DEFINITION: (For this website usage):

Temperature: A measure proportional to the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules. The flow of heat is from a high temperature region toward a lower temperature region.

– Courtesy of the references cited below but mostly based on the operational definition on the HyperPhysics pages at Georgia State University by R. Nave. and the discussion in “Traceable Temperatures” Second Ed. by Nicholas and White-See the discussions and definitions below for our rationale.

Looking at textbooks and related references and even websites for a definition of temperature can be very frustrating. Many talk around the subject and never get to the point or never state a clear definition.

Oh, yes, we can find information about the fact that temperature is not heat and what temperature scales are. We know it’s related to heat and there is a difference between them.

What is it? It’s got to be on the Web…..

Bottom line. It is.

However it is like finding the needle in a haystack; just like many topics on the web. Search engines do not help much; it takes digging by someone who understands the subject.

A serious group of experts at The American Society for Testing and Materials (ASTM) publishes standards about devices and information on how to calibrate, test and specify temperature sensors. But their standard on Terminology, ASTM E-344, is notably missing a definition for “Temperature”! Surprise!

Is it just us, or have you, too, noticed that there seems to be a real difficulty in defining or agreeing the meaning of the word and concept of “Temperature”.

(Next Bottom line: It is not a simple concept, despite all the work done on it over the past few hundred years. Yes, it can be defined, but not easily. In many cases simplified explanations, not true definitions, are used. See the HyperPhysics Web site at Georgia State University for the clearest and best explained definition we have seen to date. Their definition appears at the bottom of this page.)

Temperature-Do we know what it is?
Here are some samples of definitions that we have found:

1. “Temperature is the degree of ‘hotness’ of a body: more precisely it is the potential for heat transfer. In our everyday lives, we are aware of different temperatures through the sensation of touch, but how hot or cold something feels is subjective. We can say that the kettle is hotter than the ice-cream, but not by how much. Measurement, on the other hand, must be objective and a thermometer is used.”
National Physical Laboratory, Teddington, Middlesex, UK, TW11 0LW © Crown Copyright 2001. Reproduced by permission of the Controller of HMSO.

2. “What is Temperature?
“In a qualitative manner, we can describe the temperature of an object as that which determines the sensation of warmth or coldness felt from contact with it. It is easy to demonstrate that when two objects of the same material are placed together (physicists say when they are put in thermal contact), the object with the higher temperature cools while the cooler object becomes warmer until a point is reached after which no more change occurs, and to our senses, they feel the same. When the thermal changes have stopped, we say that the two objects (physicists define them more rigorously as systems) are in thermal equilibrium . We can then define the temperature of the system by saying that the temperature is that quantity which is the same for both systems when they are in thermal equilibrium.”
From The Popular Website “About Temperature” (since 21 Nov 1995).
“About Temperature” Disponible en espanol

3. Temperature, when measured in Kelvin degrees, is a number that is directly proportional to the average kinetic energy of the molecules in a substance. So, when the molecules of a substance have a small average kinetic energy, then the temperature of the substance is low.From The Physics and Mathematics Web Site “Zona Land”

NOTE: This website also has an interesting page that graphically illustrates the relationship between molecular motion and the temperature of a gas.

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