Kelvina

Meet Kelvina, a thermometer that starts counting at absolute zero.

The Celsius scale puts zero at the freezing point of water, which is convenient but arbitrary. The kelvin scale starts at absolute zero, the lowest possible temperature, where the thermal motion of particles is at its minimum. William Thomson, later Lord Kelvin, proposed an absolute scale in 1848. A step of one kelvin is the same size as one degree Celsius, so converting is just a shift: T_K = T_°C + 273.15.

Normal body temperature, about 37 °C, is 37 + 273.15 = 310.15 K. Absolute zero is 0 K, or −273.15 °C. Water boils at about 373.15 K at standard pressure.

Scientists use kelvins because many laws, such as the ideal gas law and the Stefan–Boltzmann law, only work with absolute temperature. A gas at 600 K has twice the average molecular kinetic energy of one at 300 K, while 20 °C is not twice as hot as 10 °C.

Kelvins are written without a degree sign. Absolute zero cannot actually be reached, though laboratories have cooled atoms to billionths of a kelvin.