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. Their increase in weight was due 500 calories to lower a sufficient air to rain or to dew, but they lost weight volume to the dewpoint to deposit a through transpiration and evaporation. gram of water, which also required In a series of measurements over a about 570 calories of heat of condensa- 6-year period, it was found that an tion. About half of the radiation, or average equivalent of 9. r inches of rain 0.1 calorie per square centimeter a was deposited throughout the year in minute, is therefore available during the form of dew—approximately 20 clear nights for dew deposition. For a percent of the total water supply came whole night that amounts to i milli- in the form of dew. When we compare meter of dew. For a whole year of this figure with the 15 inches that 365 cloudless nights it is equivalent to could theoretically be deposited during 15 inches of rain. That amount is I year of clear nights, we find that the rather independent of the relative hu- actual observations agree quite closely midity of the air, because in dry re- with the measured condensation in the gions the radiation is greater but the lysimeters. We have to take into con- amount of dew deposited per calorie sideration that on many cloudy nights is less, whereas in most climates the dew cannot be formed. radiation is less, but it is more effective in producing dew. CALCULATIONS and actual observa- These 15 inches of potential conden- tions apparently agree that dew is a po- sation from the air is in sharp contrast tentially important source of water for to the 1.5 inches measured with dew plants. As it may amount to about 10 gages. We should try to find the actual inches of precipitation a year—in line amounts of dew that are deposited on with field experiments such as those a natural cover of vegetation. That of Duvdevani—more work should be has been done in several ways. In the carried out to establish the exact con- first place, we have a set of measure- ditions under which the greatest amount iTients by C. W. Thornthwai to and of dew can be deposited. 106 Yearbook of Agriculture 1955 As I mentioned earlier, dew can be summer. Those crops develop well deposited only when the sky is clear. even without irrigation, although no There is no dew on cloudy nights; rain falls during the growing season semiarid regions therefore are more from May to October. The soil in the favorable for dew deposition than most region may contain enough moisture other climates. Not only do clouds for the first few months of growth but prevent the nightly radiation toward definitely not enough for the last few the sky; dust and haze also will pre- months. If growth and production are vent it. Dusty areas, such as the Sa- good nevertheless, that means there hara, and regions with much haze, must be some other source of moisture. like the Los Angeles area, therefore, The source can hardly be anything have very much less dew than areas else but dew or coastal fog. with clear night skies, such as the southwestern desert areas and Israel. FOGS AND MISTS are really low-hang- Considerable differences in the for- ing clouds in which the water droplets mation of dew can be expected accord- are so small that they do not settle on ing to the nature of the terrain and horizontal surfaces and thus do not velocity of air movement past the con- register in a rain gage. Consequently densing surfaces. In completely still they usually are not considered impor- air, only the amount of water vapor tant as sources of water. Yet fogs may that can diffuse toward the leaves will be important to plants. be condensed, and that amounts to less Vegetation in many places where than I percent of the greatest possible fogs are frequent differs from the vege- dew deposition. When, on the other tation in nearby localities where fogs hand, strong winds move the air too do not occur. One of the best examples fast past the leaves, their temperature of this is the northern coast of Cali- will not reach the dewpoint, and there- fornia. Redwoods occur there predom- fore no dew will be deposited at all. inantly in a narrow belt and never Optimal dew formation can be ex- range inland beyond the influence of pected when an intermediate amount sea fogs. In places where more water of air comes in contact with the leaves. is available, redwoods grow very well outside the fog belt—an indication that IN THE FUTURE we should investigate fog is important in their water econ- all conditions that affect dew deposi- omy. In this belt, fogs occur almost tion so that it might be possible to in- daily. Anyone who walks in a redwood crease the effectiveness of dew and in- forest during a fog finds that the trees crease the amount condensed. It is are dripping, although in nonforested quite possible, for instance, that by the spots nearby not a drop falls. That judicious spacing of trees and shrubs means that the redwoods are able to through natural air drainage the op- condense the fog droplets to larger timal amount of air movement can be drops. achieved so that the greatest possible amount of dew will be formed. THIS SAME DRIPPING of trees in fog Dew seems to be particularly effec- one can observe in pine forests in the tive in a number of coastal areas around mountains or under live oaks in the the world. In several such places that California foothills during autumn have too little rainfall, very heavy dew fogs. In the latter instance it can be apparently can carry the crops to ma- seen that this fog-drip water is effective turity without the help of irrigation. for the vegetation. Rain would wet An example is the coast of southern equally the ground under and around California. Immediately adjoining the the trees and therefore would not ac- ocean is a zone one-half to a few miles count for the greener color of the grass wide where tomatoes, peppers, beans, or the extensive germination of wild and other vegetables can be grown in plants under the trees. But during fogs, Fog, Mist, Dew, and Other Sources of Water 107 water drips under the trees and actu- a source of water because the conden- ally moistens the soil within the perim- sation occurs only when a sufficient eter of their crowns. If it were merely volume of air with fog droplets is shade that enables the seedlings to be- carried over the condensing surface. come started or encourages the growth Therefore the ground mist developed of grass, the green area would extend through rapid cooling at night of the north beyond the tree. That it does air nearest the soil is ineffective for not do. condensation of the fog particles. But coastal fogs formed by moisture-laden LET US QUESTION for a moment how air rising against coastal ranges, or trees can condense moisture from fogs. clouds forming against the mountain A fog consists of very small (o.oi-o.i ranges by lateral air movement which millimeter) water droplets, which are forces air up against the mountain far enough apart that they do not slopes, are ideal for fog condensation. fuse, are suspended in saturated air In regions where such fogs occur so that they do not evaporate, and are rather regularly, a tree or shrub v^cge- light enough not to settle. Further- tation grows that is rich beyond what more in a fog there is usually enough the actual precipitation records would air movement to stir up droplets about make one expect. Along the California to settle. When such a fog moves past coast, the redwood growth is phe- solid surfaces, it is deflected, and the nomenal against the western slopes, water droplets flow with the air but against the eastern slopes, with around the surface and prevent con- approximately equal amounts of rain, tact with it. If the surface is small or tree growth is much poorer. The narrow enough, the air is hardly de- difference indicates the degree to flected, and the inertia of the water which water precipitation from fogs droplets is sufliicient to carry them is effective. against the surface, where they fuse In the drier areas in the West, there with it. That means that small or nar- are series of high mountain ranges in row surfaces will act as strainers for areas with insufliicient precipitation the fog droplets. It is an interesting for tree growth. On the mountains we fact that the diameter of pine or red- find a lower tim berline, below which wood needles is such that they con- trees do not grow. If the timberline dense fog droplets very efficiently at were due to reaching the threshold the normal rate of fog movement. value of precipitation where tree growth That can best be seen in hoarfrost or was barely possible, one would expect rime, which develops when a fog a ragged and irregular timberline, touches surfaces, where the condensed with the trees gradually thinning out. fog droplets freeze immediately upon But along the eastern slopes of the contact if the temperature is below Sierra Nevada, for example, one sees freezing. Under such conditions a a sharp horizontal line of demarcation, ribbon of ice extends from telephone below which no trees occur and above and other wires in the direction from which a normal pine forest is found. which the wind came. But we also see The line coincides with the level at that hoarfrost covers all pine needles, which clouds are formed by eastern although the larger leaves of broad- winds. leaved trees have hoarfrost only along their edges and not over their surfaces. IN MANY DRY REGIONS, such as the That shows how much more eff'ective areas covered with chaparral in Cali- the needle form is for condensing fog fornia, many of the most characteristic droplets. plants, such as chamise {Adenostoma We can now return to the effective- fasciculatum) or wild buckwheat {Erio- ness of fog as a source of water in gonum Jasciculaiurn) have developed nature. Stagnant mists cannot serve as very small leaf surfaces in the form of 108 Yearbook of Agriculture 1955 needlelike leaves. They are not an found only along a strip 10 feet wide adaptation to reduce transpiration, that follows the ridge of the tableland, as is often suggested, but probably again the place where fog will hit most are effective in condensing water and where (through droplet conden- from the occasional fogs. sation) the water supply is more abun- In dry regions bordering a coast, dant. we may find so-called fog deserts, One can get an idea of the total areas with very little precipitation but amount of liquid that can be precipi- with frequent fogs. Typical of such tated by such fogs by using a rain gage, areas are southwestern Africa and the over which a set of fine wires or coast of Peru. In both, mountains rise branches is placed. The wires or the immediately behind the coast and branches will condense the fog drop- cause the formation of fog as the air lets, which then drip into the rain gage. rises against them. In each, also, a cold Such an arrangement disclosed that ocean current (the Bengucla and the during a particularly heavy storm, Humboldt Currents) limits the total when clouds w^ere swept over the Table amount of moisture present in the air, Mountain in South Africa, only 0.2 so that the total precipitation is about inch of precipitation fell in a regular I inch a year. The lowlands near the rain meter, but in a similar rain gage, coast, where no fog occurs, are prac- in which branches had been placed, tically without vegetation, but at 1,000 6 inches of precipitation was collected. feet a shrub vegetation beyond expec- It seems a worthwhile project to inves- tation occurs wehere fogs hang most of tigate this fog condensation in greater the year. Indicating the high humidity detail. during most of the day is the richness of the epiphyte vegetation. Even on THE RELATIVE HUMIDITY of the air cacti, like Trichocereus, grow many may be important in the conservation lichens and several tillandsias, which of water by plants. Under otherwise ordinarily are found in moist rain for- identical conditions, water loss by ests. And on the trees {Caesalpinia and transpiration will be proportional to Zizyphus) mosses and peperomias cover the saturation deficit of the air. As the the branches, which consequently seem saturation deficit increases beyond a several times thicker than they are. certain point, however, phenomena The importance of the actual con- (such as incipient drying) stop a further densation of fog droplets in fog areas increase in transpiration. Plants that can be seem in several places. Just themselves have a saturation deficit north of San Diego there is a small area will lose water toward the atmosphere where the Torrey pine occurs. The only if the saturation deficit of the air trees are limited to the upper parts of is greater than that of the plants. At a the slopes facing the ocean—that is, diffusion pressure deficit, or suction where the fog comes in closest contact force, of 100 atmospheres, such as can w^ith the vegetation. On the lower parts be found in arid regions, plants will be of the slopes and a few hundred feet in equilibrium with air having a rela- inland, the pines disappear. It is an tive humidity of 93 percent. That is to otherwise very dry area, as indicated say that they will lose water by tran- by the much poorer shrub vegetation spiration only when the air is drier than anywhere except on the ridges border- 93-percent relative humidity but will ing the ocean. A similar phenomenon take up water vapor from the air when can be observed along the Mediter- this is more moist than 93 percent. In ranean coast. Just behind Oran in a number of experiments, E. C. Stone, Algeria rises a plateau with a rainfall C. L. Young, and I found that several of about 15 inches—enough for some chaparral plants can take up water pines but not enough for the holly oak vapor from air with a relative humid- {Qjiercus ilex). However, this oak is ity of 85 percent or more. Conversion of Saline Waters 109 Therefore we should consider the Conversion possibility that absorption of water vapor can be a source of water for cer- of Saline tain plants. This I found to be true of Waters several tropical orchids, which hang from the branches of jungle trees and obtain liquid water only during actual David S. Jenkins, R. J. McNiesh, and rainstorms. When the relative humid- Sidney Gottley ity in the forest was 95 percent or more, the orchids took up v/ater vapor and Reclaiming water from otherwise un- increased in weight. Since the relative usable sources, by removing dissolved humidity inside such forests remains salts from saline water, is coming to be above 95 percent more than 12 hours regarded as a probable way to supple- a day, water vapor was a significant ment our traditional water supplies. source of water for the plants. The process is known as conversion or the demineralization of saline or sea F. W. WENT is professor of plant water. One of the sources is the rela- physiology and in charge of the Earhart tively static and indispensable water Plant Research Laboratory in the California of the oceans, which contain most of Institute of Technology. After he received his the world's water. Its use would sup- doctors degree from the University of plement the short supplies along coast- Utrecht^ he spent § years in Java with the al areas. Another source is the brackish Botanical Garden in Buitenzorg, He joined water now found inland—much of the institute in 1933^ which is becoming increasingly saline to a point where it can no longer be For further reference: used for some purposes without harm- S, Duvdcvani: Dew Gradients in Relation to ful effects. Climate^ Soil and Topography, in Desert Research, In either case, the salts held tena- Proceedings of the Desert Symposium, Jeru- salem, pages 136-152, 1953. ciously in solution must be removed S. Duvdevani: An Optical Method of Dew before the water becomes suitable for Estimation, Quarterly Journal of the Royal agriculture, industry, or domestic use. Meteorological Society, volume 73, pages Although sand, sediment, and other 282-296, 1947. Rudolf Geiger: The Climate Near the Ground^ foreign particles can be removed 2d edition, Harvard University Press, Gam- simply by filtration, salt cannot be bridge, Mass., 482 pages, 1950. removed so easily. L. L. Harrold and F. R. Dreibelbis: Agri- Saline water is a relatively simple cultural Hydrology as Evaluated by Monolith Lysimeters, U. S. D. A. Technical Bulletin system of inorganic salts dissolved in 1050, 1951. water. It has certain physical and Willis Linn Jepson: A Manual oj Flowering chemical properties that determine the Plants of California, University of California, various methods by which the salts Berkeley, 1925. H. Masson: Condensations Atmosphériques non may be separated from the water. The enregistrables au Pluviomètre, Bulletin de l'Insti- system, although not complex, is com- tut français d'Afrique noire, volume 10, ï8r paratively stable; because of the sta- pages, 1948. bility, separation of saline solutions E. G. Stone and H. A. Fowciis: Drought Sur- vival of Ponderosa Pine Seedlings Treated with requires relatively large quantities of Simulated Dew Survive by Month Nontreated Con- energy. trols in Greenhouse Tests, Galifornia Agriculture, Theoretical calculations show that volume 8, No. 7, page 9, I954. at least 2.8 kilowatt-hours (3.8 horse- E. G. Stone, F. W. Went, and G. L. Young: Water Absorption from the Atmosphere by Plants power-hours) of electrical energy is re- Growing in Dry Soil, Science, volume 111, quired to separate 1,000 gallons of pure pages 546-548, 1950. water from sea water. Thus, more than C. W. Thornthwaite and Benjamin Holz- goo kilowatt-hours (more than 1,200 man: Measurement of Evaporation from Land and Water Surfaces, U. S. D. A. Technical Bulletin horsepower-hours) must be used to ob- 817, 1942. tain I acre-foot of fresh water from sea