The most commonly used temperature scale in the US today is the Fahrenheit scale, abbreviated F. In this scale, water freezes at 32 degrees and boils at 212 degrees. (This only holds strictly when atmospheric pressure equals the average sea level pressure. At high altitudes, water boils at a lower temperature, as anyone who cooks in the mountains knows.)
Another common scale is the Celsius (also called Centigrade) scale. In this scale, water freezes at 0 degrees and boils at 100 degrees.
To convert between Fahrenheit and Celsius use this formula:
Fahrenheit Temperature = (Celsius Temperature)x(9/5) + 32
There are also temperature scales in which zero is absolute zero, the lowest possible temperature. (People have gotten close to absolute zero, but have never reached it. According to theory, we never will.) Absolute zero is at 273.15 Celsius, or 459.67 Fahrenheit.
The Kelvin temperature scale uses the same size degree as Celsius, but has its zero set to absolute zero. To convert from Celsius to Kelvin, add 273.15 to the Celsius reading.
The Rankine temperature scale uses the same size degree as Fahrenheit, but has its zero set to absolute zero. To convert from Fahrenheit to Rankine, add 459.67 to the Fahrenheit reading.
To convert from Kelvin to Rankine, multiply the Kelvin temperature by 9/5.
Here's one example of temperature comparisons: 68 Fahrenheit is the same as 20 Celsius, 293.15 Kelvin, and 527.67 Rankine. For other comparisons, see the table below.
Fahrenheit  Celsius  Kelvin  comments 

212  100  373.15  water boils 
32  0  273.15  water freezes 
40  40  233.15  Fahrenheit equals Celsius 
320.42  195.79  77.36  liquid nitrogen boils 
452.11  268.95  4.2  liquid helium boils 
459.67  273.15  0  absolute zero 
Our JavaScript temperature converter can give you other temperature comparisons.
Absolute zero, according to current scientific thought, is the lowest temperature that could ever be. In fact, it's so low that we can never quite reach it, although research teams have come within a fraction of a degree. So if we can never get there, how do we know it's really there?
The first clue to the existence of absolute zero came from the expansion and contraction of gasses. We know that hot air rises and cold air falls. Air rises when it's heated because it expands, so it's less dense than the cooler air around it. It has bouyancy, just like a piece of wood in a pond, which floats because it's less dense than the water. Air sinks when it cools because it contracts, so it's more dense than the warmer air around it.
Suppose we took a certain amount of air and cooled it as much as we could. How much would it shrink? When scientists first began studying the behavior of of heated and cooled gasses, they didn't have our modern cooling methods. They measured as best they could over the temperature range that they could reach. Then they plotted their data on graphs.
The graph of volume vs temperature for a sample of gas forms a straight line. (This assumes that you keep the pressure constant.) The lower the temperature, the smaller the volume. If you extend this line to low enough temperatures, it will eventually hit zero volume. Scientists noticed that, for all gasses, the temperature at which the graph said they would reach zero volume was about 273 Celsius (about 460 Fahrenheit). This temperature became known as absolute zero, and is today the zero for the Kelvin and Rankine temperature scales. Nowadays, we know that gasses do not shrink to zero volume when cooled to absolute zero, because they condense into liquids at higher temperatures. However, absolute zero remains one of the basic concepts in cryogenics to this day.
Although nothing can be colder than absolute zero, there are a few physical systems that can have what are called negative absolute temperatures. Oddly enough, such systems are hotter than some with positive temperatures!
