During the early 1700s, Danish astronomer Ole Roemer turned his attention toward developing a reliable instrument for measuring temperature. Roemer constructed glass tubes containing alcohol that expanded when heated. Building on Roemer's work, German scientist Daniel Gabriel Fahrenheit began using mercury, which has a higher boiling point than alcohol, in thermometers of his own construction. Using the more accurate mercury, Fahrenheit calculated a four-point scale, where zero represented the lowest calculable temperature, 32 as the freezing point of water, 96 as the temperature of the human body and 212 as the boiling point of water.
With the early thermometers at his disposal, the coldest temperature Fahrenheit could calculate was the freezing point of water when mixed with salt. Fahrenheit set this temperature on his scale at zero since no colder temperature could be recorded. With the many advancements made in thermometer readings over the past 300 years, temperatures much colder than Fahrenheit's zero can be recorded. Since the Fahrenheit scale is still in use in the U.S., negative numbers must be used to properly represent temperatures below the freezing point of water and salt.
Anders Celsius developed an inverted centigrade scale in 1742, six years after Fahrenheit's death, where zero is the temperature at which water boils and 100 is the temperature at which it freezes. The scale was eventually flipped and became the standard used for scientific measurements. With zero being the freezing point on the Celsius scale, any negative temperature marks a temperature below freezing. In relation to Celsius, negative 1 degree Fahrenheit equals negative 18 degrees Celsius. So a temperature of negative 1 Fahrenheit is 18 Celsius degrees below freezing.
Fahrenheit remains the standard in the U.S. for several reasons. When used for recording the weather, the Fahrenheit scale conveniently uses zero to 100 to mark a normal temperature range. Anything either above, below or near zero or 100 is considered an extreme temperature. Additionally, the Celsius scale often uses three digits, such as 36.5, when detailing a temperature. The Fahrenheit scale's use of two numbers makes gauging the weather easier for the average individual.