James Hutton and the measurement of atmospheric moisture

Measuring the amount of water in the air has been one of the most challenging of routine tasks in meteorology. From the 1650s onwards the methods involved either the uptake of moisture by organic materials such as human hair and whalebone (hygrometry) or the measurement of cooling due to loss of latent heat during evaporation (psychrometry). In 1792 James Hutton reported that a thermometer previously moistened by water records a lower temperature when exposed to cooling in the wind. This led to the development of the first condensation hygrometer by John Leslie in 1801 based on wet- and dry-bulb thermometers, whose design was refined and improved throughout the nineteenth century. Determining relative and absolute humidity values from a hygrometer required the use of a psychrometric equation, first developed by James Ivory in 1822 and later refined by Ernst Ferdinand August, James Apjohn and many others. Modern-day determinations of humidity often involve an aspirated psychrometer located in a Stevenson screen, whose origin can be traced directly back to Hutton’s observation of cooling due to evaporation. An extensive review of the literature on psychrometry records only cursory and ill informed acknowledgement of Hutton’s contribution. This article reasserts Hutton’s seminal role in the history of psychrometry.