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Meteorology; or Weather Explained

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<h2>CHAPTER II</h2> THE FORMATION OF DEW The writer of the Book of Job gravely asked the important question, “Who hath begotten the drops of dew?” We repeat the question in another form, “Whence comes the real dew? Does it fall from the heavens above, or does it rise from the earth beneath?” Until about the beginning of the seventeenth century, scientific men held the opinion of ordinary observers that dew fell from the atmosphere. But there was then a reaction from this theory, for Nardius defined it as an exhalation from the earth. Of course, it was well known that dew was formed by the precipitation of the vapour of the air upon a colder body. You can see that any day for yourself by bringing a glass of very cold water into a warm room; the outer surface of the glass is dimmed at once by the moisture from the air. M. Picket was puzzled when he saw that a thermometer, suspended five feet above the ground, marked a lower temperature on clear nights than one suspended at the height of seventy-five feet; because it was always supposed that the cold of evening descended from above. Again he was puzzled when he observed that a buried thermometer read higher than one on the surface of the ground. Until recently the greatest authority on dew was Dr. Wells, who carefully converged all the rays of scientific light upon the subject. He came to the conclusion that dew was condensed out of the air. But the discovery of the true theory was left to Dr. John Aitken, F.R.S., a distinguished observer and a practical physicist, of whom Scotland has reason to be proud. About twenty years ago he made the discovery, and it is now accepted by all scientific men on the Continent as well as in Great Britain. What first caused him to doubt Dr. Wells’ theory, so universally accepted, that dew is formed of vapour existing at the time in the air, and to suppose that dew is mostly formed of vapour rising from the ground, was the result of some observations made in summer on the temperature of the soil at a small depth under the surface, and of the air over it, after sunset and at night. He was struck with the unvarying fact that the ground, a little below the surface, was warmer than the air over it. By placing a thermometer among stems below the surface, he found that it registered 18° Fahr. higher than one on the surface. So long, then, as the surface of the ground is above the dew-point (i.e. the temperature when dew begins to be formed), vapour must rise from the ground; this moist air will mingle with the air which it enters, and its moisture will be condensed and form dew, whenever it comes in contact with a surface cooled below the dew-point. You can verify this by simple experiments. Take a thin, shallow, metal tray, painted black, and place it over the ground after sunset. On dewy nights the inside of the tray is dewed, and the grass inside is wetter than that outside. On some nights there is no dew outside the tray, and on all nights the deposit on the inner is heavier than that on the outside. If wool is used in the experiments, we are reminded of one of the forms of the dewing of Gideon’s fleece—the fleece was bedewed when all outside was dry. You therefore naturally and rightly come to the conclusion that far more vapour rises out of the ground during the night than condenses as dew on the grass, and that this vapour from the ground is trapped by the tray. Much of the rising vapour is generally carried away by the passing wind, however gentle; hence we have it condensed as dew on the roofs of houses, and other places, where you would think that it had fallen from above. The vapour rising under the tray is not diluted by the mixture with the drier air which is occasioned by the passing wind; therefore, though only cooled to the same extent as the air outside, it yields a heavier deposit of dew. If you place the tray on bare ground, you will find on a dewy night that the inside of the tray is quite wet. On a dewy night you will observe that the under part of the gravel of the road is dripping wet when the top is dry. You will find, too, that around pieces of iron and old implements in the field, there is a very marked increase of grass, owing to the deposit of moisture on these articles—moisture which has been condensed by the cold metal from the vapour-charged air, which has risen from the ground on dewy nights. But all doubt upon this important matter is removed by a most successful experiment with a fine balance, which weighs to a quarter of a grain. If vapour rises from the ground for any length of time during dewy nights, the soil which gives off the vapour must lose weight. To test this, cut from the lawn a piece of turf six inches square and a quarter of an inch thick. Place this in a shallow pan, and carefully note the weight of both turf and pan with the sensitive balance. To prevent loss by evaporation, the weighing should be done in an open shed. Then place the pan and turf at sunset in the open cut. Five hours afterwards remove and weigh them, and it will be found that the turf has lost a part of its weight. The vapour which rose from the ground during the formation of the dew accounts for the difference of weight. This weighing-test will also succeed on bare ground. When dealing with hoar-frost, which is just frozen dew, we shall find visible evidence of the rising of dew from the ground. You know the beautiful song, “Annie Laurie,” which begins with— “Maxwelton’s braes are bonnie, Where early fa’s the dew”— well, you can no longer say that the dew “falls,” for it rises from the ground. The song, however, will be sung as sweetly as ever; for the spirit of true poetry defies the cold letter of science.     <hr style="width: 50%;" /> <div style="break-after:column;"></div>

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