dew

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DEW

DEW (Heb. טַל), condensation of water vapor on an object near the ground, whose temperature has fallen below the "dew point" of the surrounding air because of radiational cooling during the night. The conditions favoring the formation of dew are clear nights, moist air, and only light winds in the surface layers of the atmosphere.

The Bible places so much importance on dew as a source of water for plant life (Hos. 14:6–8) that in its absence a drought is considered to prevail (Hag. 1:10–11). Dew, like rain, is a symbol of life and God's beneficence (Zech. 8:12). (It should be noted, however, that in biblical Hebrew טל may also refer to rain.) As a figure of speech dew expresses a source of abundance (Gen. 27:28), silent and sudden-coming (ii Sam. 17:12), and ephemeral (Hos. 13:3). Several verses referring to dew appear in the Bible, according to which the main season of dew is late spring–early summer, or harvest time (e.g., Hos. 14:6; Prov. 19:12; Isa. 18:4; Job 29:19; Song 5:2). Soon after harvest time in the Harod Valley, Gideon "wrung enough dew from the fleece to fill a bowl with water" (Judg. 6:38). This valley, however, does not receive much dew, and is situated near the hills of Gilboa which David, in his lament over the death of Saul and Jonathan, cursed to enjoy neither dew nor rain (ii Sam. 1:21).

According to modern investigations, however, the value of dew in the water balance of plants is dubious. As for the distribution of dew, it is interesting to note that the Bible refers to many of the regions, on both sides of the Jordan, in which dew occurs. As some Bible scholars have pointed out, most verses which allude to dew in different regions employ similar phrases of dewiness for them. The average frequency of dew nights in Israel, according to measurements by the Duvdevani dew gauge in the period 1945–1952, was for Tel Shalom (on the Coastal Plain) 231 nights per year; at the Tavor Agr. School (Hill Region) 163; and at Dafnah (in the Jordan Valley) 115.

The largest annual number of dew nights is found in the central Coastal Plain and the northwestern Negev. The northern coastal region and Carmel beach are not favorable for dew formation. The Hill Region is not known for much dew. Mt. Carmel, being relatively low and the nearest hill to the Mediterranean, is the dewiest hill in Israel. The quantity and frequency of dew depend much on local topography: slopes receive little dew while level and concave areas receive it in abundance. In the low and level Valley of Jezreel there are many dew nights, but its western part is dewier than the eastern part, which descends to the Harod Valley. Dew is scarce in the Jordan Valley, particularly in its southern part (Jericho). However, in the flat and concave parts of the Huleh and Beth-Shean valleys the conditions for dew formation are better.

A breakdown of the seasonal distribution of dew frequency in Israel indicates that the largest number of dew nights occurs in summer in the Coastal Plain and Hill Region, the lowest number occurs in winter, and an intermediate number occurs in spring and fall (see Table: Number of Dew Nights).

In the southern Jordan Valley, between the Beth-Shean Valley and the Dead Sea, the regimen of dew is opposite to that in the coastal and hill regions. Dew in the southern Jordan Valley is most frequent in winter, while in summer it is rare or absent in this low-lying region. In the northern Jordan Valley no one month shows a marked increase in the number of dew nights.

bibliography:

S. Duvdevani, in: Quarterly Journal of the Royal Meteorological Society, 73 (1947), 282–96; D. Ashbel, in: Geographical Review, 39 (1949), 291–7; M. Gilead and N. Rosenan, in: iej, 4 (1954), 120–3; J. Neumann, in: Archiv fuer Meteorologie, Geophysik und Bioklimatologie, 9 (1956), 197–203; J. Katsnelson, in: Enziklopedyah le-Hakla'ut, 1 (1966), 27–62; U. Mané, in: Atlas of Israel (1970), sheet iv/1., maps R-T; N. Shalem, in: Sinai, 20 (1947), 119–35.

[Jacob Katsnelson]

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dew, thin film of water that has condensed on the surface of objects near the ground. Dew forms when radiational cooling of these objects during the nighttime hours also cools the shallow layer of overlying air in contact with them, causing the condensation of some water vapor. This condensation occurs because the capacity of air to hold water vapor decreases as the air is cooled. The temperature at which condensation begins, for a sample of air with a given water vapor content, is termed the dew point. If a dew point temperature below 32°F (0°C) is reached, sublimation occurs, i.e., the water vapor converts directly to frost. Should the surface temperature drop below 32°F after the dew has already collected, the dew may freeze into so-called white dew. Most authorities account for the supply of water vapor as coming from the atmosphere, though some research suggests that it also diffuses up through the soil and then condenses on the ground surface if conditions are favorable. Dew forms most readily on those surfaces that lose heat through radiation most efficiently but are nevertheless insulated from external heat sources. Dew formation is favored by high humidity in the lowest layers of air, which either supplies the moisture or at least inhibits the evaporation of the dew already deposited. Strong winds inhibit dew formation because they mix a larger layer of air, creating a more homogeneous distribution of heat and water vapor; under such circumstances it is unlikely that a sufficiently cool and damp layer of air can form near the ground.

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dew / d(y)oō/ • n. tiny drops of water that form on cool surfaces at night, when atmospheric vapor condenses: the grass was wet with dew | [in sing.] a cold, heavy dew dripped from the leaves. ∎  [in sing.] a beaded or glistening liquid resembling such drops: her body had broken out in a fine dew of perspiration. • v. [tr.] wet (a part of someone's body) with a beaded or glistening liquid: sweat dewed her lashes.

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DEW • abbr. ∎  distant early warning.

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dew OE. dēaw = OS. dau (Du. dauw), OHG. tou (G. tau), ON. dǫgg :- Gmc. *dawwaz, -am, f. the base repr. in Skr. dhāv flow, Gr. theîn run.

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DEW (djuː) directed-energy weapon
• Military distant early-warning (as in DEW line; network of radar stations)

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dew Water droplets formed, usually at night, by condensation on vegetation and other surfaces near the ground.