![Dew Drops](https://upload.wikimedia.org/wikipedia/commons/e/ed/Dew_Drops.JPG)
Dew is
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
in the form of
droplets
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant d ...
that appears on thin, exposed objects in the
morning
Morning is the period from sunrise to noon. There are no exact times for when morning begins (also true of evening and night) because it can vary according to one's lifestyle and the hours of daylight at each time of year. However, morning s ...
or
evening
Evening is the period of a day that starts at the end of the afternoon and overlaps with the beginning of night. The exact times when evening begins and ends depend on location, time of year, and culture, but it is generally regarded as beginn ...
due to
condensation.
![Dense dew on grass](https://upload.wikimedia.org/wikipedia/commons/b/b3/Dense_dew_on_grass.jpg)
As the exposed surface cools by
radiating
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visib ...
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, called
frost.
Because
dew
Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to condensation.
As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at wh ...
is related to the temperature of surfaces, in late summer it forms most easily on surfaces that are not warmed by conducted heat from deep ground, such as grass, leaves, railings, car roofs, and bridges.
Formation
Water vapor will condense into droplets depending on the temperature. The
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
at which droplets form is called the ''
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
''. 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
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, Rain and snow mixed, sleet, snow, ice pellets, ...
(meteorological occurrences of water), which form directly in air that has cooled to its dew point (typically around
condensation nuclei), such as
fog or
clouds. The
thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...
principles of formation, however, are the same. Dew is commonly formed during select times of the day. Nights, early mornings, and early evenings are all times during which dew is likely to be found.
Occurrence
Adequate cooling of the surface typically takes place when it loses more energy by
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
radiation than it receives as
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre ( ...
from the sun, which is especially the case on clear nights. Poor
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
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, while shiny metal coated surfaces are poor infrared radiators. Preferred weather conditions include the absence of clouds and little water
vapor
In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Her ...
in the higher atmosphere to minimize greenhouse effects and sufficient humidity of the air near the ground. Typical dew nights are classically considered calm, because the wind transports (nocturnally) warmer air from higher levels to the cold surface. However, if the atmosphere is the major source of moisture (this type 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 vapor, however (this type of dew formation is called
distillation
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...
), wind always seems adverse.
The processes of dew formation do not restrict 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.
Measurement
A classical device for dew measurement is the
drosometer
A drosometer (from Classical Greek , ''drosos'', dew + , ''metron'', measure) is an apparatus for measuring the quantity of dew formed in a unit of time per unit area of surface.
Description
The surface may be either a horizontal metal plate or a ...
. 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
A lysimeter (from Greek λύσις (loosening) and the suffix ''-meter'') is a measuring device which can be used to measure the amount of actual evapotranspiration which is released by plants (usually crops or trees). By recording the amount of p ...
. 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. Some of these methods include
guttation
Guttation is the exudation of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses, and a number of fungi, which are not plants but were previously categorized as such and studied as part of botany. Guttation ...
, while others only measure dewfall and/or
distillation
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...
.
Significance
![Slovenia (12248224706)](https://upload.wikimedia.org/wikipedia/commons/a/ac/Slovenia_%2812248224706%29.jpg)
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 exceed 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
The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in th ...
and
Namib
The Namib ( ; pt, Namibe) is a coastal desert in Southern Africa. The name is of Khoekhoegowab origin and means "vast place". According to the broadest definition, the Namib stretches for more than along the Atlantic coasts of Angola, Nami ...
desert, however, is mostly attributed to
fog water. In the Negev Desert in Israel, dew has been found to account for almost half of the water found in three dominant desert species, ''
Salsola inermis,
Artemisia sieberi and
Haloxylon scoparium''.
Another effect of dew is its hydration of fungal substrates and the mycelia of species such as
Pleated Inkcaps on lawns and ''
Phytophthora infestans
''Phytophthora infestans'' is an oomycete or water mold, a fungus-like microorganism that causes the serious potato and tomato disease known as late blight or potato blight. Early blight, caused by '' Alternaria solani'', is also often called "p ...
'' which causes blight on potato plants.
Historic
The book ''
On the Universe
''On the Universe'' ( el, Περὶ Κόσμου; la, De Mundo) is a theological and scientific treatise included in the Corpus Aristotelicum but usually regarded as spurious. It was likely published between and the . The work discusses cosmol ...
'' (''De Mundo'') (composed before 250 BC or between 350 and 200 BC) stated: "Dew is moisture minute in composition falling from a clear sky; ice is water congealed in a condensed form from a clear sky;
hoar-frost is congealed dew, and 'dew-frost' is dew which is half congealed".
In Greek mythology,
Ersa is the goddess and personification of dew.
Also, according to the myth, the dew in the morning was created when
Eos
In ancient Greek mythology and religion, Eos (; Ionic and Homeric Greek ''Ēṓs'', Attic ''Héōs'', "dawn", or ; Aeolic ''Aúōs'', Doric ''Āṓs'') is the goddess and personification of the dawn, who rose each morning from her home at ...
(Ersa's aunt), goddess of the dawn, cried for
her son's death, although lately he received immortality.
Dew, known in
Hebrew
Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...
as טל (''tal''), is significant in the
Jewish religion
Judaism ( he, ''Yahăḏūṯ'') is an Abrahamic, monotheistic, and ethnic religion comprising the collective religious, cultural, and legal tradition and civilization of the Jewish people. It has its roots as an organized religion in the M ...
for agricultural and
theological
Theology is the systematic study of the nature of the divine and, more broadly, of religious belief. It is taught as an academic discipline, typically in universities and seminaries. It occupies itself with the unique content of analyzing the s ...
purposes. On the first day of
Passover
Passover, also called Pesach (; ), is a major Jewish holiday that celebrates the Biblical story of the Israelites escape from slavery in Egypt, which occurs on the 15th day of the Hebrew month of Nisan, the first month of Aviv, or spring. ...
, the ''
Chazan
A ''hazzan'' (; , lit. Hazan) or ''chazzan'' ( he, חַזָּן , plural ; Yiddish ''khazn''; Ladino ''Hasan'') is a Jewish musician or precentor trained in the vocal arts who helps lead the congregation in songful prayer.
In English, this p ...
'', dressed in a white ''
kittel
220px, A kittel
A ''kittel'' ( yi, קיטל) is a white linen or cotton robe worn by religious Ashkenazi Jews on holidays, in the synagogue or at home when leading the Passover seder. Kittels are sometimes worn by grooms. It is also customary for ...
'', 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
midrash
''Midrash'' (;["midrash"]
''Random House Webster's Unabridged Dictionary''. he, מִדְרָשׁ; ...
im that refer to dew as being the tool for ultimate resurrection.
In the Biblical
Torah
The Torah (; hbo, ''Tōrā'', "Instruction", "Teaching" or "Law") is the compilation of the first five books of the Hebrew Bible, namely the books of Genesis, Exodus, Leviticus, Numbers and Deuteronomy. In that sense, Torah means the ...
or
Old Testament, dew is used symbolically in : "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
The
harvesting of dew potentially allows water availability in areas where supporting weather conditions, such as rain, are lacking. Several man-made devices such as antique big stone piles in Ukraine, medieval
dew pond
A dew pond is an artificial pond usually sited on the top of a hill, intended for watering livestock. Dew ponds are used in areas where a natural supply of surface water may not be readily available. The name dew pond (sometimes cloud pond or mist ...
s in southern England, and 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 Organization for Dew Utilization (OPUR) 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 the
Indian Institute of Management Ahmedabad
Indian Institute of Management Ahmedabad (IIM Ahmedabad) is the world's number 1 business school, located in Ahmedabad, Gujarat, India. The school has been accorded the status of an Institute of National Importance by Ministry of Human Resourc ...
(IIMA) with the participation of OPUR in the coastal, semiarid region of
Kutch.These condensers can harvest more than 200 liters (on average) of dew water per night for about 90 nights in the October-to-May dew season. The IIMA research laboratory has shown that dew can serve as a supplementary source of water in coastal arid areas.
A large-scale dew harvesting scheme envisages circulating cold sea water in EPDM collectors near the seashore. These condense dew and fog to supply clean drinking water. Other, more recent, studies display possible roof integration for dew harvesting devices.
References
External links
{{Commons, Dew
OPUR (International Organisation for Dew Utilization)Audio Recording of Cantor Yossele Rosenblatt's Dew Servicefrom the Florida Atlantic University Judaica Sound Archives ''(Hebrew audio)''
Psychrometrics
Precipitation