Tuesday, 15 January 2013

It's cold but not twice as cold!!

Tonight if the weather forecasters are correct, which as I wrote last night, they generally are the temperature is set to fall in my local area to -4 degrees Celsius. Only reaching a minimum temperature of -2 degrees Celsius last night this will prompt some people to say that it will be twice as cold tonight compared to last.

This is of course rubbish. It is not twice as cold even though -4 is twice -2...but to understand why we might need to think about how temperature is measured.

Daniel Fahrenheit described a temperature scale in 1756. His temperature scale was based on three different points. The first was body temperature, or blood temperature. On his scale this was given the number 96 degrees. The second was a mixture of ice and water in equal quantities. This was 32 degrees on his scale. Thirdly given a value of 0 degrees was a mixture of water, ice and the salt ammonium chloride (NH4Cl). The boiling point of water was observed to be 212 degrees exactly 180 degrees higher than the melting point. Following Fahrenheit's original calculations other scientists revised the scale making body temperature approximately 98.5 degrees.

(Temperature scale, http://eo.ucar.edu/skymath/Texamp.gif)

Over the last twenty years the use of the Fahrenheit scale has been largely replaced by another scale, designed by Anders Celsius in 1742. 0 degrees Celsius being the freezing point of water and 100 degrees Celsius being the boiling point of water at pressure of 1 atmosphere. In fact Anders first proposed the scale the other way around with 0 being the boiling point and 100 being the freezing point of water. Nowadays the temperature scale Celsius is actually defined in a slightly more scientific and complicated way, too boring to go into detail here.

William Thomson the 1st Baron Kelvin proposed the primary unit of temperature in 1848. He proposed a scale based on the coldest possible temperature, absolute zero, to be zero on the Kelvin scale. Through his calculations he determined that 0 Kelvin was equivalent to -273 degrees Celsius. This has now been revised to -273.15 degrees Celsius.


Water therefore freezes at approximately 273K and boils at 373K. On the Kelvin scale (one of the seven base units in the International System of Units) the temperature last night would have been 271K and tonight 269K. 269K is clearly not twice as cold as 271K.

In fact for tonight to be twice as cold as last night the temperature would need to plummet to 135.5K or -137.5 degrees Celsius!

Now that is cold.


The coldest temperature ever recorded was -89.2 degrees Celsius and that's cold enough to freeze the nuts off a brass monkey!


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