Temperature is measuring how hot or cold something is, such as the outside air temperature, your body temperature, temperature of water for cooking, boiling, washing, etc.

So, is it cold or hot when the temperature forecast says it will be 5 degrees or it will be 30 degrees? What temperature does water freeze at? Or boil at? What temperature is comfortable for your living room?


Using the modern Celsius system of temperature measurement, we can easily determine how hot or cold it is. It was also known as Centigrade. This is because it is based on the freezing and boiling points of water with 100 degrees between those temperatures.

The main SI unit of temperature is the kelvin, which measures absolute temperatures. But, for our everyday usage, we use Celsius which is directly related to kelvins. It is set so that pure water freezes at zero degrees Celsius and boils at one hundred degrees Celsius.

This is a very easy system to learn as these two marker points are so easy to remember:

Freezing at 0 °C

Boiling at 100 °C

And with 100 degrees between those points, we can specify temperatures using 1 or 2 digit positive numbers. When the temperature is negative, i.e. below 0 °C, then we know that it is so cold that water will freeze. Water will always be ice below 0 °C, liquid from 1 to 99 °C, and steam from 100 °C upwards.

Here is a table showing some guidelines as to Celsius temperatures:

Temperature °C

What might be at this temperature

How it feels

Other points of interest

-30 (that is 30 degrees below zero)

Ice, freezer

Very cold if you are outside in this temperature, unbearable to most

Ice is water below zero degrees Celsius



Very cold still but bearable if in lots of warm clothes

cold outside at minus 10 degrees celsius


Water freezes, ice melts


Typical coldest average temperature for UK in winter




Water has its smallest volume at this temperature*





20 Room indoors Warm
25 Warm room Warm to hot
UK summers are normally about 25 degrees Celsius at the most
Typical maximum temperature for British summer
30 Hot day Feeling hot Typical temperature for hotter countries


Body temperature

Very hot

This is the average human body temperature

40 Washing machine setting for clothes for normal wash Very hot
some parts of the world are very hot at around 40 degrees Celsius
If you are outside in this temperature you will feel hot, but more so if humidity is high.
If your internal body temperature reaches this level you are in danger of death.
50 Extremely hot Not many places get this high in temperature, and this heat you would need to find shade, drink water and stay cool.
60 Washing machine setting for hot wash
washing machine would be set to 60 degrees Celsius for a hot wash
Too hot to live in Most bacteria die at this temperature
90 Washing machine setting for hottest wash Too hot to live in Humans cannot survive such high temperatures
100 Water boils in the kettle
Kettle boils at 100 degrees Celsius
Water turns to steam
1000 Lava from a volcano
Lava is very hot at about 1000 degrees Celsius
1535 Melting point of iron
Iron melts at 1535 degrees Celsius
At this temperature iron will turn into liquid form, to be used in industrial manufacturing processes
2750 Boiling point of iron The temperature at which iron boils and vapourises


Surface of the Sun

Surface of the sun is around 5500 degrees Celsius and its core is around 15 million degrees C

15 000 000

Core of the Sun

*Note that water expands as its temperature increases above 4 degrees, and as it goes colder below 4 degrees. Its volume will be at its smallest at 4 °C. This is important to remember when freezing water in the icebox or freezer, as the water will undoubtedly increase in volume as it freezes, and the colder it gets, the bigger the ice gets. So remember not to put water into a bottle that will be in the freezer for more than a minute or so, as the water inside will expand and break the bottle.

If you have to work in hot conditions, here are guidelines for rest periods needed:


Rest Period (per hour of work)

30 to 32 ºC

10 minutes

32 to 35 ºC

15 minutes

more than 35 ºC

at least 30 minutes

Remember that when you see a weather forecast on TV, in a newspaper or on the radio, that anything from 20 degrees upwards is going to be warm, above 25 degrees is hot, above 30 degrees is very hot. Below 20 is cool, below 10 degrees is cold, and below zero degrees means that it will be icy outside as the water will freeze and it will feel very cold outside.

Kelvin (for the more scientific among us)

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvin is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (0.01 °C). The Celsius temperature scale is now defined in terms of the kelvin.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin. Thus we have another metric unit named after a British person. Most metric units are not foreign at all, unlike °F which is Foreign! The degree Celsius (°C) is named after the Swedish astronomer Anders Celsius (1701–1744) but as it is now defined in terms of the kelvin, it is more British than Fahrenheit, named after a German scientist.

The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike Celsius. This is because Celsius is a scale of measurement, whereas the kelvin is a unit of measurement.

The freezing point of water, as we know from what we read above, is 0 °C, and 0 °C = 273.15 K. For every increase by 1 °C the temperature also increases by 1 K. Thus 0 K = -273.15 °C, 100 K = -173.15 °C, 273.15 K = 0 °C, and the boiling point of water is 373.15 K (100 °C).