Temperature (2024)

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 5degrees or it will be 30degrees? 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
-10	Ice	Very cold still but bearable if in lots of warm clothes
	Water freezes, ice melts	Cold	Typical coldest average temperature for UK in winter
4	Fridge	Cold	Water has its smallest volume at this temperature*
10		Cold
15		Cool
20	Room indoors	Warm
25	Warm room	Warm to hot	Typical maximum temperature for British summer
30	Hot day	Feeling hot	Typical temperature for hotter countries
37	Body temperature	Very hot	This is the average human body temperature
40	Washing machine setting for clothes for normal wash	Very hot	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	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		Water turns to steam
1000	Lava from a volcano
1535	Melting point of iron		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
5500	Surface of the Sun
15 000 000	Core of the Sun

*Note that water expands as its temperature increases above 4 degrees, and as it goes colder below 4degrees. 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:

Remember that when you see a weather forecast on TV, in a newspaper or on the radio, that anything from 20degrees upwards is going to be warm, above 25degrees is hot, above 30degrees is very hot. Below 20 is cool, below 10degrees 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=273.15K. For every increase by 1°C the temperature also increases by 1K. Thus 0K=-273.15°C, 100K=-173.15°C, 273.15K=0°C, and the boiling point of water is 373.15K (100°C).