Why Kelvin Doesn’t Have Degrees
Kelvin is different because it’s an absolute scale. 0K is absolute zero — the point at which gas molecules have no thermal energy.
There’s no negative temperature on the Kelvin temperature scale. It’s not just about the endpoint, though. Temperature is a measure of the amount of energy contained by molecules.
The Kelvin unit reflects this, where doubling the Kelvin temperature means you doubled the thermal energy.
A degree of the Celsius scale may seem like Kelvin, just bumped up 273, but if you double a Celsius temperature, you don’t double the thermal energy.
Doubling 20°C gives you 40°C, which feels a lot hotter, but isn’t very meaningful from the thermodynamic perspective.
Even worse, what if you double -40°C? Do you get -80°C or -20°C?
You can see why scientific formulas often ask for Kelvin temperature.