ELI5: Why is -40c the same as -40f

Minus 40 degrees Celsius is the exact same temperature as minus 40 degrees Fahrenheit because the two temperature scales start at different numbers and count by different step sizes, causing their measurement lines to cross at that specific point.

To understand how this happens in everyday life, imagine two people, Cathy Celsius and Frank Fahrenheit, running a race down a mountain where the air gets colder the further down they go.

• Different starting lines: When water freezes into ice, Cathy starts her scale at 0 degrees, but Frank starts his scale at 32 degrees. Frank is already 32 steps ahead of Cathy.
• Different step sizes: Cathy takes very large steps for every change in temperature, while Frank takes smaller, baby steps. For every 5 big steps Cathy takes downward, Frank has to take 9 smaller steps just to keep up.

The exact spot where they are standing right next to each other on the mountain is at -40.

In daily life, this means if you visit a freezing place like Alaska or northern Canada during the middle of winter and the thermometer reads -40, it does not matter if your thermometer uses Celsius or Fahrenheit. At that extreme chill:

• Boiling water thrown into the air instantly turns into snow on both scales.


• Your breath freezes immediately into ice crystals on your eyelashes.


• You do not need to convert the weather forecast because minus 40 is equally freezing cold to everyone in the world.