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What is temperature really?

Temperature is one of the most important physical quantities that shows how much "heat energy" is in a body or medium. In reality, it's not heat itself, but its measure: how fast atoms and molecules move. The faster they move, the higher the temperature.

In everyday life, we measure temperature on different scales – most commonly in Celsius (°C), Fahrenheit (°F), and Kelvin (K). But why are there so many different scales, and how can we convert between them?

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1. A brief history of the three main temperature scales

Celsius scale (°C)

Swedish astronomer Anders Celsius proposed the scale still used today in 1742. In his system, he set the freezing point of water at 0 °C and the boiling point at 100 °C at normal atmospheric pressure. This linear scale is still used in most countries today, especially in everyday life and science.

Fahrenheit scale (°F)

German-born Daniel Gabriel Fahrenheit created his own system earlier, in 1724. He set the freezing point of water at 32 °F and the boiling point at 212 °F. The scale is divided into 180 units between these two points, so the degrees are "denser" compared to Celsius. In the United States, this is still the official temperature scale today.

Kelvin scale (K)

British physicist Lord Kelvin (William Thomson) introduced the thermodynamic scale in the mid-1800s. Its zero point is absolute zero, where the molecules of matter completely stop (–273.15 °C). The Kelvin scale is used primarily in science and engineering practice, as this scale is directly related to the measure of energy.

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2. Why do the scales differ?

The difference can be traced back to historical and practical reasons.

• Fahrenheit wanted a scale based on his own experiments where everyday life temperatures would remain in the positive range.
• Celsius chose the two basic points of water (freezing and boiling) to make it simple and easy to measure.
• Kelvin, however, started from physical absolutes to measure temperature universally and scientifically.

Therefore:

• Celsius and Fahrenheit are relative scales, adjusted to water.
• Kelvin is an absolute scale, adjusted to molecular motion.

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3. The formulas – how do we convert between scales?

Conversion between the three scales is done with linear formulas.

Celsius ↔ Fahrenheit:

°F = (°C × 9/5) + 32
°C = (°F - 32) × 5/9

Celsius ↔ Kelvin:

K = °C + 273.15
°C = K - 273.15

Fahrenheit ↔ Kelvin:

K = (°F - 32) × 5/9 + 273.15
°F = (K - 273.15) × 9/5 + 32

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4. Practical examples – how we encounter them in everyday life

Weather

In Europe and Hungary, temperature is given in Celsius. If a summer day is 30 °C, that means 86 °F in Fahrenheit:

(30 × 9/5) + 32 = 86 °F

So in the United States, if the weather report says 86 °F, it's just as warm as a 30-degree day for us.

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Baking and cooking

Oven temperature is one of the most common situations when conversion is needed.
If an American recipe calls for 350 °F, that's in Celsius:

(350 - 32) × 5/9 ≈ 176.7 °C

So you need to set the oven to about 180 °C.

Conversely: if a Hungarian recipe says 200 °C, that's 392 °F.

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Scientific measurements

Physicists and engineers prefer to use the Kelvin scale.
For example, the freezing point of water:

0 °C = 273.15 K

Its boiling point:

100 °C = 373.15 K

Astronomers often express star temperatures in thousands or millions of Kelvin (e.g., the Sun's surface is about 5778 K).

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5. A short story – the meeting of the three scales

Imagine three scientists – Celsius, Fahrenheit, and Kelvin – meeting on a cold morning.

Fahrenheit complains:

"It's only 20 degrees outside, I'm going to freeze!"

Celsius laughs:

"Come on, that's only –6 °C. It's not that cold!"

Kelvin quietly remarks:

"You're both wrong, it's around 267 K, and the particles are still moving."

This little dialogue well illustrates that the same physical reality can be described in three different ways – depending on which scale we use.

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6. Special point – when the scales meet

There's a point where Celsius and Fahrenheit values match:

°C = °F = -40

This is the intersection of the two scales. So –40 °C = –40 °F.
The Kelvin scale naturally differs here too: –40 °C = 233.15 K.

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7. Summary – what's worth remembering

• Celsius is tied to water, Fahrenheit to everyday temperature sensation, and Kelvin to absolute physical zero.
• Conversion is always linear, done with simple formulas.
• In science, Kelvin is most important, in everyday life Celsius, and in the United States Fahrenheit.
• Interestingly, at –40 degrees, all three systems "meet" at a special point.

Temperature conversion is therefore not only a practical calculation, but also an insight into history and the foundations of physics. It shows that the world can be measured in many ways – the point is that we know how to speak the same language with numbers.