temperature

Temperature is a physical quantity that expresses the relative hotness or coldness of a system. It is a measure of the average kinetic energy of its particles, though not a direct measure of energy itself. Temperature governs the direction of heat transfer, from warmer to cooler bodies, until thermal equilibrium is reached.

Common scales include Celsius, Fahrenheit, and Kelvin. In Celsius, 0 °C is the freezing point and 100 °C is the boiling point of water at one atmosphere. In Fahrenheit, water freezes at 32 °F and boils at 212 °F under the same conditions. Kelvin is an absolute scale with 0 K at absolute zero, the theoretical limit of cold.

Measurement relies on thermometers that use physical properties that change with temperature, such as liquid expansion (mercury or alcohol), electrical resistance (thermistors and RTDs), or infrared emission. Accurate readings require calibration against fixed reference points, and international standards define procedures and uncertainty estimates.

Temperature is essential in science and daily life. It affects chemical reaction rates, phase changes, material properties, and biological processes. Temperature data underpin meteorology, climate studies, industrial control, medical diagnostics, and many engineering applications.

Key concepts include absolute zero (0 K), the idea that temperature is not energy itself but a measure of particle motion, and the distinction between temperature and heat. Thermal equilibrium occurs when all parts of a system share the same temperature.