Fahrenheit to Reaumur

The United States adopted Fahrenheit in the colonial era when it was the dominant scientific scale. When the metric system spread through Europe in the late 18th and 19th centuries, the US did not follow. Multiple metrication attempts (notably in the 1970s) failed to gain public support, leaving Fahrenheit entrenched in daily life and infrastructure.

Fahrenheit and Celsius are equal at −40°. At that point −40°C = −40°F. This is the only temperature where the two scales intersect, arising naturally from the conversion formula: °F = °C × 1.8 + 32, which equals °C only when both sides solve to −40.

Fahrenheit set his 0°F at the coldest temperature he could reliably reproduce — a salt-ice-water brine — so that everyday weather readings would never go negative in temperate Europe. Water's freezing point landed at 32°F and body temperature at 96°F (later corrected to 98.6°F). The seemingly odd number 32 is simply where water freezing fell on his scale after he anchored zero to the brine. The 32-to-212 span (180 degrees) also divides neatly by many factors, which suited 18th-century instrument makers.

Internal body temperature above 104°F (40°C) is heat stroke territory — a medical emergency. Externally, a wet-bulb temperature of 95°F (35°C) is the theoretical limit of human survival because the body can no longer cool itself by sweating. Air temperatures above 130°F (54°C) have been recorded in Death Valley, but low humidity keeps wet-bulb values survivable there. In humid climates, even 110°F (43°C) air temperature can be lethal because moisture prevents sweat evaporation.

98.6°F is the Fahrenheit equivalent of 37°C, which is itself a rounded metric value. Fahrenheit introduced his scale before Celsius, so 37°C converting to a non-round Fahrenheit value is an artifact of the conversion arithmetic. In practice, normal body temperature varies from about 97°F to 99°F (36.1–37.2°C) across individuals and times of day.

The Réaumur scale was proposed by René Antoine Ferchault de Réaumur in 1730. Réaumur was a French scientist primarily known for his contributions to natural history, particularly entomology. His thermometer used dilute alcohol and was calibrated between the freezing and boiling points of water, with the scale divided according to the actual expansion of the alcohol.

Réaumur calibrated his scale based on the physical expansion of his thermometric fluid — a specific dilute alcohol mixture. Between water's freezing and boiling points, the fluid expanded by exactly 80 parts in 1000 of its volume at freezing. He used this empirical measurement directly as the degree count, making 80°Ré the boiling point. The number 80 was not chosen arbitrarily but measured.

The French Revolution's embrace of the metric system in the 1790s demanded decimal-friendly units. Celsius's 0-to-100 scale fit the decimal philosophy perfectly; Réaumur's 0-to-80 did not. The French Academy of Sciences standardized Celsius for scientific work, and it spread through Napoleonic Europe. Réaumur lingered in German-speaking regions and Russia into the mid-19th century but eventually yielded everywhere. The irony is that Réaumur was French — his own country was the first to abandon his scale.

Multiply the Réaumur value by 5/4 (or 1.25). For example, 16°Ré × 1.25 = 20°C. To convert Celsius to Réaumur, multiply by 4/5 (or 0.8). The two scales share the same zero (0°C = 0°Ré); they differ only in degree size, since 100°C = 80°Ré.

Classic French confectionery texts list sugar syrup stages in Réaumur: "thread" stage at 80°Ré (100°C), "soft ball" at 94°Ré (117°C), "hard crack" at 124°Ré (155°C). Pastry chefs memorized these benchmarks and tested with a thermometer or by dripping syrup into cold water. Some vintage French and Swiss recipe books still reference Réaumur temperatures — modern reprints add Celsius equivalents in brackets. It is the last practical domain where Réaumur had a lingering foothold.