Fog, Mist, Dew, and Other Sources of Water 103 Fog9 Mist, Dew3 In the first place, the dew water could run ofi" the leaves and collect in und Other Sources the soil on which it drips. The amount of Wuter of dew, however, is too small to wet more than a few millimeters of the soil. In the second place, it is possible F. W. Went that the dew water is absorbed by the leaf surface, on which it condenses, and The main sources of water for crops thus is used directly by the plant. That in most agricultural areas are rain or was found to be the case; young leaves irrigation. In many parts of the world, especially can absorb the dew as rapid- however, neither rain nor ground wa- ly as it forms. One can often observe ter seems to be enough to account for that after a night of heavy dew the the development of the naturally oc- young leaves of shrubs and herbs are curring vegetation. In those places we quite dry and the older ones are cov- have to look for other sources of wa- ered with dew. This absorbed water ter—fog, mist, and dew—and their can be moved to other parts of the possible contribution to plant growth. plant and can even be excreted by the On many days and in many locali- roots into the soil. Such water would ties, plants are covered with dew in be available again the next day. It was the early morning. Because dew dis- actually found that the soil around root appears soon after sunrise, people often systems of plants is more moist when overlooked it as a possible source of they are subjected to dew than around water, which they consider to be most roots of plants that do not receive dew, necessary in midday. The question, despite the fact that the dew-covered however, is not whether water droplets plants are larger and must take more stay on the leaves for a long time but water from the soil. whether the water can contribute in In the third place, water saturation any way to the growth of plants. In of a plant is essential for its growth. As other words, does enough of the dew growth in most plants occurs during water reach the soil or is a sufficient night, a water supply would be most amount absorbed by the plant itself eñ'ective at night. These observations to improve the whole water supply? therefore make it very likely that dew The question has been investigated can contribute to the better develop- by S. Duvdevani, w^ho compared two ment of plants, at least in semiarid field plots of plants that were treated regions. identically except for the possibility Experiments with the yellow pine by of dew formation at night. By placing E. C. Stone and H. A. Powells have a canopy over one plot between sunset shown that when yellow pines are and sunrise, he prevented formation of grown in very dry soil, which is unable dew on the plot without interfering to supply water to plants like sun- with the temperature or the movement flower, they will survive for a limited of air over the plants. He conducted number of weeks (on the average, 3) ; the experiments in the coastal plain of however, when such plants are sprayed Israel, where dew occurs frequently every night with a fine mist simulating and sometimes is rather heavy. The dew, they can survive for 7 weeks with- tests showed that most plants, such as out any further water supply through squash and corn, grew about twice as their roots. That is another indication much when they received dew during of the possible importance of dew in the the night. Therefore it would seem life of plants. that in a semiarid area, dew has con- Field observations also tend to indi- siderable importance in the growth of cate that there must be another source plants. Three possible reasons come to of water for plants in semiarid regions mind. where rainfall is insufficient for normal 104 Yearbook of Agriculture 1955 growth of plants. As an example, the in the morning according to the pat- growth of a number of annual plants tern of dew droplets deposited. The during the dry season in southern Cali- heavier the dew, the more the droplets fornia can be mentioned. Some plants, have coalesced. By comparing the dew among them the wild buckwheat patterns with photographs, one can {Eriogonum fasciculatum), some gilias, standardize the observations. With the and Stephanomeria^ manage to continue gages Dr. Duvdevani studied the dis- development many months after they tribution of dew in Israel and found have received the last rain. four interesting relationships. If they had an extensive root system, they might be able to extract enough FIRST, THE AMOUNT of dew is higher water from the drying soil, but these in the dry summers than in the wet plants and others have only restricted winters. root systems and few active rootlets. Second, even in the dry and hot Jor- They certainly must have exhausted in dan Valley dew is a common phenom- a short time the small amount of soil enon and occurs on nearly half the with which they are in contact. When nights. In the Southern Desert (Negev) such plants still manage to keep green it is even more frequent. and even grow, it means that there Third, considerable dififerences exist must be another source of water than in dew deposition according to the what is stored in the soil around their topography of the land. small root system. Because their water Fourth, deposition of dew in summer requirement cannot be satisfied from increases with distance from the below, it must come as atmospheric ground, but in winter the opposite dew condensation. gradient is found—proof that the water Is dew a frequent enough phenome- deposited as dew did not originate from non and is it of sufficient intensity to the ground but was of atmospheric ori- account for its apparent role in plant gin. The total amount of dew precipi- growth as demonstrated in the exam- tation in Israel amounted to approxi- ples I mentioned? mately I inch a year. To get an answer to that question, Other investigators found a similar many investigators have devised meth- figure in other parts of the world and ods to measure dew under natural con- therefore concluded generally that dew ditions. They have used various gages. could not account for more than 1.5 The one most extensively used and ac- inches of precipitation. That amount cepted is that of Dr. Duvdevani. It is a undoubtedly is too small to explain the wooden block with a standardized considerable increase in growth that painted surface, which is exposed to observers have attributed to dew. dew at night and on which the occur- Instead of measuring dew deposition rence and quantity of dew can be read with gages, we can also calculate how much dew could be produced theoreti- cally. Dew is produced when leaf and other surfaces are cooled to a tempera- ture below the dewpoint of the air. This cooling is accomplished by radia- tion of heat toward the sky. A large amount of heat is radiated by the sun during the day. The radiation amounts to approximately 1.2 calories per square centimeter a minute. It is transformed partly into long wave- lengths and partly into heat of evapo- ration. Some of the radiation heats the soil. Fog, Mist, Dew, and Other Sources of Water IDS Because, on the average, the earth does Benjamin Holzman, who measured the not become warmer, all radiation ab- water vapor gradient from soil toward sorbed by the ground has to be reradi- the air. By taking into account the rate ated by the ground at night in the form of air movement, they could calculate of long-wave heat radiation. That ra- how much water vapor was lost or diation is partly absorbed by water condensed on the basis of this water vapor in the atmosphere. The drier the vapor gradient, and they calculated atmosphere, therefore, the greater the that a total of 1.81 inches was con- amount of outgoing heat radiation and densed on a meadow surface for a 10- the stronger the cooling of the surface month period in 1939 in Arlington, will be. On clear nights this radiation Va. During that time, a total of 26.3 amounts to 0.16 calorie per square inches was received in the form of rain centimeter a minute at a relative hu- and snow, which would indicate that midity of 100 percent; 0.20 at 40 per- not more than 7 percent of the total cent; and 0.25 at 15 percent. As leaves water supply to this meadow had come and other surfaces lose heat through in the form of dew. the radiation, they are cooled and ulti- By actual weighings of an area of mately may reach the dewpoint of the soil covered with grass, corn, wheat, air. As soon as that happens, dew is or other crop plants, the amount of deposited and no further drop occurs nightly condensation can be measured. in the temperature of the leaves. That was done in lysimeter experi- ments at Coshocton, Ohio, by L. L. FROM THE AMOUNT of heat exchange Harrold and F. R. Dreibelbis. Hourly we can calculate the amount of dew weighings would indicate the increase that can be deposited. At a relative or decrease in weight of the blocks of humidity of 40 percent, it takes about soil.