THE formula which underpins the relationship between temperature, humidity and dew point - the basis of all air conditioning calculations - marks its 100th anniversary today.
It was on this day in 1911, less than a decade after the invention of air conditioning, Dr Willis Carrier's Rationale Psychrometric Formulae finally cracked the precise correlation between temperature and humidity to create a year-round comfortable environment.

Carrier presented the psychrometric formulae on December 8, 1911 at the annual meeting of the American Society of Mechanical Engineers and initiated the field of scientific air-conditioning design. His invitation to the meeting recognized air conditioning as a formal branch of engineering.

The formula still stands today as the basis in all fundamental calculations for the air conditioning industry. Carrier said he received his 'flash of genius' while waiting for a train. It was a foggy night and he was going over in his mind the problem of temperature and humidity control. By the time the train arrived, Carrier had an understanding of the relationship between temperature, humidity and dew point.