What is temperature?

Why can’t we trust our hands to tell hot from cold?

Try this at home with an adult. Keep one hand in a bowl of cold water from the fridge and the other in a bowl of warm (not hot) water for a minute. Now put both hands into a bowl of ordinary tap water.

The hand that was in cold water says the tap water is warm. The hand that was in warm water says the same tap water is cool. The water has only one temperature, but each hand compares it with what it felt just before.

So our sense of touch can fool us. To know exactly how hot or cold something is, we need a measurement: its temperature.

So what does temperature tell us?

Temperature is a reliable measure of how hot or cold a body is.

• A hotter body has a higher temperature.
• A colder body has a lower temperature.
• The difference between two temperatures tells us how much hotter one body is than the other.

The device that measures temperature is called a thermometer. You will meet two kinds in this chapter: the clinical thermometer for body temperature and the laboratory thermometer for almost everything else. See clinical vs laboratory thermometer.

What are the units of temperature?

There are three scales you should know. Each is named after the scientist who developed it.

Scale (unit)	Symbol	Where it is used
Celsius (degree Celsius)	°C	Most thermometers at home and in school
Fahrenheit (degree Fahrenheit)	°F	Some digital thermometers; not most scientific work
Kelvin (kelvin)	K	Science. The kelvin is the SI unit of temperature

A healthy body at 37.0 °C reads 98.6 °F on a Fahrenheit thermometer. It is the same temperature, written on a different scale.

How do you change °C into kelvin?

Add 273.15 to the Celsius value:

Temperature in K = temperature in °C + 273.15

• 37.0 °C → 37.0 + 273.15 = 310.15 K
• 25 °C → 25 + 273.15 = 298.15 K
• 0 °C → 0 + 273.15 = 273.15 K

To go back from kelvin to Celsius, subtract 273.15. For example, 300 K − 273.15 = 26.85 °C.

How should you write a temperature?

These small rules come straight from good scientific writing:

1. Leave a space between the number and the unit: 37 °C, not 37°C.
2. The names of the scales start with a capital letter: Celsius, Fahrenheit, Kelvin.
3. In the unit names, “degree” starts with a small letter (degree Celsius), and “kelvin” is written with a small k.
4. The symbols are °C, °F and K. There is no degree sign with K.
5. Use “degrees” for more than one: 25 degrees Celsius.

What is normal body temperature?

The normal temperature of a healthy human body is taken as 37.0 °C. This number is the average of a large number of healthy people, so a perfectly healthy person may be a little above or below it. Your temperature also changes slightly with your age, the time of day and how active you have been.

Human body temperature does not normally go below 35 °C or above 42 °C. When it is higher than normal, we say the person has a fever, and a thermometer, not a touch on the forehead, tells us how high it is.