Роса: формирование, свойства и значение.

Water in the form of droplets that appears on thin, exposed objects in the morning or evening

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 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 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.

Water vapor will condense into droplets depending on the temperature. The temperature at which droplets 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 form directly in air that has cooled to its dew point (typically around condensation nuclei), such as fog or clouds. The thermodynamic 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.

Adequate 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. Poor thermal conductivity restricts the replacement of such losses from deeper ground layers, which are typically warmer at night. Another effect of dew is its hydration of fungal substrates and the mycelia of species such as pleated inkcaps, often found on lawns, and Phytophthora infestans which causes blight on potato plants.

The book On the Universe (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 (Ersa's aunt), goddess of the dawn, cried for her son's death, although later he received immortality. Dew, known in Hebrew as טל (tal), is significant 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. "Dewy" or "my father is the morning dew" are approximate etymologies of the Hebrew given name, Avital. In the Biblical Torah or Old Testament, dew is used symbolically in Deuteronomy 32:2 (KJV): "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." In the Catholic Mass in the Western Rite, whenever the Second Eucharistic Prayer is used, the priest prays over bread and wine, to God the Father; ‘Make holy, therefore, these gifts, we pray, by sending down your Spirit upon them like the dewfall, so that they may become for us the Body and Blood of our Lord Jesus Christ.’ The idea that the Holy Spirit enters the world and our lives in a quiet, undramatic way, ‘like the dewfall’, has great appeal for many Christians.

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 ponds 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 (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.