Dew

Tau is a fog precipitate formed from liquid water. By cooling the air below the dew point of water vapor in the air condenses on ground-level objects. This happens especially in the morning, because ( to be warmed because of the albedo, the thermal radiation of the earth without the sun ) then the temperatures are the lowest; this is what we call dew morning dew. If there is then a freezing of the drops, then one speaks of frozen dew, in the Resublimation of water vapor to ice is called hoar frost, which can go over to frost or rime.

Dew formation

The maximum amount of water vapor that the air can be positively and exponentially dependent on its temperature. The dependent on the water vapor content temperature at which the air is saturated with water vapor, that is, attach as many water molecules from the ambient air to existing droplets, as it turned out released from the droplets is called the dew point and the dew point temperature ( curve ). Once the temperature of the earth's surface nearest to air layers is below the dew point, the water vapor in the air condenses on the cooled objects to small water beads, also called pearls of dew.

Colloquially speaking to the fact that " the dew falls ," but this is not right as shown. Condensation products, which are not formed directly in the air, so unlike the dew on surfaces, are responsible for haze, fog, and at some distance from the Earth's surface also clouds. The basic principles of their formation differ only slightly.

The energy released in the condensation latent energy of condensation counteracts rapid temperature decrease and attenuates the effect of cooling from hence. This leads to Taupunktregel, indicating that the minimum temperature in the night as measured up to the dew point temperature decreases previous afternoon. However, this rule represents only a rough guide

The temperatures drop after formation of dew under the freezing point of water, so in pure water below 0 ° C, the pearls of dew may freeze. Frozen dew but should not be confused with frost that forms at such low temperatures without going through liquid dew. Dew is formed by condensation, frost by Resublimation. Even with fog eaves there is a likelihood of confusion, it is, however, closely linked to the presence of fog and occurs mainly in needles.

In particular, in the English literature of the dew is still occasionally differed in dewfall and distillation. The latter is the (re - ) condensation rising water vapor, which is previously evaporated in deeper (and warmer at night ) soil layers at the surface or the plants. Dewfall referred contrast, a net gain from the atmosphere.

Is generally not counted as such for the rope of the hygroscopic media (such as dry bottom surfaces ) observed adsorption of water from the air with an increase in relative humidity. For this purpose, the surface must not fall below the dew point temperature.

Always disconnect the dew is guttation, namely the elimination of liquid water from the plant interior through openings ( hydathodes ) in the leaves of some plant species, such as many grasses. Since it occurs at high humidities ( close to 100 % in the leaf next air layer), and these conditions can also be caused by a nocturnal decrease in surface temperature, guttation is difficult to distinguish on the basis of conditions of dew formation and often occurs simultaneously.

Occurrence

A sufficiently strong cooling of the earth's surface and the lower atmosphere begins whenever a strong charisma can take place. This is after sunset, so at night, the case, wherein the air temperature in the morning usually reaches its minimum. Among the conditions necessary for this purpose include, in particular a clear, cloudless sky, a still possible air and ground cover, which enables good heat transmission.

This is for example lawns and leaves the case. Body with low emissivity are less suitable for dew formation. Anything that prevents the nocturnal emission or decreased prevents, or reduces the formation of dew. Most also wind poverty is described as a prerequisite. Motionless air is therefore beneficial for the dew, as the wind would otherwise always introduce anew warm air that would impede upon contact with the ground a cooling down to below the dew point. However, this applies strictly only for those cases in which condensed water vapor from the moist, warm soil already can be easily replaced, as is the case in Central Europe lawns usually the case. In dry ground, however, a certain amount of ventilation to maintain the downward transport of water vapor in the atmosphere is necessary, which in turn may owe to the evaporation of the previous day or nearby moisture sources its steam supply. Dew formation at high wind speeds was accordingly found on arid islands. In general, the occurrence of dew is not subject to certain climates, it may, however, differ significantly in years of transition and meaning.

The observations of the atmospheric dew formation due to air cooling are universal nature, dew is not necessarily linked to specific times of day, surfaces or environments. The term is usually applied but only to the case of morning condensation on meadows and other natural surfaces. Even if it is so in the mist of a window or glasses by the same basic process, also termed usually not of tau, but condensation. This applies in particular to industrial processes and condensation within buildings.

Measurement

The measuring device for determining the due dew rainfall is referred to as Drosometer. It contains a pointer located on a fine scale, covered with fine, fuzzy wool plate which covers at night with dew, and their weight gain indicates the Taustärke. The results obtained in this way, but for now still lack the necessary accuracy.

In addition, these mainly middle of the 20th century spread measurements on artificial surfaces only provide evidence of the potential that exists due to the state of the atmosphere ( temperature, humidity, wind and atmospheric long-wave back radiation ) for the formation of dew. However, the actual Taumenge also depends to a large extent on properties of the surface itself or its subsoil such as thermal conductivity and heat capacity, emissivity and soil temperature. Therefore, studies use the actual Taumenge at a particular study site increasingly small weighable lysimeter, but the weighing platforms must meet high accuracy requirements. In these provide a representative section of the surrounding surface ( bare soil, grass or small plants) is then used.

Other measurement methods are the assessment of the drop image by comparison with predetermined photographs or volumetric or gravimetric determination of the amount of water that can be obtained by swabbing the surface of interest. It should be noted that depending on the method used, different portion sizes of dew or even completely different phenomena are measured. Dab eg measures with guttation. Lysimeters provide the integrated water content over the depth of the soil column used, so that when sufficient size only the net profit from the atmosphere is measured ( dewfall ), but not condensation that comes from the smallest water circuit between ground and surface ( distillation ).

Importance

Statistical data

The proportion of dew on the total rainfall is low in Central Europe with typical amounts of 0.1 to 0.2 mm per night, which is about 2 to 5 %. Exceptions are amounts of 0.5 mm, more of a theoretical nature are maximum values ​​of about 0.8 mm. Quantitatively, particularly important is the formation of dew in the tropical climates where the air contains a lot of water vapor and quickly learns by the heat radiation, a strong cooling.

Effect on vegetation

However, its special significance at the dew in that it preferably forms at the vegetation surface and the water supply can thereby improve despite the rather low absolute amounts. This has been demonstrated at least for several desert plants, including lichens, and pine seedlings. The extent to which large-scale irrigation effects eg in fog deserts such as the Namib or Atacama by Tau alone possible, however, is controversial, since most of the fog precipitation plays a much bigger role.

Recovery of condensate

Various man- suspected devices for the recovery of condensation such as antique stone hill in the Ukraine, the medieval " Dew Ponds " of southern England and northern Germany or the mulching of fields with volcanic rock particles in Lanzarote are usually interpreted differently according to detailed investigations, today (eg as effective traps for fog precipitation or rain).

An organization with the name Esme is working on the development of economically usable Tausammler. Such a film has been developed, which, thanks to special surface properties collects a great deal of dew. A prototypical system has been built in a very dry region in India with 360 m² of this film. Per Night 50 liters of water are thus obtained. This Tausammler might be suitable for those areas where fog or rain is not available in sufficient quantities. (see more links).

Agriculture

Apart from its hydrological significance is dew for plants and in particular to agriculture still significant from a different angle, namely in relation to plant diseases. This shows that fungal spores often can only thrive if the plant surface is feuchtigkeitsbenetzt. An example is Phytophthora infestans, a fungus that is responsible for late blight in potatoes and caused extensive damage worldwide.

As fog or frost is dew is an indication of low temperatures and thus an increased risk of frost.

Mythology

In Greek mythology, the dew with Ersa has its own goddess.