Why is the thermodynamic temperature scale K (Kelvin) not written as °K?

The Kelvin scale is not written with the degree symbol because it is an absolute thermodynamic temperature scale defined by a fixed point and an increment, not by arbitrary reference points like the Celsius or Fahrenheit scales. This distinction is formalized by the International System of Units (SI), which designates the kelvin (symbol K) as a base unit for thermodynamic temperature. The defining shift occurred in 1967-1968 when the General Conference on Weights and Measures (CGPM) redefined the kelvin from "degree Kelvin" (°K) to simply "kelvin" (K). This change was not merely stylistic; it underscored that the unit is an absolute measure starting from absolute zero, a fundamental physical minimum, rather than being a relative "degree" of difference between two arbitrary states. Consequently, stating "300 K" correctly implies an absolute quantity of thermal energy, analogous to stating a length as "3 meters," where no degree symbol is used.

The removal of the degree symbol reinforces the conceptual framework of thermodynamics, where temperature is a measure of the average kinetic energy of particles. The kelvin is defined by fixing the numerical value of the Boltzmann constant, linking it directly to the joule, the unit of energy. This makes the kelvin a unit of measurement on an absolute scale where zero represents the complete absence of thermal motion. In contrast, scales like Celsius, which use the degree symbol, are relative; 0°C and 100°C are defined by the properties of water at specific pressures, making them convenient for everyday use but derivative from the absolute scale. Writing "°K" would conceptually conflate this absolute unit with a relative one, blurring a critical scientific distinction. The notation aligns with usage for other SI base units, such as the meter (m) or second (s), which also stand without modifiers, promoting consistency and clarity in scientific communication.

Adherence to this convention has practical implications in scientific and technical contexts, preventing potential errors in interpretation, particularly in fields like cryogenics, astrophysics, and physical chemistry where absolute temperatures are fundamental. For instance, equations in statistical mechanics and thermodynamics, such as the ideal gas law or the Planck radiation law, require temperature input in an absolute scale; the consistent "K" notation immediately signals this requirement. While the older "°K" notation persists in some historical literature or informal usage, its formal abandonment over five decades ago is universally upheld in modern metrology, academic publishing, and standards documentation. The persistence of the degree symbol with Celsius and Fahrenheit is a legacy of their historical development as relative scales, a legacy deliberately not extended to the kelvin to emphasize its foundational role in the SI system and in expressing fundamental physical laws.