DEW. The word "dew" (O.E. deaw; cf. Ger. Tau), is a very ancient one. The New English Dictionary gives "the moisture de posited in minute drops upon any cool surface by condensation of the vapour of the atmosphere ; formed after a hot day, during or towards night and plentiful in the early morning." If the deposit is in the form of ice it is called hoarfrost. Aitken's researches (Trans. Roy. Soc. of Edinburgh, xxiii., and "Nature" vol. xxxiii.) suggest that the words "by condensation of the vapour in the at mosphere" might be omitted because the large dewdrops on the leaves of plants, the most characteristic of all dew phenomena, are largely an exudation of water from the plant itself through the leaf-pores and are merely the continuation of the plant's irriga tion process for supplying the leaves with water from the soil. The action, set up in full vigour in the daytime, is intended to maintain tolerable thermal conditions of the leaf surface in the hot sun, and continues after sunset.
Nevertheless, the typical experiment to illustrate dew formation is the production of a deposit of minute drops of moisture upon the exterior surface of a glass or polished metal vessel by the cool ing of a liquid which it contains. The usual liquids are water, cooled by pieces of ice ; or ether, volatilised by bubbling air through it. No deposit is formed by this process until the temperature is reduced to a certain critical level which depends upon the state of the surrounding air. The physical analogy between the natural formation of dew and this artificial production is considered so complete that the critical level below which the temperature of a surface must be reduced in order to obtain the deposit is known as the "dew-point" (q.v.).
Physicists consider the dew-point to be the temperature at which, by being cooled without change of pressure, air becomes saturated with water vapour, on account of a reduced capacity of the air at the lower temperature for holding water. It is a well established proposition that the pressure of the existing water vapour remains constant if the air is cooled without change of its total external pressure, hence the saturation pressure at initial dew-point gives the pressure of the water vapour. Tables of pressures of saturated water vapour for various temperatures have been compiled, thus this mode of determining the dew-point is a recognized method of measuring the pressure, and amount of water vapour in the atmosphere; it is of fundamental importance in hygrometry.
The dew-point is of vital consequence in the matter of the oppressiveness of the atmosphere. High temperature with a low dew-point does not matter, but conditions become unpleasant when the dew-point begins to approach the normal temperature of the human body.
The physical explanation of dew formation is arrived at by determining the manner in which dew-laden objects in the open air have become cooled "below the dew-point." Formerly, from the time of Aristotle at least, dew was sup posed to "fall" ("Meteorologica," Bk. I., ch. Io), and that view of the process was not extinct at the time of Wordsworth. To Charles Wells, Physician to St. Thomas's Hospital, London, be longs the credit of introducing the study of radiation as a meteor ological agency. By numerous and careful experiments described in his "Essay on Dew," London, 1814, he made it clear that the deposition of dew could be satisfactorily explained by the cooling of exposed objects on clear nights and his theory of automatic cooling by radiation has found a place in all text-books of physics. The process as represented by Wells is a simple one. All bodies which are at a higher temperature than their surroundings are con stantly radiating heat, and cool unless they receive a corresponding amount of heat from other bodies. Good radiators, which are at the same time bad conductors of heat, such as blades of grass, on a clear night, become cooled below the dew-point of the atmo sphere by radiation of heat to the surroundings.
The whole question of radiation was very fully studied by Melloni (1798-1854) and by other physicists, but little was added to the explanation given by Wells until in 188o, when John Aitken showed that condensation did not take place even when the air was cooled below dew-point if all nuclei for condensation had been removed, but his most important contribution on this matter was in 1885 when he called attention to the question whether the water of dewdrops on plants or stones came from the air or the earth, either by plants exuding dewdrops, or by tion and subsequent condensation in the lowest layer of the atmosphere. Aitken's views at least showed that the physical processes operative in the evolution of meteorological phenomena are generally complex. The conditions favourable for the formation of dew are (1) a good ating surface, (2) a still atmosphere, (3 ) a clear sky, (4) thermal insulation of the radiating surface, (5) warm moist ground or some other supply of moisture in the surface layers of air. Aitken's main tribution showed that moisture of the ground as well as that of the air was tant and that the temperature of both had to be considered. Of the five conditions, the first four are essential, but the fifth is very important for securing a copious deposit. It can hardly be maintained that no dew could form unless there were a supply of water by evaporation from warm ground, but the limited process of condensation which deprives the air of its moisture is soon terminable, while the process of distillation goes on as long as conditions are maintained. This indicates that wet soil can partly protect young plants against night frost. If tillation between the ground and the leaves is set up, the ture of the leaves cannot fall much below the original dew-point because the requisite supply of water for condensation is kept up, otherwise the dew-point will get progressively lower as the rnois ure is deposited and the cooling of the plant will continue.
In dew formation comparatively large changes take place within 1, layer a few inches thick, and for an adequate comprehension of he conditions close consideration is required. Figs. i and 2 show he condition of affairs, according to one of Aitken's observations. [he vertical scale gives heights in feet, while the horizontal scale ,hows air temperatures, and the dew-points measured by an ordi iary dry-bulb thermometer ; their variations with height are repre tnted by the curved lines. The line marked o is an arbitrary ;round level, a rather indefinite quantity when the surface is grass. [he special phenomena which are being considered take place in he layer which represents the rapid transition between the tem )erature of the ground 3 in. below the surface and that of the air a .ew inches higher.
If the dew-point curve and the "dry-bulb" curve cut above the ;urf ace, mist will result ; if they cut at the surface, dew will be formed. Below the surface, it may be assumed that the air is Saturated with moisture and any difference in temperature of the dew-point is accompanied by dis tillation. Incidentally, such dis tillation between soil layers of different temperatures must transfer considerable quantities of water either upward or down ward, the direction varying ac cording to the time of year.
The diagrams illustrate the importance of the warmth and moisture of the ground. At the surface there is a continual loss of radiated heat and a continual supply of warmth and moisture from below ; but while the heat can escape, the moisture cannot. The dry-bulb line is thus de flected to the left as it approaches the surface from above and the dew-point is deflected to the right, consequently the effect of the moisture of the ground is to cause the lines to converge. Fig. 1 shows this and indicates that dew will be deposited. In the case of grass, fig. 2, the deviation of the dry bulb line shows a sharp minimum of temperature at the surface. The dew-point line is also shown diverted to the left; but that could happen only in the unusual circumstance of so copious a con densation from the atmosphere as to make the air drier at the sur face than up above. Along the underground line there must be a gradual creeping of heat and moisture towards the surface by dis tillation, becoming more rapid for a greater temperature gradient.
The amount of dew deposited may be considerable, and, in tropical countries, is sometimes sufficient to be collected by gut ters, but it is not generally regarded as a large percentage of the total rainfall. Loesche probably overestimates a single night's dew deposition on the Loango coast at 3 mm., but measurements show that the aggregate annual deposit of dew corresponds to a depth of 1 to 1.5 in. of water near London (G. Dines), 1.2 in. at Munich (Wollny), 0.3 in. at Montpellier (Crova) and 1.6 in. at Tenbury, Worcestershire (Badgley).
The maintenance of "dew ponds" is intimately associated with this matter of total amount. "Dew-ponds" are certain isolated ponds on the upper levels of the chalk downs of the south of Eng land and elsewhere used as a water-supply for cattle. Some of them are very ancient, as the title of a book on Neolithic Dew ponds, London, 1904, 1907, by A. J. and G. Hubbard indicates. Their name implies that they depend upon dew and not entirely upon rain for their maintenance. Though the question has not been settled, the balance of evidence suggests that dew deposits do not make any important contribution to the water supply. The con struction of dew ponds is, however, still practised on traditional lines. Although there is some difference of opinion on the matter, it seems necessary for a new dew pond to be filled artificially first as it will not function by a natural accumulation of water in the impervious basin.