Common Temperature Scales

• The three commonly used temperature scales are defined using water. David Jordan Williams/Stockbyte/Getty Images

  In simple terms, temperature describes how "hot" a substance is. In scientific terms, temperature is somewhat more complex but can loosely be described as the average energy of the particles within a substance. Thus, higher energy correlates to a higher temperature. The Kelvin, Celsius, and Fahrenheit temperature scales are each employed to describe temperature using a reference value. However, each scale differs significantly, and a clear understanding of their differences can mean the difference between a lukewarm 90 degree Fahrenheit shower and a scalding 90 degree Celsius shower.

Kelvin

• The Kelvin temperature scale is the temperature scale employed by the International System of Units (SI). The reference point for this scale is the triple point of water -- the temperature at normal pressure in which water is in equilibrium between its gas, liquid, and solid states -- which is defined as 273.16 K, and one kelvin (abbreviated K) "is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water." This places zero K at a point known as absolute zero, where there is no thermal energy left in the substance. The freezing point of water is then 273.15 K.

Celsius

• The Celsius scale is commonly used as an alternative to the Kelvin scale in many countries that have adopted and implemented SI, and is indeed an accepted derivative system within SI. Like the Kelvin scale, the Celsius scale uses the triple point of water as its reference point, assigning a zero value to this temperature. Since the triple point and freezing point of water are nearly the same, the freezing point of water can be defined as zero degrees Celsius. Each degree of Celsius corresponds to a unit of Kelvin; thus, converting between Celsius and Kelvin temperatures simply follows the formula: Kelvin minus 273.15 equals degrees Celsius (K -- 273.15 = degrees C).

Fahrenheit

• The Fahrenheit scale continues to find extensive use for everyday purposes in the United States, but has little use outside the U.S. The Fahrenheit scale was initially defined with the freezing point of water at 32 degrees Fahrenheit and the boiling point of water at 212 degrees Fahrenheit. Since the SI has become the official system of measure throughout much of the world including the United States, the Fahrenheit scale today is actually defined in SI terms with the degrees Fahrenheit equal to 9/5 degrees Celsius plus 32 (degrees F = 9/5 degrees C + 32).