds-rain-snow-cloud-families).

**Exercise 7.3 ★.**

[Rain](#def-g4-clouds-rain-snow-precipitation), [snow](#def-g4-clouds-rain-snow-precipitation) or [hail](#def-g4-clouds-rain-snow-precipitation)? (a) balls of ice that fall from a storm [cloud](#def-g4-clouds-rain-snow-cloud); (b) drops of liquid water; (c) flakes made of ice crystals.

**Solution of Exercise 7.3.**

(a) [Hail](#def-g4-clouds-rain-snow-precipitation). (b) [Rain](#def-g4-clouds-rain-snow-precipitation). (c) [Snow](#def-g4-clouds-rain-snow-precipitation).

**Exercise 7.4 ★.**

Which instrument measures: (a) the [rain](#def-g4-clouds-rain-snow-precipitation); (b) the temperature; (c) where the wind comes from?

**Solution of Exercise 7.4.**

(a) A [rain gauge](#def-g4-clouds-rain-snow-rain-gauge). (b) A thermometer. (c) A [wind vane](#def-g4-clouds-rain-snow-wind-vane).

**Exercise 7.5 ★.**

In the [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) drawn on page [45](#fig-g4-clouds-rain-snow-gauge), the water now reaches the mark 17. How many millimetres of [rain](#def-g4-clouds-rain-snow-precipitation) have fallen? How many more millimetres would bring it to 20?

**Solution of Exercise 7.5.**

17 millimetres of [rain](#def-g4-clouds-rain-snow-precipitation) have fallen. $20 - 17 = 3$: 3 more millimetres would bring the water to the mark 20.

**Exercise 7.6 ★.**

Look at the bar chart on page [46](#fig-g4-clouds-rain-snow-bars). Which day was the wettest? On which days did it not [rain](#def-g4-clouds-rain-snow-precipitation)? Which day had 7 millimetres?

**Solution of Exercise 7.6.**

The wettest day was Wednesday (12 millimetres). It did not [rain](#def-g4-clouds-rain-snow-precipitation) on Monday and on Friday. Thursday had 7 millimetres.

**Exercise 7.7 ★★.**

On the morning of a snowy day, the snowflakes reach the ground in the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) village, but in the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) below it [rains](#def-g4-clouds-rain-snow-precipitation). Explain the difference, using the temperature of the air.

**Solution of Exercise 7.7.**

The snowflakes form high in the cold [cloud](#def-g4-clouds-rain-snow-cloud). Up in the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) village the air is freezing all the way down, so they arrive as [snow](#def-g4-clouds-rain-snow-precipitation). Lower down, in the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the air is warmer than $0\,{}^{\circ}\mathrm{C}$: on the way down through it the flakes melt, and they reach the ground as [rain](#def-g4-clouds-rain-snow-precipitation).

**Exercise 7.8 ★★.**

Another week, the class finds: Monday 3, Tuesday 0, Wednesday 1, Thursday 9, Friday 6, Saturday 0, Sunday 1 millimetres. Draw the bar chart. How many millimetres fell in the week? Which fraction of the week’s [rain](#def-g4-clouds-rain-snow-precipitation) fell on Thursday?

**Solution of Exercise 7.8.**

The bar chart has seven bars of heights 3, 0, 1, 9, 6, 0 and 1 (no bar on Tuesday and Saturday). In the week, $3 + 0 + 1 + 9 + 6 + 0 + 1 = 20$ millimetres fell. Thursday brought 9 of the 20 millimetres: $\frac{9}{20}$ of the week’s [rain](#def-g4-clouds-rain-snow-precipitation).

**Exercise 7.9 ★★.**

Why must the [rain gauge](#def-g4-clouds-rain-snow-rain-gauge) stand in the open, far from walls and trees? Why is the thermometer kept in a white box in the shade, and not in full sun?

**Solution of Exercise 7.9.**

Walls and trees shelter the ground near them from the [rain](#def-g4-clouds-rain-snow-precipitation), especially when the wind blows the [rain](#def-g4-clouds-rain-snow-precipitation) sideways, and drops can drip from leaves into the gauge: near them the gauge would catch too little, or too much. The thermometer has to measure the temperature of the air; in full sun it would be warmed by the sunshine itself and would read too high. The white slatted box keeps the sun off and lets the air through.

**Exercise 7.10 ★★.**

The [wind vane](#def-g4-clouds-rain-snow-wind-vane) points to the west. Where does the wind come from? Where is it going?

**Solution of Exercise 7.10.**

A [wind vane](#def-g4-clouds-rain-snow-wind-vane) points to where the wind comes from: the wind comes from the west. It is going towards the east.

**Exercise 7.11 ★★★.**

On a playground of 500 square metres, 8 millimetres of [rain](#def-g4-clouds-rain-snow-precipitation) fall. Using [Remark 7.8](#rem-g4-clouds-rain-snow-litre), how many litres of water fell on the playground? A watering can holds 10 litres: how many watering cans is that?

**Solution of Exercise 7.11.**

One millimetre of [rain](#def-g4-clouds-rain-snow-precipitation) on one square metre is one litre. On one square metre, 8 millimetres give 8 litres. On 500 square metres: $500 \times 8 = 4000$ litres. With watering cans of 10 litres: $4000 \div 10 = 400$ watering cans.