What Is Temperature?

Temperature is a scalar physical quantity that characterizes the thermal state of a system and determines the direction of spontaneous heat flow between bodies in thermal contact. At the microscopic level, temperature reflects the mean kinetic energy of the constituent particles of a substance, and at the macroscopic level it is formally defined through the laws of thermodynamics: the zeroth law establishes that thermal equilibrium is a transitive relation, which guarantees the existence of a consistent temperature scale, while the second law connects temperature to entropy and the irreversibility of natural processes.

The SI unit of thermodynamic temperature is the kelvin (K), defined since the 2019 revision of the SI by fixing the numerical value of the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. This definition connects temperature to energy in a fundamental way and eliminates the earlier dependence on the triple point of water as a single fixed point. Celsius (°C) and Fahrenheit (°F) scales remain in widespread practical use, with Celsius defined as T(K) − 273.15.

International Temperature Scale and Measurement Standards

Practical temperature measurement relies on the International Temperature Scale of 1990 (ITS-90), a globally adopted convention maintained by national metrology institutes. NIST's thermometry program describes how ITS-90 spans from 0.65 K to 1357.77 K using 17 reproducible fixed points, mostly phase transitions of pure substances, and defined interpolating instruments between them. For temperatures below 273 K, standard platinum resistance thermometers serve as the reference instruments; at high temperatures above 1234 K, radiation thermometers calibrated against Planck's radiation law take over.

ITS-90 is a practical approximation rather than a direct measurement of thermodynamic temperature. Small but measurable differences between ITS-90 values and true thermodynamic temperature have been characterized by metrology laboratories and are periodically updated, as documented in NIST publications on the differences between thermodynamic temperature and ITS-90. For most engineering applications these differences are negligible, but in precision metrology and physics experiments they must be accounted for.

Temperature Measurement

Temperature measurement in engineering relies on a hierarchy of transducer types, each suited to a different range and application. Resistance temperature detectors (RTDs), which exploit the predictable increase of electrical resistance with temperature in metals such as platinum, provide high accuracy from about -200 °C to 850 °C. Thermocouples use the Seebeck effect at the junction of two dissimilar metals to generate a millivolt-level signal and cover ranges from cryogenic temperatures to above 1700 °C. Thermistors, fabricated from semiconducting oxides, offer higher sensitivity than RTDs but over narrower ranges, typically -50 °C to 150 °C.

Non-contact methods are essential when direct access is impractical or would disturb the measurement. Radiation thermometers and infrared pyrometers measure thermal emission from a target surface and infer temperature from the Stefan-Boltzmann relation, provided the emissivity of the surface is known or estimated. Fiber-optic distributed temperature sensing uses Raman backscattering in optical fibers to profile temperature along lengths of hundreds of meters, a technique applied in power cables, oil wells, and structural monitoring.

Temperature Control

In engineering systems, controlling temperature within prescribed bounds is as important as measuring it accurately. Feedback control loops using proportional-integral-derivative (PID) regulators connected to temperature sensors and actuators such as heaters, Peltier modules, or cooling valves are the standard architecture. IEEE Xplore publications on precision temperature control systems document techniques including reverse Seebeck-effect control for sub-millikelvin stability in laboratory instruments.

Applications

Temperature has applications in a range of fields, including:

• Industrial process control in chemical, petrochemical, and food manufacturing
• Medical diagnostics and patient monitoring via body temperature tracking
• Semiconductor fabrication, where thermal budgets govern diffusion, oxidation, and annealing
• Energy systems, including turbine efficiency optimization and heat exchanger design
• Meteorology and climate science through atmospheric and oceanic temperature profiling