Celsius, Fahrenheit and Kelvin

How do °C, °F and K line up?

Three scales

Drag the slider or tap a landmark. All three scales read the same thermometer.

• ISA tropopause The ISA temperature from 11 km to 20 km. It is −69.7 °F and about 217 K.
• Where C meets F The one temperature with the same number on both scales: −40 °C = −40 °F. A favourite exam question.
• Ice melts 0 °C = 32 °F = 273 K. Fahrenheit's fixed points are 32 and 212, Celsius's are 0 and 100.
• ISA sea level The standard atmosphere's sea-level temperature: 59 °F, 288 K.
• 68 °F A worked exam conversion: 68 °F → C = 5/9 × (68 − 32) = 20 °C → K = 20 + 273 = 293 K.
• Water boils At normal pressure: 100 °C = 212 °F = 373 K. Absolute zero, the coldest anything can be, is about −273 °C (0 K).

Memory hook: −40 is −40 on both. Add 273 for Kelvin.

Takeaway: F = 9C/5 + 32 and K = C + 273. The two scales cross at −40. Aviation and science use Celsius.

How cold can anything get?

Absolute zero

Cool a substance and watch its molecules.

1. Heat is molecules moving Heat is a form of energy. In a warm substance the molecules move about fast and at random.
2. Cool it, they slow down Take heat away and the molecules move more slowly and settle into a more orderly pattern.
3. Absolute zero Keep extracting heat and you reach a state where the molecules hardly move at all and no more heat can be taken out. That lowest possible temperature is the same for all substances: absolute zero, 0 K = about −273 °C.
4. Three scales Celsius (used in aviation and science) and Fahrenheit are fixed by melting ice and boiling water: 0 and 100 °C, 32 and 212 °F. Kelvin starts at absolute zero. Ambient temperature: the temperature of the surroundings. Virtual temperature: the temperature dry air would need to have the same density as a moist parcel at the same pressure, so the dry-air equation can be used for moist air. K = C + 273 · F = 9C/5 + 32 · C = 5/9 (F − 32) · −40 °C = −40 °F.

Memory hook: 0 K = −273 °C: nothing colder.

Takeaway: Temperature measures how fast molecules move. Take heat away and they slow down and line up, until at about −273 °C (0 K, absolute zero) there is no heat left to take. It is the same for every substance.

Isn't heat the same as temperature?

Heat or temperature?

Each pair looks alike. The rule underneath tells them apart.

• Heat: The sum total of the kinetic energy of all the molecules. A warm bath holds more heat than a cup of boiling water. Temperature: The average kinetic energy of the molecules. The boiling cup is hotter than the bath. Heat: sum total. Temperature: average. The thermosphere is very hot but has almost no heat, because its molecules are so few.
• Specific heat: Heat to warm 1 unit of mass by 1 °C. Water 1, ice 0.5, soil 0.2. Latent heat: Heat taken in or given out when a substance changes state (ice ↔ water ↔ vapour) with no change in temperature. Specific heat changes the temperature. Latent heat changes the state.
• Solar radiation: Short waves from the very hot sun. Comes in by day only. Terrestrial radiation: Long waves (infra-red) from the much cooler Earth. Goes out day and night; at night it is called nocturnal radiation. Hot sun, short waves. Cool Earth, long waves.
• Ambient temperature: The temperature of the surroundings. Virtual temperature: The temperature dry air would need to have the same pressure and density as a given parcel of moist air. Virtual temperature lets the dry-air equations work for moist air.