Understanding temperature
Three scales, two anchors.
Ice melts. Water boils. Almost everything else is arithmetic.
The three scales.
Celsius, Fahrenheit, and Kelvin all measure the same physical thing — the average kinetic energy of molecules — but they start counting from different places, so the same warmth gets different numbers depending on which scale is on the thermometer.
Anchors worth memorising
- 0 °C = 32 °F = 273.15 K — water freezes
- 100 °C = 212 °F = 373.15 K — water boils at sea level
- 37 °C = 98.6 °F — human body
- −40 °C = −40 °F — where the two scales cross
- 0 K = −273.15 °C = −459.67 °F — absolute zero
The formulas.
Unlike length or weight, temperature isn't multiplicative. You can't double 20 °C and call it 40 °C of heat — there's an offset baked in. Each conversion has a rate (how fast one scale ticks relative to another) and an offset (where the zeros line up).
°C → °F
°F = °C × 9/5 + 32
°F → °C
°C = (°F − 32) × 5/9
°C → K
K = °C + 273.15
0 °C ≡ 273.15 K · 0 °C ≡ 32 °F
Three worked conversions.
Apply the rate first, the offset last (going one way) — or subtract the offset first, then apply the rate (going back). That's the whole craft.
25 °C to Fahrenheit
°F = °C × 9/5 + 32
Multiply by the rate first (9/5), then add the offset (32).
25 × 9/5 + 32 = 45 + 32 = 77
= 77 °F
70 °F to Celsius
°C = (°F − 32) × 5/9
Subtract the offset first (32), then multiply by the inverse rate (5/9).
(70 − 32) × 5/9 = 38 × 5/9 ≈ 21.111
= 21.111 °C
100 °C to Kelvin
K = °C + 273.15
Celsius and Kelvin tick at the same rate; only the offset differs.
100 + 273.15 = 373.15
= 373.15 K
A note on absolute zero.
The kelvin starts where motion stops. At 0 K (−273.15 °C), the molecules in a substance are in their lowest possible energy state — there's nothing colder. That's why scientists use Kelvin: a doubling of Kelvin really is a doubling of thermal energy, something that isn't true on either Celsius or Fahrenheit. For everyday weather and cooking, Celsius and Fahrenheit are fine; for thermodynamics, Kelvin is the honest scale.
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