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Some dew on a wildflower in Sequoia National Park
Dew on a spider web
Dew on a snipe fly (Rhagio scolopaceus)
OPUR Dew Recovering Roof in Croatia
Dew recovering roof of OPUR in Croatia

Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.

When temperatures are low enough, dew takes the form of ice; this form is called frost.

Because dew is related to the temperature of surfaces, in late summer it is formed most easily on surfaces which are not warmed by conducted heat from deep ground, such as: grass, leaves, railings, car roofs, and bridges.

Dew should not be confused with guttation, which is the process by which plants release excess water from the tips of their leaves.



Water will condense into droplets depending on the temperature. The temperature at which droplets can form is called the Dew Point. When surface temperature drops, eventually reaching the dew point, atmospheric water vapor condenses to form small droplets on the surface. This process distinguishes dew from those hydrometeors (meteorological occurrences of water) which are formed directly in air cooling to its dew point (typically around condensation nuclei) such as fog or clouds. The thermodynamic principles of formation, however, are virtually the same.


Sufficient cooling of the surface typically takes place when it loses more energy by infrared radiation than it receives as solar radiation from the sun, which is especially the case on clear nights. As another important point, poor thermal conductivity restricts the replacement of such losses from deeper ground layers which are typically warmer at night. Preferred objects of dew formation are thus poor conducting or well isolated from the ground, and non-metallic or coated as shiny metal surfaces are poor infrared radiators. Preferred weather conditions include the absence of clouds and little water vapor in the higher atmosphere to minimize greenhouse effects and sufficient humidity of the air near the ground. Typical dew nights are classically considered to be calm because the wind transports (nocturnally) warmer air from higher levels to the cold surface. But, if the atmosphere is the major source of moisture (this part of dew is called dewfall), a certain amount of ventilation is needed to replace the vapor that is already condensed. The highest optimum wind speeds could be found on arid islands. If the wet soil beneath is the major source of vapour, however (this part of dew is called distillation), wind always seems to be adverse.

The principles of dew formation do not strictly constrict its occurrence to the night and the outdoors. They are also working when eyeglasses get steamy in a warm, wet room or in industrial processes. However, the term condensation is preferred in these cases.


A classical device for dew measurement is the drosometer. A small, artificial condenser surface is suspended from an arm attached to a pointer or a pen that records the weight changes of the condenser on a drum. Besides being very wind sensitive, however, this, like all artificial surface devices, only provides a measure of the meteorological potential for dew formation. The actual amount of dew in a specific place is strongly dependent on surface properties. For its measurement, plants, leaves, or whole soil columns are placed on a balance with their surface at the same height and in the same surroundings as would occur naturally, thus providing a small lysimeter. Further methods include estimation by means of comparing the droplets to standardized photographs, or volumetric measurement of the amount of water wiped from the surface. It has to be kept in mind that some of these methods include guttation, while others only measure dewfall and/or distillation.


Due to its dependence on radiation balance, dew amounts can reach a theoretical maximum of about 0.8 mm per night, measured values, however, rarely exceeding 0.5 mm. In most climates of the world, the annual average is too small to compete with rain. In regions with considerable dry seasons, adapted plants like lichen or pine seedlings benefit from dew. Large-scale, natural irrigation without rainfall, such as in the Atacama Desert and Namib desert, however, is mostly attributed to fog water.

Another effect of dew on plants is its role as a habitat for pathogens such as the fungus Phytophthora infestans which infects potato plants.

In Greek mythology, Ersa is the goddess of dew.

Dew, known in Hebrew as טל (tal), is very important in the Jewish religion for agricultural and theological purposes. On the first day of Passover, the Chazan, dressed in a white kittel, leads a service in which he prays for dew between that point and Sukkot. During the rainy season between December and Passover there are also additions in the Amidah for blessed dew to come together with rain. There are many midrashim that refer to dew as being the tool for ultimate resurrection.[1]

In the Biblical Old Testament dew is used symbolically in Deuteronomy 32:3: "My doctrine shall drop as the rain, my speech shall distill as the dew, as the small rain upon the tender herb, and as the showers upon the grass:"

Artificial harvesting

Several man-made devices such as antique, big stone piles in Ukraine, medieval "dew ponds" in southern England, or volcanic stone covers on the fields of Lanzarote have been thought to be dew-catching devices, but could be shown to work on other principles. At present, the International Organisation for Dew Utilization is working on effective, foil-based condensers for regions where rain or fog cannot cover water needs throughout the year.

Large scale dew harvesting systems have been made by Indian Institute of Management Ahmedabad (IIMA) with the participation of the International Organisation for Dew Utilization (OPUR) at coastal semi arid region Kutch. These condensers can harvest more than 200 litres (on average) of dew water per night for about 90 nights in the dew season October-May. The research lab of IIMA has shown that dew can serve as a supplementary source of water in coastal arid areas.


  1. ^ Jewish Encyclopedia: Resurrection. Accessed 21 Dec, 2008.

2. Publications on Dew Harvesting Systems

External links

Source material

Up to date as of January 22, 2010

From Wikisource

This is a disambiguation page, which lists works which share the same title. If an article link referred you here, please consider editing it to point directly to the intended page.

Dew may refer to:

  • Dew, a poem by Sara Teasdale published in Helen of Troy and Other Poems.
  • Dew, a poem by Sara Teasdale published in Love Songs.

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

DEW. The word " dew " (O.E. deaw; cf. Ger. Tau) is a very ancient one and its meaning must therefore be defined on historical principles. According to the New English Dictionary, it means " the moisture deposited 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." Huxley in his Physiography makes the addition " without production of mist." The formation of mist is not necessary for the formation of dew, nor does it necessarily prevent it. If the deposit of moisture is in the form of ice instead of water it is called hoarfrost. The researches of Aitken suggest that the words " by condensation of the vapour in the atmosphere " might be omitted from the definition. He has given reasons for believing that the large dewdrops on the leaves of plants, the most characteristic of all the phenomena of dew, are to be accounted for, in large measure at least, by the exuding of drops of water from the plant through the pores of the leaves themselves. The formation of dewdrops in such cases is the continuation of the irrigation process of the plant for supplying the leaves with water from the soil. The process is set up in full vigour in the daytime to maintain tolerable thermal conditions at the surface of the leaf in the hot sun, and continued after the sun has gone.

On the other hand, the most typical physical experiment illustrating the formation of dew is the production of a deposit of moisture, in minute drops, upon the exterior surface of a glass or polished metal vessel by the cooling of a liquid contained in the vessel. If the liquid is water, it can be cooled by pieces of ice; if volatile like ether, by bubbling air through it. No deposit is formed by this process until the temperature is reduced to a point which, from that circumstance, has received a special name, although it depends upon the state of the air round the vessel. So generally accepted is the physical analogy between the natural formation of dew and its artificial production in the manner described, that the point below which the temperature of a surface must be reduced in order to obtain the deposit is known as the " dew-point." In the view of physicists the dew-point is the temperature at which, by being cooled without change of pressure, the air becomes saturated with water vapour, not on account of any increase of supply of that compound, but by the diminution of the capacity of the air for holding it in the gaseous condition. Thus, when the dew-point temperature has been determined, the pressure of water vapour in the atmosphere at the time of the deposit is given by reference to a table of saturation pressures of water vapour at different temperatures. As it is a well-established proposition that the pressure of the water vapour in the air does not vary while the air is being cooled without change of its total external pressure, the saturation pressure at the dew-point gives the pressure of water vapour in the air when the cooling commenced. Thus the artificial formation of dew and consequent determination of the dew-point is a recognized method of measuring the pressure, and thence the amount of water vapour in the atmosphere. The dew-point method is indeed in some ways a fundamental method of hygrometry.

The dew-point is a matter of really vital consequence in the question of the oppressiveness of the atmosphere or its reverse. So long as the dew-point is low, high temperature does not matter, but when the dew-point begins to approach the normal temperature of the human body the atmosphere becomes insupportable.

The physical explanation of the formation of dew consists practically in determining the process or processes by which leaves, blades of grass, stones, and other objects in the open air upon which dew may be observed, become cooled " below the dew-point." Formerly, from the time of Aristotle at least, dew was supposed to " fall." That view of the process was not extinct at the time of Wordsworth and poets might even now use the figure without reproach. To Dr Charles Wells of London belongs the credit of bringing to a focus the ideas which originated with the study of radiation at the beginning of the 19th century, and which are expressed by saying that the cooling necessary to produce dew on exposed surfaces is to be attributed to the radiation from the surfaces to a clear sky. He gave an account of the theory of automatic cooling by radiation, which has found a place in all text-books of physics, in his first Essay on Dew published in 1818. The theory is supported in that and in a second essay, by a number of well-planned observations, and the essays are indeed models of scientific method. The process of the formation of dew as represented by Wells is a simple one. It starts from the point of view that all bodies are constantly radiating heat, and cool automatically unless they receive a corresponding amount of heat from other bodies by radiation or conduction. Good radiators, which are at the same time bad conductors of heat, such as blades of grass, lose heat rapidly on a clear night by radiation to the sky and become cooled below the dew-point of the atmosphere.

The question was very fully studied by Melloni and others, but little more was added to the explanation given by Wells until 1885, when John Aitken of Falkirk called attention to the question whether the water of dewdrops on plants or stones came from the air or the earth, and described a number of experiments to show that under the conditions of observation in Scotland, it was the earth from which the moisture was probably obtained, either by the operation of the vascular system of plants in the formation of exuded dewdrops, or by evaporation and subsequent condensation in the lowest layer of the atmosphere. Some controversy was excited by the publication of Aitken's views, and it is interesting to revert to it because it illustrates a proposition which is of general application in meteorological questions, namely, that the physical processes operative in the evolution of meteorological phenomena are generally complex. It is not radiation alone that is necessary to produce dew, nor even radiation from a body which does not conduct heat. The body must be surrounded by an atmosphere so fully supplied with moisture that the dewpoint can be passed by the cooling due to radiation. Thus the conditions favourable for the formation of dew are (I) a good radiating surface, (2) a still atmosphere, (3) a clear sky, (4) thermal insulation of the radiating surface, (5) warm moist ground or some other provision to produce a supply of moisture in the surface layers of air.

Aitken's contribution to the theory of dew shows that in considering the supply of moisture we must take into consideration the ground as well as the air and concern ourselves with the temperature of both. Of the five conditions mentioned, the first four may be considered necessary, 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, when such a supply is forthcoming, it is evident that in place of the limited process of condensation which deprives the air of its moisture and is therefore soon terminable, we have the process of distillation which goes on as long as conditions are maintained. This distinction is of some practical importance for it indicates the protecting power of wet soil in favour of young plants as against night frost. If distillation between the ground and the leaves is set up, the temperature of the leaves cannot fall much below the original dew-point because the supply of water for condensation is kept up; but if the compensation for loss of heat by radiation is dependent simply on the condensation of water from the atmosphere, without renewal of the supply, the dew-point will gradually get lower as the moisture is deposited and the process of cooling will go on.

In these questions we have to deal with comparatively large changes taking place within a small range of level. It is with the layer a few inches thick on either side of the surface that we are principally concerned, and for an adequate comprehension of the conditions close consideration is required. To illustrate this point reference may be made to figs. I and 2, which represent the condition of affairs at 10 40 P.M. on about the 10th of October 1885, according to observations by Aitken. Vertical distances represent heights in feet, while the temperatures of the air and the dew-point are represented by horizontal distances and their variations with height by the curved lines of the diagram. The line marked o is the ground level itself, a rather indefinite quantity when the surface is grass. The whole vertical distance represented is from 4 ft. above ground to i ft. below ground, and the special phenomena soil Grass which we are considering take place in the layer which represents the rapid transition lieo tween the temperature 211 of the ground 3 in.

below the surface and that of the air a few inches above ground. Ground The point of interest 0 is to determine where the dew-point curve and dry-bulb curve will cut.

If they cut above the FIG. I.

Earth ljempera lure 30° 40° surface, 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 distillation. It may be remarked, by the way, that such distillation between soil layers of different temperatures must be productive of the transference of large quantities of water between different levels in the soil either upward or downward according to the time of year.

These diagrams illustrate the importance of the warmth and moisture of the ground in the phenomena which have been considered. From the surface there is a continual loss of heat going on by radiation and a continual supply of warmth and moisture from below. But while the heat can escape, the moisture cannot. Thus the dry-bulb line is deflected to the left as it approaches the surface, the dew-point line to the right. Thus the effect of the moisture of the ground is to cause the lines to approach. In the case of grass, fig. 2, the deviation of the dry-bulb line to the left to form a sharp minimum of temperature at the surface is well shown. The dew-point line is also shown diverted to the left to the same point as the dry-bulb; but that could only happen if there were so copious a condensation from the atmosphere as actually to make the air drier at the surface than up above. In diagram 1, for soil, the effect on air temperature and moisture is shown; the two lines converge to cut at the surface where a dew deposit will be formed. Along the underground line there must be a gradual creeping of heat and moisture towards the surface by distillation, the more rapid the greater the temperature gradient.

The amount of dew deposited is considerable, and, in tropical countries, is sometimes sufficiently heavy to be collected by gutters and spouts, but it is not generally regarded as a large percentage of the total rainfall. Loesche estimates the amount of dew for a single night on the Loango coast at 3 mm., but the estimate seems a high one. Measurements go to show that the depth of water corresponding with the aggregate annual deposit of dew is I in. to 1.5 in. near London (G. Dines), 1.2 in. at Munich (Wollny), 0.3 in. at Montpellier (Crova), 1.6 in. at Tenbury, Worcestershire (Badgley).

With the question of the amount of water collected as dew, that of the maintenance of " dew ponds " is intimately associated. The name is given to certain isolated ponds on the upper levels of the chalk downs of the south of England and elsewhere. Some of these ponds are very ancient, as the title of a work on Neolithic Dewponds by A. J. and G. Hubbard indicates. Their name seems to imply the hypothesis that they depend upon dew and not entirely upon rain for their maintenance as a source of water supply for cattle, for which they are used. The question has been discussed a good deal, but not settled; the balance of evidence seems to be against the view that dew deposits make any important contribution to the supply of water. The construction of dew ponds is, however, still practised on traditional lines, and it is said that a new dew pond has first to be filled artificially.

13"  ?

evel 50F It does not come into existence by the gradual accumulation of water in an impervious basin.


- For Dew, see the two essays by Dr Charles Wells (London, 1818), also " An Essay on Dew," edited by Casella (London, 1866), Longmans', with additions by Strachan; Melloni, Pogg. Ann. lxxi. pp. 416, 424 and lxxiii. p. 467; Jamin, " Complements a la theorie de la rosee," Journal de physique, viii. p. 41; J. Aitken, on " Dew," Trans. Roy. Soc. of Edinburgh, xxxiii., part i. 2, and " Nature," vol. xxxiii. p. 256; C. Tomlinson, " Remarks on a new Theory of Dew," Phil. Mag. (1886), 5th series, vol. 21, p. 483 and vol. 22, p. 270; Russell, Nature, vol. 47, p. 210; also Met. Zeit. (1893), p. 39 0; Homen, Bodenphysikalische and meteorologische Beobachtungen (Berlin, 1894), iii.; Taubildung, p. 88, &c.; Rubenson, " Die Temperaturand Feuchtigkeitsverhaltnisse in den unteren Luftschichten bei der Taubildung," Met. Zeit. xi. (1876), p. 65; H. E. Hamberg, " Temperature et humidite de l'air a differentes hauteurs a Upsal," Soc. R. des sciences d'Upsal (1876); review in Met. Zeit. xii. (1877), p. 105.

For Dew Ponds, see Stephen Hales, Statical Essays, vol. i., experiment xix., pp. 52-57 (2nd ed., London, 1731); Gilbert White, Natural History and Antiquities of Selborne, letter xxix. (London, 1789); Dr C. Wells, An Essay on Dew (London, 1818, 1821 and 1866); Rev. J. C. Clutterbuck, " Prize Essay on Water Supply," Journ. Roy. Agric. Soc., 2nd series, vol. i. pp. 271-287 (1865); Field and Symons, " Evaporation from the Surface of Water," Brit. Assoc. Rep. (1869), sect., pp. 25, 26; J. Lucas, " Hydrogeology: One of the Developments of Modern Practical Geology," Trans. Inst. Surveyors, vol. ix. pp. 153-232 (1877); H. P. Slade, " A Short Practical Treatise on Dew Ponds " (London, 1877); Clement Reid, " The Natural History of Isolated Ponds," Trans. Norfolk and Norwich Naturalists' Society, vol. v. pp. 272-286 (1892); Professor G. S. Brady, On the Nature and Origin of Freshwater Faunas (1899); Professor L. C. Miall, " Dew Ponds," Reports of the British Association (Bradford Meeting, 1900), pp. 579-5 8 5; A. J. and G. Hubbard, " Neolithic Dewponds and Cattle-Ways " (London, 1904, 1907). (W. N. S.)

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Bible wiki

Up to date as of January 23, 2010

From BibleWiki

"There is no dew properly so called in Palestine, for there is no moisture in the hot summer air to be chilled into dew-drops by the coldness of the night. From May till October rain is unknown, the sun shining with unclouded brightness day after day. The heat becomes intense, the ground hard, and vegetation would perish but for the moist west winds that come each night from the sea. The bright skies cause the heat of the day to radiate very quickly into space, so that the nights are as cold as the day is the reverse, a peculiarity of climate from which poor Jacob suffered thousands of years ago (Gen 31:40). To this coldness of the night air the indispensable watering of all plant-life is due. The winds, loaded with moisture, are robbed of it as they pass over the land, the cold air condensing it into drops of water, which fall in a gracious rain of mist on every thirsty blade. In the morning the fog thus created rests like a sea over the plains, and far up the sides of the hills, which raise their heads above it like so many islands. At sunrise, however, the scene speedily changes. By the kindling light the mist is transformed into vast snow-white clouds, which presently break into separate masses and rise up the mountain-sides, to disappear in the blue above, dissipated by the increasing heat. These are 'the morning clouds and the early dew that go away' of which Hosea (6:4; 13:3) speaks so touchingly" (Geikie's The Holy Land, etc., i., p. 72). Dew is a source of great fertility (Gen 27:28; Deut 33:13; Zech 8:12), and its withdrawal is regarded as a curse from God (2 Sam 1:21; 1 Kg 17:1). It is the symbol of a multitude (2 Sam 17:12; Ps 1103); and from its refreshing influence it is an emblem of brotherly love and harmony (Ps 1333), and of rich spiritual blessings (Hos 14:5).

This entry includes text from Easton's Bible Dictionary, 1897.

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This article needs to be merged with DEW (Jewish Encyclopedia).

Simple English

File:Dew on nasturtium
Dew on nasturtium leaf
File:Dew on a Equisetum fluviatile Luc
Dew on a Equisetum fluviatile Luc Viatour.

Dew is a thin film of water that has condensed on the surface of objects near the ground in the morning or evening.[1] These objects cool in the night. When they cool, the thin layer of air around them cools too. This makes some water vapor condense on the object.[1] The condensation happens because the air is able to hold less water when it is cooled. The temperature when condensation begins is called the dew point. If the dew point temperature becomes lower than 32 °F (0 °C), the dew turns into frost. Most people agree that the water in the dew comes from the atmosphere.[1] However, some studies suggest that it also comes up through the soil and then condenses on the ground.[1] This happens, though, only when conditions are right.

Dew forms best when the atmosphere is clean, the air is calm, and there is not much wind.[2] It does not always form in the night and outdoors, though. They also work when eyeglasses get steamy in warm, wet rooms. Dew may also happen in industrial processes. However, people use the word condensation more often than dew.

In the Old Testament dew is used as a symbol. Deuteronomy 32:3 says: " speech shall distill as the dew, as the small rain upon the tender herb, and as the showers upon the grass." (Deuteronomy 32 3)


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