Common temperature units:

• Celsius (°C): metric standard
• Fahrenheit (°F): US standard
• Kelvin (K): SI unit (science)
• Rankine (°R): engineering

Key reference points:

• Water freezes: 0°C = 32°F = 273.15K
• Body temp: 37°C = 98.6°F = 310.15K
• Water boils: 100°C = 212°F = 373.15K
• Absolute zero: -273.15°C = -459.67°F = 0K

Celsius:

Invented by Swedish astronomer Anders Celsius in 1742. Originally defined with 0° as boiling and 100° as freezing, later reversed by Carl Linnaeus. Based on the freezing and boiling points of water at standard pressure.

Fahrenheit:

Developed by German physicist Daniel Gabriel Fahrenheit in 1724. He set 0°F as the temperature of a brine solution, 32°F as the freezing point of water, and 96°F as body temperature (later refined to 98.6°F).

Kelvin:

Proposed by Lord Kelvin (William Thomson) in 1848. Based on the concept of absolute zero, the theoretical point where all thermal motion ceases. The Kelvin scale uses the same increment size as Celsius but starts at absolute zero (0K = -273.15°C).

Celsius is used for:

• Weather forecasts (most countries)
• Cooking and food safety
• Medical thermometers (most countries)
• Industrial processes

Fahrenheit is used for:

• Weather forecasts (US)
• Cooking (US recipes)
• Medical thermometers (US)
• HVAC systems (US)

Kelvin is used for:

• Scientific research and publications
• Color temperature (photography, lighting)
• Thermodynamic calculations
• Astrophysics and cosmology