A typical Celsius thermometer measures a winter day temperature of -17°C.
A temperature is an objective comparative measure of hot or cold. It is measured by a thermometer, which may work through the bulk behavior of a thermometric material, detection of thermal radiation, or particle kinetic energy. Several scales and units exist for measuring temperature, the most common being Celsius (denoted °C; formerly called centigrade), Fahrenheit (denoted °F), and, especially in science, Kelvin (denoted K). In today's Union, Celsius is the most commonly used scale, followed by the Kelvin scale. The fahrenheit scale is rarely used except on Terra, although celsius is still the most used there.
The coldest theoretical temperature is absolute zero, at which the thermal motion in matter would be zero. However, an actual physical system or object can never attain a temperature of absolute zero. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 F on the Fahrenheit scale.
The kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic systems. It indicates the absolute temperature as proportional to the average kinetic energy of the random microscopic motions of their constituent microscopic particles such as electrons, atoms, and molecules.
Temperature is important in all fields of natural science, including physics, geology, chemistry, atmospheric sciences, medicine, and biology—as well as most aspects of daily life.