Fahrenheit to Delisle

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.

Delisle calibrated his thermometer by observing how mercury contracted as it cooled from boiling water. He measured degrees of cooling rather than degrees of warmth — the scale counted how far the temperature had dropped from the boiling point. This made 0°De the starting reference (boiling) and larger numbers represent lower temperatures, the opposite of every other common scale.

The freezing point of water is 150°De. The boiling point is 0°De. The scale spans 150 degrees between freezing and boiling — contrast with 100 degrees in Celsius or 180 degrees in Fahrenheit. To convert: °De = (100 − °C) × 1.5, so 0°C gives (100 − 0) × 1.5 = 150°De.

Joseph-Nicolas Delisle invented the scale in St. Petersburg in 1732, where he had been invited to work by Peter the Great. The scale was adopted by Russian scientists and used throughout the 18th century, particularly by Mikhail Lomonosov. It was later standardized by Joseph-Nicolas's colleague Joseph-Adam Braun, who fixed the two reference points.

Subtract the Delisle value from 100, then multiply by 2/3. Formula: °C = 100 − (°De × 2/3). For example, 150°De → 100 − (150 × 2/3) = 100 − 100 = 0°C (freezing). For 0°De → 100 − 0 = 100°C (boiling). Remember the scale is inverted: higher Delisle = colder temperature.

Delisle is the only well-known inverted temperature scale, but the concept appears elsewhere. Astronomical magnitude runs backwards (brighter stars have lower numbers). The Scoville scale for chilli heat is not inverted but is logarithmically compressed in a way that confuses people similarly. In thermometry, Delisle stands alone — every other scale (Celsius, Fahrenheit, Kelvin, Rankine, Newton, Rømer, Réaumur) increases with heat. The inversion made Delisle unintuitive, which is a key reason it lost out to Celsius.