The kelvin (symbol K) is the base unit of thermodynamic temperature in the International System of Units (SI). Unlike Celsius and Fahrenheit, the kelvin scale starts at absolute zero (0 K = −273.15 °C), the theoretical point where all molecular motion ceases. One kelvin increment equals exactly one degree Celsius — only the starting point differs — making conversions simple: T(K) = T(°C) + 273.15.
In meteorology, kelvin is used extensively in scientific calculations: radiative transfer equations, the Clausius-Clapeyron relation for water vapour capacity, potential temperature computations, and satellite-derived brightness temperatures all work in kelvin. Weather models run their thermodynamics in kelvin internally, converting to Celsius only for output. The concept of potential temperature (θ) — the temperature an air parcel would have if brought adiabatically to 1000 hPa — is always expressed in kelvin and is fundamental for identifying air masses.
For everyday meteorology, the practical importance of kelvin lies in understanding why scientists and models use it: it eliminates the complications of negative numbers in physical equations and provides a true ratio scale (200 K contains half the thermal energy of 400 K). Surface temperatures on Earth range from roughly 184 K (−89 °C, Antarctica record) to 330 K (57 °C, Death Valley record). The Sun's surface radiates at about 5,778 K, determining the spectrum of solar radiation that drives our weather.