Dew. The question of the origin of dew has been discussed since very early times, and many theories have been advanced on the subject. Among more recent writers, Gersten, followed by Du Fay, held that dew rose from the ground. Others believed that it fell from above, but no really systematic treatment of the subject appeared until the publication of Wells's Essay on Dew in 1814. Wells combatted the opinion of Du Fay and Gersten, and showed that all the phenomena described by them could be equally well explained by his theory. He asserted that the dew was condensed out of the air in contact with surfaces below a certain temperature. For every definite pressure and temperature of the atmosphere there is a definite amount of water-vapour per unit-volume which can be held in suspension. Hence, when air containing a certain amount of water-vapour is cooled below a particular temperature, the vapour is condensed. This temperature is called the dew-point of the atmosphere under the given conditions as to pressure and quantity of vapour. At night the earth and objects at its surface are being cooled by radiation. If the rate of loss of heat by radiation is sufficiently rapid, the temperature of the earth's surface, and consequently of the air in immediate contact with it, may fall below the dew-point. When this occurs, the moisture condenses on the surface in the form of dew. This is Dr Wells's theory regarding the formation of dew. He considered that the vapour had risen from the ground during the previous day, and that very little dew was formed from vapour which had just risen from the ground, although he admitted that some might be so produced.
Wells's theory of the formation of dew is undoubtedly correct, but, in a paper read in 1885 before the Royal Society of Edinburgh, Mr John Aitken has adduced evidence which shows that the greater part of dew is formed from vapour which has just risen from the ground and been trapped by the grass and other objects. Mr Aitken was led to this conclusion by three independent experimental methods. One of these consists in removing at sunset a portion of turf from the ground, placing it in a shallow metal pan and weighing it. The turf, still contained in the pan, is then replaced in the ground in good heat-communication with it. When reweighed after some time, it is invariably found to have lost weight. If the turf be covered by a metal tray so as to prevent escape of moisture, this loss of weight is largely obviated. Similarly, it is shown that moisture rises always during the night from bare soil, from roads, and from the driest earth.
Certain atmospheric conditions are necessary for the copious formation of dew. The sky must be clear, otherwise the clouds will radiate back much of the heat given off from the earth, and so will prevent the surface objects from cooling below the dew-point. The atmosphere must be calm, or the air in immediate contact with the ground will be carried away before it is completely saturated.
When the temperature of objects falls below the freezing-point, the dew is deposited in a solid state, and is known as hour-frost. See EVAPORATION, METEOROLOGY.