The word 'precipitation'

E. Linacre and B. Geerts


The word ‘precipitation’ was first used by Charles Le Roy in the 18th century. He believed that water dissolved into air when evaporation took place, like salt into water, and that condensation occurred when the air was supersaturated with dissolved water, in the same way that salt is precipitated from a supersaturated solution (1). Hence the word 'precipitation', i.e. the appearance out of a solution. Both rainfall and dew where evidence of this 'precipitation process', according to Le Roy.

Charles Le Roy did not know that water in gaseous form is just one of the types of molecules in the air. Evaporation can occur into a vacuum, and water vapour can condense without air, i.e. the saturation vapour pressure is independent of the ambient air pressure. So the processes of condensation and solute precipitation are actually quite different. Despite this, the word precipitation continues to be used as the label for rain, snow, hail, i.e. any type of water falling towards the ground. Dewfall is not included in precipitation statistics, although it is an atmospheric source of water for some flora and fauna.

Cloud physicists often use the word 'hydrometeors' to refer to precipitation. Rain, drizzle, snow, sleet (frozen rain), graupel and hail are hydrometeors, whereas cloud droplets and cloud ice are not - the latter have a negligible fallspeed and therefore they are not 'meteors' (falling objects).



(1) Sorbjan, Z. 1996. Hands-On Meteorology (Amer. Meteor. Soc.) 306pp.