Why do you have 3 temperature scales?

Answer 1

The reason why there are 3 temperature scales is that people have introduced new temperature scales in order to improve on existing scales. Fahrenheit was first. There was no temperature scale before that. It is a bit awkward because it defines 100o as the normal temperature of the interior of the human body, which is not that convenient to measure (and oral thermometers read 98.6o because they are not quite in the interior of the human body, even though they do go into an orifice of the body). So the Celsius scale is easier to calibrate since it is based on the freezing point of water, at zero degrees, and the boiling point of water at a hundred degrees; these phenomena are relatively easy to observe. But then, there is an even more useful zero point for a temperature scale, which is the point at which there is actually no heat at all, in other words absolute zero, so this gives us degrees Kelvin.

It would be perfectly feasible to use only the Kelvin scale for everything, but then, we are used to using both of the other two scales, which have become well established, and people are reluctant to give them up. So we have 3 scales.