Temperature and Thermometers

What is temperature?

Temperature tells us how hot or cold something is. We measure it in degrees Celsius, written °C. A warm summer day might be 25 °C, a cold winter morning might be 1 °C, and your body is normally about 37 °C.

We read temperature using a thermometer. Most school thermometers have a thin tube of coloured liquid. When it gets warmer, the liquid expands and rises up the tube. When it gets colder, the liquid shrinks back down. The number next to the top of the liquid is the temperature.

A thermometer is a vertical scale

A thermometer is really just a number line standing up, so reading one uses the same skill as any other scale. The first job is always to work out what one small mark is worth. You may already know this idea from Reading Scales and Measuring.

To find the value of one mark:

1. Find two numbered marks.
2. Subtract to get the gap.
3. Count the spaces between them.
4. Divide the gap by the number of spaces.

Worked example 1: simple marks

A thermometer is labelled 0, 10, 20, 30, with 10 small spaces between each pair of numbers.

1. Gap: 10 − 0 = 10.
2. Spaces: 10.
3. One space: 10 ÷ 10 = 1 °C.

If the liquid stops on the 4th mark above 20, the temperature is 20 + 4 = 24 °C.

Worked example 2: marks worth more than one

A thermometer shows 0, 20, 40, with 5 spaces between each pair.

1. Gap: 20 − 0 = 20.
2. Spaces: 5.
3. One space: 20 ÷ 5 = 4 °C.

If the liquid reaches the 3rd mark above 20, count on in fours: 20, 24, 28, 32. The temperature is 32 °C.

Two temperatures worth remembering

Celsius was cleverly built around water:

• 0 °C — water freezes into ice.
• 100 °C — water boils into steam.

These two facts make it easy to judge whether a temperature is icy or boiling. Anything near 0 °C is cold enough for frost. Anything near 100 °C would scald you.

Below zero: negative temperatures

On a very cold day, the temperature can drop below zero. We write these with a minus sign:

• −1 °C, −5 °C, −10 °C

The further below zero, the colder it is. So −10 °C is colder than −2 °C, even though 10 is a bigger number than 2. On a thermometer, the negative numbers sit below the zero mark, going down. This is the same idea you meet in Introduction to Integers.

Temperature	What it feels like
−10 °C	Freezing, deep frost
0 °C	Icy, water turning to ice
10 °C	Cool, need a coat
20 °C	Comfortable, mild
30 °C	Hot summer day
37 °C	Normal body temperature
100 °C	Boiling water

Working out a change in temperature

Sometimes you need the difference between two temperatures. Count along the scale from one to the other.

The morning temperature was −3 °C. By the afternoon it was 6 °C. How much did it rise?

1. From −3 up to 0 is 3 degrees.
2. From 0 up to 6 is 6 degrees.
3. Total rise: 3 + 6 = 9 °C.

Counting through zero in two parts keeps you from making mistakes with the minus sign.

Why thermometers matter

Reading temperature accurately keeps us safe and healthy. Nurses check if you have a fever, weather forecasters warn of ice on the roads, and cooks make sure food is hot enough to be safe. Because a thermometer is just a scale, the very same "find one interval" skill you use here will help you read fuel gauges, speedometers and measuring jugs for the rest of your life.

Try it yourself

If you have a thermometer at home (even a weather one), try this:

1. Work out what one small mark is worth.
2. Read the temperature indoors.
3. Read it again outdoors or near a window.
4. Find the difference between the two readings.

No thermometer? Look up today's forecast and write down the highest and lowest temperatures, then find the difference.

Well done!

You can now read a thermometer by finding the value of one mark, you know that water freezes at 0 °C and boils at 100 °C, and you understand negative temperatures below zero. Strengthen the negative-number idea in Introduction to Integers, or practise more scales in Reading Scales and Measuring.