Celsius

Formal Definition

The degree Celsius (symbol: °C) is a unit of temperature on the Celsius scale, which is a temperature scale originally defined by two fixed points: the freezing point of water at 0 °C and the boiling point of water at 100 °C, both at standard atmospheric pressure (101.325 kPa). The scale divides this range into 100 equal intervals, making it intuitive for everyday use: the entire range of common weather temperatures, cooking processes, and biological conditions falls within a span of roughly -40 °C to 250 °C.

Relationship to the Kelvin Scale

In modern SI usage, the Celsius scale is defined in relation to the kelvin (K), the SI base unit of temperature. The degree Celsius is exactly equal in magnitude to the kelvin — a temperature difference of 1 °C is the same as a difference of 1 K. The two scales are offset by 273.15: the temperature in Celsius equals the temperature in kelvins minus 273.15 (i.e., °C = K − 273.15). This means that absolute zero (0 K), the lowest possible temperature, corresponds to −273.15 °C.

Since the 2019 SI redefinition, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. Because the Celsius scale is defined relative to the kelvin, this redefinition also implicitly fixes the Celsius scale. The Boltzmann constant relates the average kinetic energy of particles in a gas to the thermodynamic temperature, making it a fundamental bridge between the microscopic and macroscopic worlds.

The Celsius scale is sometimes informally called the "centigrade" scale, particularly in older texts and in some languages. However, the name "Celsius" has been the official designation since 1948, when the 9th CGPM adopted it to honor the scale's inventor and to avoid confusion with the centesimal grade unit used in angular measurement.

Named After Anders Celsius

The Celsius scale is named after Anders Celsius (1701–1744), a Swedish astronomer, physicist, and mathematician who was a professor at Uppsala University. Celsius proposed his temperature scale in 1742, presenting it in a paper to the Royal Swedish Academy of Sciences. The adoption of his name for the scale was formalized in 1948 by the 9th General Conference on Weights and Measures, replacing the earlier name "centigrade."

From Centigrade to Celsius

The word "centigrade," which was used for the scale for nearly two centuries before the name change, derives from the Latin "centum" (hundred) and "gradus" (step or degree). This name described the scale's defining feature: 100 equal divisions between the freezing and boiling points of water. The rename was necessitated because "centigrade" was also used in France and Spain to refer to the centesimal grade (gradian), a unit of angular measurement where a right angle equals 100 grades. Using "centigrade" for both temperature and angle created ambiguity in scientific communication.

Anders Celsius himself was a distinguished scientist beyond his thermometric work. He was one of the first to observe the relationship between the aurora borealis and Earth's magnetic field, and he participated in a French geodetic expedition to northern Sweden in 1736 that confirmed Isaac Newton's prediction that the Earth was flattened at the poles. He founded the Uppsala Astronomical Observatory in 1741, just a year before proposing his temperature scale. Celsius died of tuberculosis in 1744, aged only 42, just two years after publishing his scale.

The Original Inverted Scale

The Celsius scale was devised in 1742 by Swedish astronomer Anders Celsius (1701–1744), who was a professor at Uppsala University. In his original proposal, published as an appendix to a paper presented to the Royal Swedish Academy of Sciences titled "Observations on two persistent degrees on a thermometer," Celsius defined the scale with the boiling point of water at 0 degrees and the freezing point at 100 degrees — the reverse of the modern convention. He carefully documented the boiling point's dependence on atmospheric pressure, noting that it varied with barometric readings.

After Celsius's death from tuberculosis in 1744, the scale was inverted to its modern form, placing 0 at the freezing point and 100 at the boiling point. The reversal is commonly attributed to the Swedish botanist Carl Linnaeus (Carolus Linnaeus), who reportedly used the inverted scale in his greenhouse at Uppsala from 1745 onward. However, historical evidence is not conclusive — some sources credit the instrument maker Daniel Ekström, who built thermometers for Linnaeus, while others point to Celsius's colleague and successor Mårten Strömer, who may have made the change as early as 1744.

The Centigrade Era

For nearly two centuries, the scale was known internationally as the "centigrade" scale, from the Latin "centum" (hundred) and "gradus" (step), because it divided the range between the freezing and boiling points of water into 100 equal degrees. This name was used in scientific literature, textbooks, and everyday speech across Europe and much of the world. In French, the scale was called "centigrade," and this name persists informally in some French-speaking countries today.

In 1948, the 9th General Conference on Weights and Measures (CGPM) officially renamed the scale from "centigrade" to "Celsius." The primary motivation was to eliminate confusion with the centesimal grade (gradian), a unit of angular measurement in which a right angle equals 100 grades. In French and Spanish, the term "centigrade" could refer to either temperature or angle, creating genuine ambiguity in scientific communication. The new name honored the inventor while resolving the linguistic conflict.

Anchoring to Absolute Zero

In 1954, the 10th CGPM fundamentally redefined the Celsius scale. Instead of relying on the freezing and boiling points of water — which are sensitive to atmospheric pressure and water purity — the scale was anchored to the triple point of water (0.01 °C = 273.16 K exactly) and absolute zero (−273.15 °C = 0 K). This made the Celsius scale a derived scale, formally dependent on the kelvin rather than independently defined. The boiling point of water at standard atmospheric pressure became a measured value (99.974 °C under ITS-90) rather than a defining point.

The 2019 Redefinition

The 2019 SI redefinition further shifted the foundation. The kelvin is now defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K, rather than by the triple point of water. Since the Celsius scale is defined as K − 273.15, this change propagated to Celsius as well. The triple point of water became a measured quantity (still very close to 273.16 K) rather than an exact defined value. In practice, the change had no effect on temperature measurements — the values are the same within the uncertainty of the best measurements — but it placed the definition on a more fundamental footing.

Throughout its history, the Celsius scale has remained the most practical and widely used temperature scale for human activities. Its design — placing 0 at the freezing point of water and 100 at the boiling point — creates an intuitive framework for weather, cooking, industrial processes, and biological science. The transition from centigrade to Celsius, from water-based to constant-based definitions, has been essentially invisible to everyday users while satisfying the demands of precision metrology.

In Weather and Daily Life

The Celsius scale is the most widely used temperature scale in the world for everyday purposes. It is the standard for weather forecasts in virtually every country except the United States, where temperatures are reported in Fahrenheit. A winter day might be described as −5 °C, a comfortable spring day as 20 °C, and a hot summer day as 35 °C. These values are intuitive and widely understood: below 0 means freezing conditions, around 20 means comfortable, and above 35 means uncomfortably hot.

In Cooking and Food Safety

In cooking and food safety, Celsius is the standard reference in most countries. Water boils at 100 °C at sea level, bread bakes at 180 to 220 °C, and refrigerators should be set at 3 to 5 °C. Food safety guidelines specify that perishable foods should not be held between 5 °C and 60 °C (the "danger zone" for bacterial growth) for more than two hours. Internal cooking temperatures for food safety — chicken at 74 °C, ground beef at 71 °C — are specified in Celsius throughout the EU, Asia, and most other regions. Oven temperatures in metric countries are displayed in Celsius, and recipe books outside the US use Celsius exclusively.

In Science and Medicine

In science and engineering, Celsius is used alongside kelvin for most practical measurements. Laboratory instruments — thermocouples, RTDs, infrared thermometers — typically display temperatures in Celsius. Scientific papers commonly report experimental temperatures in Celsius, with kelvin used for thermodynamic calculations and when absolute temperatures are needed. In medicine, body temperature is measured in Celsius in most countries, with the normal range being approximately 36.1 to 37.2 °C. Fever is generally defined as a body temperature above 38.0 °C. Hospital and laboratory equipment worldwide is calibrated in Celsius.

The United States remains the only major country where Fahrenheit is the primary temperature scale for daily life, including weather reports, cooking, and thermostat settings. However, American scientists, engineers, and medical professionals frequently use Celsius, and US weather services provide Celsius readings alongside Fahrenheit for international audiences. In Canada, Celsius was officially adopted in 1977 as part of metrication, though some Canadians informally reference Fahrenheit for cooking or swimming pool temperatures.

Weather and Planning

For weather and daily planning, the Celsius scale provides an intuitive framework that most of the world takes for granted. Temperatures below 0 °C mean ice and frost — roads may be slippery, pipes can freeze, and warm clothing is essential. Temperatures from 0 to 10 °C are cold, requiring a coat; 10 to 20 °C is mild and comfortable; 20 to 30 °C is warm; and above 30 °C is hot. This simple mental model serves billions of people daily when checking forecasts, deciding what to wear, or planning outdoor activities. Weather records in Celsius include the highest reliably recorded air temperature: 56.7 °C in Death Valley, California, on 10 July 1913.

In the Kitchen

In the kitchen, Celsius governs cooking processes. Water simmers at around 85 to 95 °C and boils at 100 °C at sea level (dropping about 1 °C for every 300 meters of elevation). Sugar stages in candy-making are measured in Celsius: thread stage at 110 °C, soft ball at 113-116 °C, hard crack at 149-154 °C, and caramel at 170 °C. Bread yeast is most active at 35 to 40 °C and dies above 60 °C. Chocolate must be tempered at precise temperatures — dark chocolate at 31-32 °C — to achieve proper crystal structure. Deep-frying oil is typically heated to 175-190 °C.

Health and Body Temperature

For health monitoring, Celsius is the standard in most healthcare systems worldwide. Normal human body temperature is approximately 37 °C (ranging from about 36.1 to 37.2 °C depending on the measurement site and time of day). A mild fever begins at 37.5-38.0 °C, a moderate fever at 38.1-39.0 °C, and a high fever above 39.0 °C. Hypothermia sets in when core body temperature drops below 35 °C and becomes life-threatening below 28 °C. These reference points are taught in first-aid courses and medical schools throughout the metric world.

Fitness and Recreation

In fitness and outdoor recreation, temperature awareness in Celsius guides safe activity choices. Running in temperatures above 30 °C significantly increases the risk of heat-related illness. Swimming pools are typically maintained at 25-28 °C for recreation and 28-30 °C for competition. Sauna temperatures range from 70 to 100 °C. Skiing conditions require temperatures at or below 0 °C for natural snow, though snowmaking equipment can operate at temperatures down to about -2 °C. Understanding wind chill — which describes how cold air feels on exposed skin — requires familiarity with the Celsius scale in most countries.

Chemistry

In chemistry, temperature in Celsius (or kelvin) is a critical parameter in virtually every reaction and process. Reaction rates typically double for every 10 °C increase in temperature, a principle known as the Q₁₀ rule or van 't Hoff rule. Chemical equilibrium constants are temperature-dependent, described by the van 't Hoff equation. Phase diagrams — which map the states of matter (solid, liquid, gas) as functions of temperature and pressure — use Celsius or kelvin on the temperature axis. The Celsius scale is particularly convenient for aqueous chemistry because water's phase transitions bracket the 0–100 °C range.

Biology and Life Sciences

In biology and life sciences, temperature control in Celsius is essential for experimental reproducibility. Enzyme activity is highly temperature-dependent: most human enzymes function optimally at approximately 37 °C (body temperature) and denature above 40-45 °C. PCR (polymerase chain reaction), the foundational technique of molecular biology, cycles between 94 °C (DNA denaturation), 50-65 °C (primer annealing), and 72 °C (DNA extension). Cell cultures are maintained at 37 °C in CO₂ incubators. Cryopreservation of biological samples uses temperatures from -80 °C (dry ice) to -196 °C (liquid nitrogen). Pasteurization of milk requires heating to 72 °C for 15 seconds (HTST method).

Physics and Engineering

In physics and engineering, Celsius is used for practical temperature measurements, while kelvin is preferred for thermodynamic calculations. Materials science specifies heat treatment temperatures in Celsius: steel is quenched from about 800-900 °C, aluminum alloys are solution-treated at 460-530 °C, and glass transition temperatures of polymers are reported in Celsius. In electronics, semiconductor operating temperature ranges are specified in Celsius — a typical commercial IC operates from 0 to 70 °C, while military-grade components are rated from -55 to 125 °C. Superconductivity occurs at extremely low temperatures: conventional superconductors require temperatures near absolute zero, while high-temperature superconductors (like YBCO) achieve superconductivity below about -180 °C (93 K).

Meteorology and Climate Science

In meteorology and climate science, all temperature records and climate models use Celsius (or equivalently, kelvin for absolute values). Global average surface temperature is approximately 15 °C, and climate change is measured in fractions of a degree Celsius. The Paris Agreement aims to limit global warming to 1.5 °C above pre-industrial levels. Ocean temperatures, atmospheric profiles, and ice core data are all recorded and reported in Celsius. The World Meteorological Organization maintains global temperature records in Celsius, enabling consistent comparisons across countries and decades.

Global Celsius Dominance

The Celsius scale dominates temperature measurement globally. In Europe, Asia, Africa, South America, Oceania, and most of North America (Canada), Celsius is used for weather forecasts, cooking, medical measurements, industrial processes, and scientific work. When a European weather forecast says "tomorrow's high will be 28°," everyone understands this means Celsius. Oven settings, thermostat dials, and thermometer readings are all in Celsius. The European Union requires Celsius for all official temperature measurements, and EU product safety standards specify temperature limits in Celsius.

The United States

The United States is the sole major holdout. Americans describe a hot day as "95 degrees" (meaning Fahrenheit), set their ovens to "350°F" for baking, and consider "98.6°F" as normal body temperature. This creates a persistent cultural divide: American travelers abroad must mentally convert temperatures, and international visitors to the US face the reverse challenge. Despite periodic metrication efforts — the Metric Conversion Act of 1975 and the Omnibus Trade and Competitiveness Act of 1988 — Fahrenheit remains deeply entrenched in American daily life. However, US scientists universally use Celsius (and kelvin), and American medical devices increasingly display temperatures in both scales.

Other Countries and Territories

A few other countries and territories maintain partial Fahrenheit usage alongside Celsius. In the Bahamas, Belize, the Cayman Islands, and Palau, Fahrenheit is sometimes used due to American cultural influence, though Celsius is increasingly standard in official and scientific contexts. In the United Kingdom, weather forecasts are given in Celsius, but older generations sometimes reference Fahrenheit for hot weather ("It was 100 degrees!" meaning Fahrenheit). In practice, Celsius has been the official standard in the UK since the 1960s, and younger Britons are generally unfamiliar with Fahrenheit. Canada officially adopted Celsius in 1977, but proximity to the US means some Canadians still understand Fahrenheit references, particularly for cooking temperatures and swimming pool settings.