Dew (precipitation)
Dew is a misting precipitate of liquid water . By cooling the air below the dew point, the water vapor in the air condenses on objects close to the ground. This happens especially in the early morning, because then the temperatures are lowest due to the advanced night heat radiation of the earth ( morning dew ). If the droplets freeze afterwards , one speaks of frozen dew, in the case of the resublimation of water vapor to ice one speaks of hoarfrost , which can turn into frost or rough ice (rough frost).
Dew formation
The maximum amount of water vapor that the air can absorb increases exponentially with the air temperature. The temperature at which the air is saturated with water vapor , which is dependent on the water vapor content, i.e. exactly as many water molecules from the ambient air are attached to existing droplets as are released from the droplets, is called the dew point . As soon as the temperature of the air layers on the surface of the earth falls below the dew point, the water vapor in the air condenses on the cooled objects to form small water globules , also called dew pearls .
Colloquially it is said that “the dew falls” , but this is not correct as shown. Condensation products that are formed directly in the air, i.e. not on surfaces in contrast to dew, are responsible for haze , fog and, at some distance from the earth's surface, also clouds . The basic principles of their creation differ only insignificantly.
The latent condensation energy released during condensation counteracts a rapid decrease in temperature and therefore dampens the cooling effect. This leads to the dew point rule, which states that the lowest temperature of the night falls roughly to the dew point temperature measured the previous afternoon. However, this rule is only a rough guide.
If the temperatures drop below the freezing point of the water after the dew has formed , the dew pearls can freeze. Frozen dew should not be confused with frost , which forms at such low temperatures without going through liquid dew. Dew is created by condensation, frost by resublimation . There is also a risk of confusion with fog eaves , but it is closely linked to the presence of fog and occurs primarily on needles.
In the English-language specialist literature in particular, a distinction is occasionally made between dewfall and distillation . The latter is the (re) condensation of rising water vapor, which has previously evaporated in deeper (and thus warmer at night) soil layers, on the surface or on the plants. Dewfall , on the other hand, describes a net gain from the atmosphere.
The adsorption of water from the air that can be observed in hygroscopic media (e.g. dry soil surfaces) when the relative humidity rises is usually not counted as dew in the actual sense. For this, the surface does not have to fall below the dew point temperature.
The guttation must always be separated from the rope . H. the excretion of liquid water from inside the plant through openings ( hydathodes ) in the leaves of some plant species, e.g. B. many grasses. Since it occurs at high humidity (close to 100% in the air layer closest to the leaf) and these conditions can also be caused by a nightly decrease in surface temperature, guttation is difficult to distinguish from dew formation based on the weather conditions and often occurs simultaneously.
Occur
A sufficiently strong cooling of the earth's surface and the lower layers of the air always sets in when a strong radiation can take place. This is the case after sunset , i.e. at night , with the air temperature usually reaching its minimum in the early morning. The necessary conditions for this include above all a clear, cloudy sky, the air that is as motionless as possible, and a ground cover that enables good heat transfer .
This is the case, for example, with lawns and leaves. Bodies with a low level of radiation are less suitable for dew formation. Everything that hinders or reduces the nocturnal radiation also hinders or reduces the formation of dew. Usually, lack of wind is also described as a requirement. An unmoved air is therefore advantageous for the formation of dew, because otherwise the wind would always bring in new warm air, which would hinder cooling down to below the dew point if it came into contact with the ground. However, this strictly only applies to those cases in which already condensed water vapor from the moist, warmer soil can be replaced without any problems, as is usually the case with central European lawns. If the ground is dry, however, a certain amount of ventilation is necessary to maintain the downward transport of water vapor in the atmosphere, which in turn can owe its vapor reserve to the evaporation of the previous day or to nearby sources of moisture. Accordingly, dew formation at high wind speeds was found on arid islands. In general, the occurrence of dew is not linked to specific climatic zones , but it can differ considerably in terms of the year and significance.
The considerations of atmospheric dew formation due to air cooling are of a generally valid nature, so dew is not necessarily linked to specific times of the day, surfaces or surroundings. However, the term is mostly only applied to the case of morning droplets forming on meadows and other natural surfaces. So even if the fogging of a window or glasses involves the same basic process, one usually speaks of condensation rather than dew . This applies in particular to industrial processes and condensation inside buildings.
Measurement
The measuring device for determining the amount of precipitation caused by the dew is called a drosometer . It contains a plate on a fine pointer scale, covered with fine, fluffy wool, which is covered with dew at night, and the increase in weight indicates the thickness of the dew. The results obtained in this way, however, still lack the necessary precision for the time being.
In addition, these measurements on artificial surfaces, which were particularly widespread in the middle of the 20th century, only provide information on the potential for dew formation due to the state of the atmosphere ( temperature , humidity , wind and long-wave atmospheric counter-radiation ). The actual amount of dew also depends to a considerable extent on the properties of the surface itself or its subsurface, such as thermal conductivity and heat capacity , emissivity and soil temperature. For this reason, studies on the actual amount of dew at a specific examination site are increasingly using small, weighable lysimeters whose weighing platforms, however, have to meet high accuracy requirements. A representative section of the surrounding surface (bare ground, lawn or smaller plants) is then inserted into this.
Further measuring methods are the assessment of the drop image by comparison with given photographs or the volumetric or gravimetric determination of the amount of water that can be obtained by dabbing the surface of interest. It should be noted that, depending on the method used, different partial sizes of the rope or completely different phenomena are measured. Dab z. B. measures the guttation . Lysimeters provide the integrated water content over the depth of the soil column used, so that if it is large enough, only the net gain from the atmosphere is measured ( dewfall ), but not condensation water that comes from the smallest water cycle between the soil and the surface ( distillation ).
Interpretation - meaning
Statistical data
The proportion of dew in total precipitation in Central Europe is low with typical amounts of 0.1 to 0.2 mm per night and thus around 2 to 5%. The exceptions are quantities of 0.5 mm; maximum values of around 0.8 mm are more of a theoretical nature. In terms of quantity, dew formation is particularly important in tropical climates, where the air contains a lot of water vapor and is rapidly cooled down by the thermal radiation.
Effect on vegetation
However, dew is particularly important because it forms primarily on the surface of the vegetation and can thus improve its water supply despite the rather small absolute quantities. This could at least be proven for various desert plants, including lichens, and pine seedlings. To what extent large-scale irrigation effects z. B. in foggy deserts such as the Namib or Atacama are possible due to dew alone, is however controversial, since the fog precipitation usually plays a much larger role here.
Extraction of condensation water
Various man-made devices for the extraction of condensation water such as ancient stone mounds in the Ukraine, the medieval " dew ponds" of southern England and northern Germany or the mulching of fields with volcanic rock particles on Lanzarote are mostly interpreted differently today (e.g. as effective traps for mist or rain).
An organization called Esme is working on the development of economically viable harvesters. A film was developed which, thanks to special surface properties, collects a particularly large amount of dew. A prototype plant was built in a very dry region in India with 360 m² of this film. 50 liters of water are obtained per night. These dew collectors might be suitable for areas where there is insufficient fog or rain.
Agriculture
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Apart from its hydrological importance, dew is important for plants and especially for agriculture from another point of view, namely in relation to plant diseases . This shows that fungus spores can often only thrive when the plant surface is moistened with moisture. One example is Phytophthora infestans , a fungus that is responsible for late blight and tuber blight in potatoes and causes great damage worldwide.
mythology
In Greek mythology, the rope has its own goddess with Ersa .
manna
The doctors of the Arab and Latin Middle Ages understood the "manna" to be a dew ("ros") that falls on stones and trees, is sweet and congeals like honey. The manna should take on the nature of what it falls on. It should soften the abdomen, clear acute fevers, be useful to the chest and lungs as well as to the choleric and hot natures .
Sources of the Arab Middle Ages on "Manna" (selection)
- Avicenna 10th-11th --- Constantine the African 11th century
- Around instans 12th century --- Pseudo-Serapion 13th century
- Abu Muhammad ibn al-Baitar 13th century
Sources of the Latin Middle Ages on "Manna" (selection)
- Konrad von Megenberg 14th century
- Herbarius Moguntinus Mainz 1484 --- Gart der Gesundheit 1485 --- Hortus sanitatis 1491
May dew
In 1500 Hieronymus Brunschwig wrote in his small distilling book about "meigen dow". By this he meant the dew, which in mid-May, with the waxing moon on a bright, clear and light morning, before the sun rose completely and it had not rained the day before or during the night, was brushed with white sheets of meadows that were full of noble meadows Flowers stood far from damp ends and were as close as possible to mountains. The cloths were squeezed out and the water obtained was distilled with an alembic in Marienbad . Brunschwig recommended this water against skin impurities. It was also used as a solvent to make distillates from dried plants.
A 15th century southwest German manuscript recommended collecting this water on the night of the summer solstice :
- “For all wetagen yn leib, that's okay. Item an ſand iohan's day practicing gee except at night vnd vahe the tawe yn ſnice towels vnd wint dye outside yn a candy and ſeihs because ſchön vnd nym des ye in the morning a spoonful. When a fraw ſwanger iſt ſo eats ir useful. Also useful for ale gift. "
May dew served the alchemists to make the materia prima . They viewed it as "water impregnated with astral seed".
See also
Web links
- International Organization for Dew Utilization
- International Conference on Fog, Fog Collection and Dew
Individual evidence
- ^ Avicenna . 10-11 Century canon of medicine . Book II. Simple Medicines. Translation and editing by Gerhard von Cremona and Arnaldus de Villanova . Revision by Andrea Alpago (1450–1521). Venice 1555, p. 272: Manna (digitized version)
- ↑ Constantine the African . 11th century Liber de gradibus simplicium = translation of the Liber de gradibus simplicium of Ibn al-Jazzar . 10th century, print. Opera . Basel 1536, p. 347: Manna Digitalisat
- ↑ Approximately instans . 12th century print. Venice 1497, sheet 202r: Manna (digitized)
- ↑ Pseudo-Serapion . 13th century print. Venice 1497, sheet 106r: Manna (digitized)
- ↑ Abu Muhammad ibn al-Baitar . 13th century Kitāb al-jāmiʿ li-mufradāt al-adwiya wa al-aghdhiya. Translation. Joseph Sontheimer under the title Large compilation on the powers of the well-known simple healing and food. Hallberger, Volume II, Stuttgart 1842, p. 533 Manna (digitized version)
- ↑ Konrad von Megenberg . 14th century. Main source: Thomas of Cantimpré , Liber de natura rerum . Output. Franz Pfeiffer . Konrad von Megenberg. Book of nature. Aue, Stuttgart 1861, SS 90–91: Himelprot (digitized version )
- ↑ Herbarius Moguntinus . Peter Schöffer , Mainz 1484, Part II, Chapter 7 Manna (digitized version)
- ↑ Gart der Gesundheit . Peter Schöffer , Mainz 1485, chapter 267: Manna hymmeldauwe (digitized version )
- ^ Hortus sanitatis . Jacobus Meydenbach, Mainz 1491, Chapter 275: Manna (digitized version)
- ↑ Small distilling book, sheet 76v (digitized version )
- ↑ Small distilling book, sheets 10v-11r (digitized version )
- ↑ Small distilling book, sheet 107r (digitized version )
- ↑ Solstice
- ↑ Heidelberg. Cpg 551. Collective manuscript, Southwest Germany, 15th century, sheet 116r – 183r: Medical recipe collection. (Digitized version)
- ↑ Karin Figala. In: Claus Priesner and Karin Figala (eds.): Alchemie. Lexicon of Hermetic Science . Beck, Munich 1998, p. 239.