Appendix I: Measuring Up - Temperature


Temperature is the measure of how rapidly molecules are vibrating (The "vibration" has to do with thermodynamics, my friend). The coldest a substance can be is called absolute zero, where there is no more vibration. Scientists use the Kelvin scale to measure from absolute zero upward, all the way to the hottest stars. Earth temperatures are several hundred degrees above 0K. Water is a good substance to use as an example for Earth temperatures, because it is so common and everybody knows it well. Water is a solid (ice) when the temperature is less than 273K, it is a liquid (water) between 273K and 373K, and it is a gas (water vapor) above 373K. Earth temperatures are fairly low down on the temperature scale: the Sun has a surface temperature of about 6000K, but it is not so hot, compared to the hottest stars, which are about 30,000K.


So, we use the Kelvin scale to describe the complete range of temperatures possible.


Several other temperature scales have been used to describe the range of Earth temperatures in everyday use. These are the Celsius (Centigrade) and Fahrenheit scales. The Fahrenheit scale has been used in the United States, but the Celsius scale is catching on. The Celsius scale is easier to use because 0C is where water freezes/melts and 100C is where water condenses/vaporizes. The Fahrenheit scale is harder to use in science, because it was defined on less clear boundaries. We are stuck with both, though, and need to learn to convert between them.


Conversion between C and F : Keying on water, here are the basic facts:


                                                            # of units between

     Freezing/Melting             Condensing/Vaporizing     freezing and vaporizing


     273K                         373K                      100

     0C                           100C                      100

     32F (0F is below freezing)   212F                      180


Why is Celsius better? Because:


There are 100 units between freezing and vaporizing. That makes calculations involving water much cleaner. For the Fahrenheit Scale, 212F - 32F = 180 units, as compared to the cleaner Celsius Scale, 100C - 0C = 100 units.


The starting point for the Celsius scale is the freezing/melting point of water, which is a fundamental natural boundary. For the Fahrenheit Scale, 0F is 32 degrees below freezing, which ends up complicating things. It is easy to think about the Celsius scale and what it represents: 0 for freezing/melting and 100 for condensing/vaporizing.


The Celsius units are the same size as those of the Kelvin scale (common basis is water), which comes into play when scientists work with the broader picture.


How do you convert between Celsius and Fahrenheit?


1C = 1.8F


1F = .5556C


To convert C to F, multiply C by 1.8, then add 32 degrees (0C = 32F).


To convert F to C, subtract 32 from F, then divide by 1.8.


Or, you may have seen the 1.8 factor expressed as a fraction, 9/5, which gives the following use of whole number multiplication and division:


To convert C to F, multiply C by 9, then divide by 5, then add 32


To convert F to C, subtract 32 from F, then multiply by 5, then divide by 9.


Keep in your head that there are 100 units (C scale: 100 - 0 = 100 units) vs. 180 units (F scale: 212 - 32 = 180 units), and you can figure it out, even if you have to resort to drawing a number line labeling the C and F values and then spotting your known temperature to do a rough conversion, something like: :


       Freezing/Melting                                               Condensing/Vaporizing

       |                                                                               |

                                            100 units


       0C      10      20      30      40      50      60      70      80      90     100C

       |       |       |       |       |       |       |       |       |       |       |


       |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |

       32F 41  50  59  68  77  86  95  104 113 122 131 140 149 158 167 176 185 194 203 212F


                                            180 units


For reference, consider the extremes in Earth temperatures:


     Coldest (Vostok, Antarctica)            Hottest (Death Valley, California)


     183K                                    330K

     -89.2C                                  56.7C

     -129F                                   134F


Web Sources


Coldest Temperature on Earth


Hottest Temperature on Earth