Celsius
In 1742, Anders Celsius made a "reversed" version of the modern Celsius temperature scale. In this scale, zero was the boiling point of water and 100 was the melting point of ice. In his paper Observations of two persistent degrees on a thermometer, he wrote about his experiments. He showed that the melting point of ice was basically unaffected by air pressure.  Ice would turn into water at the same temperature, whether it was at sea level or on a mountain.  This was not the case for the boiling point of water.  It would boil easier with less pressure (on a mountain). He decided that zero on his temperature scale (water's boiling point) would be set at the standard barometric pressure at  sea level.  This pressure is known as one atmosphere.  In 1954, Resolution 4 of the 10th CGPM  (the General Conference on Weights and Measures) set what exactly is one standard atmosphere ().