A scale for measuring temperature. On the Celsius scale, which is named for the Swedish astronomer that proposed it, pure water freezes at 0° and boils at 100°. Impurities and minerals in water (such as salt) change the temperature at which water freezes or boils. The United States, which uses the Fahrenheit scale, is one of the few countries in the world that has not made the conversion to Celsius. To convert a Fahrenheit temperature reading to Celsius, subtract 32 and multiply the remainder by 5/9. Celsius is also known as the centigrade scale.