In the SI system, temperature is measured in what unit?

In the SI (International System of Units) system, temperature is measured in Kelvin (K). This unit is part of the metric system and is used universally in scientific contexts because it provides an absolute scale starting at absolute zero, the point at which molecular motion ceases.

Celsius (C) is widely used in everyday contexts and scientific measurements as well, but it is technically not the SI base unit for temperature. While it aligns closely with Kelvin (with the freezing point of water at 273.15 K), Kelvin is favored in scientific applications because it allows for calculations involving thermal energy and thermodynamics that require absolute temperatures.

Fahrenheit and Rankine are less common in scientific practice. Fahrenheit is primarily used in the United States for non-scientific applications, and Rankine is used primarily in engineering fields, especially in thermodynamics, where calculations involving absolute temperature are needed but in a scale based off Fahrenheit.

Thus, while Celsius might be what many think of for everyday temperature measurement, Kelvin is the standard SI unit for temperature, making it the correct answer in this context.