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
56°C / 133°F
Hottest air temp ever 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.
Part C · Fahrenheit ↔ Celsius — the conversion tricks
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.
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 on Earth (Antarctica, 1983)
0°C to 100°C
Liquid water — life's zone
~200°C
Standard oven
~600°C
Wood fire, glass softens
~1,000°C
Iron melts (~1,538°C)
~3,500°C
Tungsten melts — highest melting point of any 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 and need higher pressure.
Part E · energy and calories — the other half of this module
The calorie confusion — kcal vs cal
Food "calories" are actually kilocalories (kcal). 1 food calorie = 1,000 physics calories. When a label says "250 calories," it means 250 kcal. This is one of the most widespread unit confusions in everyday life. From here on, "calorie" means food calorie = kcal.
Human daily energy need
~2,000 kcal
Average adult. ~2,500 for men, ~2,000 for women. Athletes can need 3,500–5,000 kcal.
In watts (power output)
~80–100 W
A resting human radiates about as much heat as a 100W light bulb. Your body is literally a heater.
1 gram of fat
9 kcal
More than double carbs or protein (4 kcal/g each). Fat is the body's most dense energy store.
1 gram of carbohydrate
4 kcal
Same as protein. Sugar = carbohydrate. 1 teaspoon of sugar (~4g) = 16 kcal.
How far does 100 kcal go? (exercise to burn it off)
Running (fast)
~8 min
Cycling
~15 min
Brisk walking
~25 min
Sitting at a desk
~75 min
100 kcal = roughly a small apple, a plain rice cake with peanut butter, or 3 squares of dark chocolate. Exercise burns less than most people think — that croissant (250 kcal) needs 30 min of running to offset.
Part F · energy in the world — from food to nuclear weapons
Energy released — logarithmic scale (in joules, J)
Each step up is ~1,000× more energy. 1 food calorie = 4,184 joules.
Lifting 1 kg by 1 metre
~10 J
1 food calorie (1 kcal)
~4,200 J
Human daily food (2,000 kcal)
~8.4 MJ
1 litre of petrol (burned)
~34 MJ
Lightning bolt
~1–5 GJ (1–5 billion J)
Full tank of car petrol (~50L)
~1.7 GJ
Hiroshima bomb (Little Boy)
~63 TJ (63 trillion J)
Largest H-bomb ever (Tsar Bomba)
~210 PJ — 3,300× Hiroshima
Surprising fact: a lightning bolt (~1 billion joules) contains less total energy than a full car petrol tank (~1.7 billion joules). The difference is delivery speed — a lightning bolt releases its energy in microseconds. Your petrol tank releases it over hours. Energy rate (power) matters as much as total energy.
Part G · interactive calorie calculator
How long to burn off any food?
Part H · 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.
37°C
Human body temperature
38.5°C+ = fever. 40°C+ = dangerous. 42°C = fatal.
190–220°C
Standard oven temperature range
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.
2,000 kcal
Average human daily energy need
Fat = 9 kcal/g. Carbs & protein = 4 kcal/g each.
~80 W
Resting human heat output
A human body radiates as much as a dim light bulb, continuously.
Part I · 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. A person eats a 550 kcal Big Mac and then goes for a 30-minute jog. Have they burned it off?
Not quite. Running burns roughly 10–12 kcal per minute at moderate pace, so 30 minutes = ~300–360 kcal. The Big Mac has 550 kcal — so you'd need closer to 50 minutes of running to fully offset it. This is the key insight: it is much faster to consume calories than to burn them. A 2-second bite of a burger takes 4 minutes of running to undo. This is why diet (what you eat) has far more impact on weight than exercise alone.
3. 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.
4. A nuclear power plant produces about 1,000 megawatts of electricity. How many humans would you need to generate the same power by cycling?
About 13 million cyclists. A person cycling hard generates roughly 75–80 watts of electrical power. 1,000 megawatts = 1,000,000,000 watts. 1,000,000,000 ÷ 75 ≈ 13,300,000 people. One nuclear plant replaces 13 million people pedalling flat out, continuously, 24 hours a day. This illustrates why energy density matters so much — a lump of uranium the size of a golf ball contains the energy equivalent of about 800 tonnes of coal, or 600,000 litres of oil.
5. 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.