Evaporation in nature

A P O R A T I O N plays some part in most human activity directed toward the production of food, clothing, shelter and comfort, but whereas industrial operations usually have the process under control, in nature it is as uncontrollable as the weather.I n many of its aspects the study of natural evaporation is part of meteorology-" the most desperate branch of that desperate science " (Symons, 1867)-but it is a broad subject overlapping the borders of several other fields of scientific interest.Evaporation from oceans and land surfaces plays a large part in the heat balances of both; the precipitation and re-evaporation of this water demands the attention of meteorologists, hydrologists and all attempting to grow crops ; and along with other weather factors the cycle of rain and evaporation maintained over geological time plays an important part in the formation of soil types and their world distribution.Aristotle posed a fundamental question : " Is wind or sun more important in promoting natural evaporation?" His own answer was in favour of wind because it carried away the vapour, and although this answer has rarely been challenged, the question remained open while ideas about the nature of vapours and of evaporation were vague and techniques for measuring relevant physical magnitudes were inadequate or non-existent.T h e beginnings of a scientific study of evaporation are due to Dalton, who not only initiated much in the technique of evaporation measurements, but also provided the indispensable requisite of progress, clear ideas on the nature of the process.Famous names are associated with earlier ideas : Halley and Benjamin Franklin believed evaporation to be due to solution of water in air; Lavoisier regarded it as the combination of fire with water.Dalton (1801-2) stated clearly that the space above a water surface could contain only a limited amount of water yapour, the maximum partial pressure of which was dependent on temperature and independent of the presence of other gases or vapours, and showed experimentally that where the partial pressure was less than the maximum value at the surface of the water then evaporation would take place at a rate directly proportional to the partial pressure difference, the " constant " in the relation increasing with increasing wind-speed.He gave what appears to be the earliest estimate of annual evaporation from Great Britain, based on the difference between the total rainfall and total river discharge, and was one of the first to sst up what is in effect a micro-catchment area for the study of local and seasonal variations in evaporation from soil surfaces.A cylinder with impermeable -walls was filled with soil so that the measured rain falling on the surface could only be lost as evaporation from the surface or as drainage through the bottom :