Celsius, Fahrenheit, and Kelvin: Understanding the World's Temperature Scales

Celsius: The Water Scale

In 1742, Swedish astronomer Anders Celsius proposed a temperature scale based on two easy-to-reproduce reference points: the freezing and boiling points of water at standard atmospheric pressure.

• 0°C = water freezes
• 100°C = water boils

Fun fact: Celsius originally defined the scale upside down — 100° for freezing and 0° for boiling. It was reversed after his death by Carl Linnaeus (the famous botanist). The scale was called "centigrade" until 1948, when it was officially renamed "Celsius" to avoid confusion with the angular measurement unit.

Today, Celsius is used by virtually every country except the United States for everyday temperature. It's intuitive for weather, cooking, and science because its reference points — water freezing and boiling — are universally relevant.

Fahrenheit: The Human Scale

Daniel Fahrenheit, a German-Dutch physicist, created his scale in 1724. His reference points were more… creative:

• 0°F = the coldest temperature Fahrenheit could achieve with a mixture of ice, water, and ammonium chloride (salt)
• 32°F = water freezes
• 96°F = human body temperature (later measured more precisely as 98.6°F / 37°C)
• 212°F = water boils

Fahrenheit defenders argue it's actually better for weather and human comfort. The 0–100°F range roughly maps to "extremely cold to extremely hot" for outdoor temperatures in temperate climates. In Celsius, weather ranges from about −20°C to 40°C, which is a narrower and less intuitive range.

Kelvin: The Absolute Scale

In 1848, Lord Kelvin (William Thomson) proposed a scale starting at absolute zero — the theoretical lowest possible temperature where all molecular motion stops.

• 0 K = absolute zero (−273.15°C / −459.67°F)
• 273.15 K = water freezes
• 373.15 K = water boils

Kelvin uses the same degree size as Celsius — the only difference is the starting point. A change of 1 K equals a change of 1°C. Kelvin never uses a degree symbol — you write "300 K," not "300°K."

Scientists prefer Kelvin because:

1. No negative numbers — You can't go below 0 K, which simplifies equations
2. Proportional — 200 K is genuinely twice as hot as 100 K (in terms of molecular kinetic energy). This isn't true for Celsius or Fahrenheit
3. Required for gas laws — PV = nRT only works with absolute temperature

Conversion Formulas

Key Reference Points

Note: −40° is the only temperature that reads the same in both Celsius and Fahrenheit. It's a favourite trivia question.

Who Uses What?

• Celsius: Used by ~95% of the world's population for everyday weather, cooking, and general purposes
• Fahrenheit: Used for everyday temperature in the United States, Bahamas, Cayman Islands, Liberia, Palau, and the Federated States of Micronesia
• Kelvin: Used in science worldwide. Also used in colour temperature (photography lighting, monitor calibration) and some engineering applications

Some countries use both: the UK officially uses Celsius, but many Britons still refer to warm weather in Fahrenheit ("It's going to be 80° this weekend!") while using Celsius for cold weather and cooking.

Why Does the US Still Use Fahrenheit?

The same reason it still uses miles, pounds, and gallons: cultural inertia. The US attempted metrication in the 1970s, but the effort was voluntary and fizzled out. Americans grew up thinking of 72°F as comfortable and 100°F as a scorching day — switching to 22°C and 38°C feels unintuitive to them.

That said, American scientists, doctors, and the military all use Celsius/Kelvin. US weather forecasts remain one of the last major holdouts.

Other Temperature Scales

Several other scales exist, mostly of historical interest:

• Rankine (°R): Like Kelvin, starts at absolute zero, but uses Fahrenheit-sized degrees. Used in some US engineering contexts. K × 1.8 = °R.
• Réaumur (°Ré): 0° at water's freezing point, 80° at boiling. Used in parts of Europe until the mid-20th century. Still seen on some antique thermometers.
• Newton (°N): Isaac Newton's scale — 0° at water's freezing point, 33° at boiling. Never widely adopted.
• Rømer (°Rø): The scale Fahrenheit was inspired by — 7.5° at water's freezing point, 60° at boiling.

Temperature in Science: Beyond the Basics

In physics, temperature is a measure of the average kinetic energy of particles in a substance. This is why Kelvin is preferred — it directly relates to energy.

• Absolute zero (0 K) has been approached but never reached. The closest: ~38 picokelvin (3.8 × 10⁻¹¹ K) in a laboratory at the University of Bremen, 2021.
• At temperatures near absolute zero, matter exhibits bizarre quantum effects: superconductivity (zero electrical resistance) and superfluidity (zero viscosity).
• The hottest temperature ever created by humans: ~5.5 trillion K at CERN's Large Hadron Collider — about 360,000 times hotter than the Sun's core.
• Colour temperature in photography uses Kelvin: warm light is ~2,700 K, neutral daylight is ~5,500 K, and overcast sky is ~6,500 K. Higher K = cooler/bluer light (counterintuitive!).