Part A · the temperature scale — interactive thermometer
Drag to explore any temperature. What does it mean?
−273°C5,778°C
Part B · the practical daily range — what temps feel like
Weather and environment — what you actually experience
-40°C / -40°F
Extreme cold — F = C here
-20°C / -4°F
Siberian winter, Arctic
-10°C / 14°F
Harsh European winter
0°C / 32°F
Freezing — water/ice boundary
10°C / 50°F
Cool — coat needed
20°C / 68°F
Comfortable room temp
28°C / 82°F
Warm summer day
37°C / 98.6°F
Human body temperature
40°C / 104°F
Dangerous fever / extreme heat wave
54.4°C / 130°F
Hottest air temp recorded — Death Valley, 2020
100°C / 212°F
Water boils at sea level
The magic number: -40°C = -40°F. The only point where Celsius and Fahrenheit are identical. A useful anchor for the F↔C conversion. The all-time official record is 56.7°C / 134°F, also at Death Valley — but that was set on July 10, 1913.
Part C · Fahrenheit ↔ Celsius — the conversion tricks
Type any temperature and get both the exact answer and the mental shortcut
The mental shortcut that works 80% of the time
°C → °F: Double it, then add 30. (Exact: ×1.8 + 32)
20°C → 40 + 30 = 70°F. Actual: 68°F. Close enough.
°F → °C: Subtract 30, then halve it.
80°F → 50 ÷ 2 = 25°C. Actual: 26.7°C. Works well for weather conversations.
The three temperature scales — side by side
Why Kelvin? Scientists use Kelvin because it starts at absolute zero — the point where all thermal motion stops. This makes equations clean: doubling the Kelvin value actually doubles the thermal energy. You can't say that about Celsius or Fahrenheit. 0°C is just "where water freezes," not "zero energy." The size of one kelvin degree is identical to one Celsius degree — only the starting point differs.
Part D · the full temperature scale — beyond daily life
From absolute zero to the Big Bang
-273.15°C (0 K)
Absolute zero — no motion
-196°C
Liquid nitrogen
-89°C
Coldest station record — Vostok, Antarctica 1983
0°C to 100°C
Liquid water — life's zone
~200°C
Standard oven
~600°C
Wood fire, glass softens
~1,538°C
Iron melts
~3,422°C
Tungsten melts — highest melting point of any pure metal
5,500°C
Surface of the Sun
15,000,000°C
Sun's core — nuclear fusion
For reference: the Big Bang was ~10³² °C. Nuclear fusion reactors aim for ~150,000,000°C — ten times hotter than the Sun's core, because they use different fuel (deuterium-tritium) and need higher plasma pressure to compensate for Earth-scale confinement.
Note: in 2013, satellite measurements recorded −93.2°C on the East Antarctic plateau — lower than the Vostok station record, but taken from orbit rather than a ground thermometer. The station-based record remains −89.2°C.
Part E · anchor numbers to memorize
-273°C
Absolute zero — coldest possible temperature
No heat energy at all. Nothing in the universe reaches this.
-40°
Where Fahrenheit = Celsius
The one anchor that unlocks F↔C mental arithmetic.
0° / 100°C
Water freezes / boils — life's boundaries
The entire range of liquid water is just 100 degrees.
Fan ovens run ~20°C hotter effectively. Above 250°C = very hot.
5,500°C
Surface of the Sun
Its core is 15,000,000°C. Nuclear fusion reactors aim for 150,000,000°C.
Part F · test yourself
1. A weather forecast says it will be 95°F tomorrow. Is that hot, mild, or cold? What is it in Celsius?
Hot — a scorching summer day. Using the mental shortcut: 95 − 30 = 65, then ÷ 2 = 32.5°C. The exact answer is (95 − 32) × 5/9 = 35°C. At 35°C you're approaching heat-wave territory. Human core temperature is only 2 degrees higher at 37°C, which is why such days feel so oppressive — the air is nearly as hot as your own body.
2. Why does 20°C water feel cold but 20°C air feels comfortable?
Thermal conductivity. Water conducts heat away from your body about 25× faster than air. At 20°C both are below your body temperature (37°C), so both are drawing heat away — but water does it so efficiently that your body can't compensate, and you feel cold. This is why falling into cold water is far more dangerous than being in cold air at the same temperature. Wet clothing similarly conducts heat away much faster than dry clothing, which is why "cotton kills" in outdoors survival — wet cotton conducts heat almost as well as water.
3. At the top of Mount Everest (altitude ~8,849 m), water boils at about 70°C instead of 100°C. Why — and why does this matter for cooking?
Boiling point is not a fixed property of water — it depends on atmospheric pressure. At sea level, air pressure is ~101 kPa, and water boils at 100°C. At Everest's summit, pressure drops to ~33 kPa — about one third — and water boils at roughly 70°C. This matters enormously for cooking: boiling at 70°C instead of 100°C means far less thermal energy is transferred to food. A "soft-boiled" egg at Everest base camp would need nearly twice as long. Pasta takes much longer. Mountaineers use pressure cookers (which raise the boiling point by increasing internal pressure) for any serious cooking at altitude.
4. Body temperature is 37°C. A fever is 39°C. That's only 2 degrees — why is it dangerous?
Because the human body is an extraordinarily finely tuned machine that operates within an extremely narrow temperature band. The proteins and enzymes that run every chemical reaction in your body are designed to work at 37°C. Even small deviations denature (unfold and break) them. At 40°C, enzymes begin failing. At 41°C, brain damage risk rises sharply. At 42°C+, it becomes life-threatening. Similarly, hypothermia becomes dangerous around 35°C — just 2 degrees below normal. The "safe range" for the human body is only about ±3°C around 37°C. For comparison, a kitchen oven works anywhere from 100°C to 250°C — a 150°C range. Biology is far less forgiving.
5. Steel expands when heated. A steel railway rail is 25 metres long at 0°C. How much longer is it on a 40°C summer day?
About 12 mm. Steel has a linear thermal expansion coefficient of approximately 12 × 10⁻⁶ per °C. The formula is: ΔL = L × α × ΔT = 25,000 mm × 0.000012 × 40 = 12 mm. This is exactly why railway tracks have small gaps between rails — called expansion joints. Without them, rails would buckle and warp in summer heat. Modern continuously welded rail (CWR) solves this by welding rails together and pre-stressing them at a neutral temperature (~27°C), so they are under tension in winter and compression in summer, both within safe limits.
Part G · how heat moves — the three mechanisms
Temperature differences drive heat flow — but the mechanism matters enormously. The same temperature difference can transfer wildly different amounts of heat depending on which of the three pathways is active.
🔗
Conduction
Direct contact. Vibrating atoms pass energy to their neighbours. Metals are excellent conductors — electrons carry energy quickly. Wood, air, and foam are poor conductors (insulators).
Metal spoon in hot soup heats up fast. Wooden spoon stays cool.
🌊
Convection
Moving fluid (liquid or gas) carries heat. Hot fluid rises, cool fluid sinks — creating circulation. Forced convection (fan ovens, wind) is far more efficient than natural convection.
Fan oven cooks food ~20°C hotter effectively than same-temperature still oven.
☀️
Radiation
Electromagnetic waves — no medium needed. Every object above absolute zero emits infrared radiation. Works through vacuum. Rate scales with T⁴ (Stefan-Boltzmann law) — doubling temperature sends 16× more radiation.
The Sun heats Earth through 150 million km of empty space.
Thermal conductivity — how fast different materials transfer heat
Diamond
~2,200 W/m·K — best conductor known
Silver
429 W/m·K
Copper
401 W/m·K — why copper pipes and cookware
Aluminium
205 W/m·K
Iron / Steel
50–80 W/m·K
Glass
1.0 W/m·K
Wood
0.1–0.4 W/m·K
Aerogel
0.015
Air (still)
0.025
Copper conducts heat 16,000× faster than still air. This is why a metal object and a wooden object at the same room temperature feel different to the touch — the metal feels cooler because it conducts heat away from your fingertips faster. They are at the same temperature; only the rate of transfer differs.
The Stefan-Boltzmann law — radiation power scales with T⁴
Every object radiates heat as infrared light. The power radiated is proportional to the fourth power of absolute temperature (Kelvin). This is why small temperature increases can cause dramatic radiation increases.
Object temperature (°C)
300°C
Part H · thermal expansion — how materials grow with heat
Almost all solids expand when heated. The amount is tiny — but at engineering scales, it adds up to real consequences. Bridges, rails, pipelines, and buildings all need expansion joints to accommodate this.
Thermal expansion calculator
Real-world examples
Eiffel Tower — 300 m of iron. Grows by ~18 cm between winter and summer. The top sways up to 6 cm in the wind.
Railway track — gaps every 25 m allow ~12 mm expansion per section. Modern welded rail uses pre-stress instead.
Power lines — deliberately strung with sag in summer so they don't snap taut and break in winter cold.
Invar alloy (iron + 36% nickel) was invented specifically for watch springs and surveying rods because it barely expands at all.
Part I · the human body — a thermal machine
Your body generates about 80 watts of heat at rest — roughly the same as an incandescent light bulb. It must constantly shed this heat or overheat within minutes. The entire system is engineered for a target of exactly 37°C.
≥ 42°CPotentially fatal. Proteins denature. Multi-organ failure. Medical emergency.
The fever paradox: A moderate fever (38–39°C) is actually beneficial — it's your immune system deliberately raising temperature to slow bacterial replication and speed up immune cell activity. Suppressing a low-grade fever too quickly can prolong illness. The danger zone starts above 40°C, where the cure becomes worse than the disease.
How the body maintains 37°C — the four cooling mechanisms
💧 Sweating
Primary mechanism. Evaporation removes ~580 kcal per litre of sweat. Peak: ~1.5–2 litres/hour in extreme heat. This is why humid heat is more dangerous than dry heat — sweat can't evaporate.
🩸 Vasodilation
Blood vessels near the skin dilate, increasing blood flow to the surface so heat can radiate away. This is why you go red when hot. Skin blood flow can increase from 0.5 to 8 litres/minute.
🫁 Breathing
Each exhaled breath carries warm, moist air. Dogs exploit this heavily (panting). In humans it's a minor contribution at rest but meaningful during hard exercise.
🥶 Shivering
The cold response: rapid involuntary muscle contractions generate heat. Can temporarily triple metabolic heat output. Stops when core temperature drops below ~32°C — a dangerous sign.
Part J · temperature in cooking — explorer
Cooking is applied thermodynamics. Each food has critical temperature thresholds — safety temperatures where pathogens die, texture temperatures where proteins or starches transform, and flavour temperatures where chemical reactions create taste.
Select a food to see its key temperatures
Click any item above to learn about its key temperatures.
Oven temperature guide — what actually happens at each setting
120°C / 250°F
Very slow / drying (meringue, jerky)
150°C / 300°F
Slow cook — collagen melts to gelatin
180°C / 356°F
Standard baking — Maillard reaction begins
200°C / 392°F
Roasting — browning and crisping
220°C / 428°F
Hot roast / pizza — rapid crust formation
250°C / 482°F
Maximum — near-char, Neapolitan pizza
~233°C / 451°F
Fahrenheit 451 — paper can ignite
The Maillard reaction is the set of chemical reactions between amino acids and reducing sugars that begins around 140–165°C and creates hundreds of flavour compounds. It is not caramelisation (which involves only sugars, starting around 160°C) — they are distinct processes, but both require removing moisture and reaching the right temperature. Note: 451°F (233°C) is the temperature Ray Bradbury used as paper's ignition point for his novel's title — the actual autoignition range of paper varies from about 218–246°C depending on paper type.
Part K · estimation game — what temperature is it?
Read the scenario. Pick the best temperature estimate.
Building real-world temperature intuition, one scenario at a time.