How the Temperature Scales Differ
Fahrenheit, Celsius, Kelvin, and Rankine all measure temperature, but they use different starting points and degree sizes. Fahrenheit is commonly used for everyday temperatures in the United States, while Celsius is widely used throughout most of the world and in many scientific and technical applications.
Kelvin and Rankine are absolute temperature scales. Unlike Fahrenheit and Celsius, their zero points represent absolute zero, the theoretical lowest possible temperature.
Fahrenheit and Celsius
The Fahrenheit and Celsius scales use different degree sizes and different zero points. Water freezes at 32°F and boils at 212°F at standard atmospheric pressure, while the corresponding Celsius temperatures are 0°C and 100°C.
One interesting point shared by the two scales is -40 degrees. At that temperature, Fahrenheit and Celsius have the same numerical value: -40°F equals -40°C.
Kelvin and Rankine
Kelvin is based on the same degree interval as Celsius, while Rankine uses the same degree interval as Fahrenheit. The difference is where each scale begins.
Absolute zero is 0 K, which equals -273.15°C and -459.67°F. Because Kelvin begins at absolute zero, temperatures are written simply as K rather than degrees Kelvin or °K.
Rankine works similarly but uses Fahrenheit-sized degrees. Absolute zero is therefore 0°R.
Actual Temperatures vs. Temperature Changes
Converting an actual temperature is different from converting a change in temperature. The familiar 32-degree offset between Fahrenheit and Celsius applies to actual temperature values, but it does not apply to temperature differences.
For example, an increase of 1°C is the same size temperature change as an increase of 1 K and is equivalent to an increase of 1.8°F.
Why Boiling Temperatures Can Vary
The commonly quoted boiling point of water—212°F or 100°C—assumes standard atmospheric pressure. Water can boil at different temperatures when atmospheric pressure changes.
At higher elevations, lower atmospheric pressure allows water to boil at a lower temperature. This is why cooking and boiling times can sometimes differ significantly with elevation.