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.

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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?

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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 °CWhat might be at this temperatureHow it feelsOther points of interest

-30 (that is 30 degrees below zero)

Ice, freezer

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

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-10

Ice

Very cold still but bearable if in lots of warm clothes


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0

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

20Room indoorsWarm
25Warm roomWarm to hot
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Typical maximum temperature for British summer
30Hot dayFeeling hotTypical temperature for hotter countries

37

Body temperature

Very hot

This is the average human body temperature

40Washing machine setting for clothes for normal washVery hot
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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.
50Extremely hotNot many places get this high in temperature, and this heat you would need to find shade, drink water and stay cool.
60Washing machine setting for hot wash
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Too hot to live inMost bacteria die at this temperature
90Washing machine setting for hottest washToo hot to live inHumans cannot survive such high temperatures
100Water boils in the kettle
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Water turns to steam
1000Lava from a volcano
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1535Melting point of iron
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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:

TemperatureRest 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.

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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).