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

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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

| | | | | | | | | | |

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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)

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183K 330K

-89.2C 56.7C

-129F 134F