Handbook of Meteorology/Evaporation and Condensation
THE MOISTURE OF THE AIR: EVAPORATION AND
CONDENSATION
Water vapor and floating dust are components of the air which vary from day to day and even from hour to hour. All the waters of the land are derived from the water vapor of the air; and this in turn is brought from the oceans. Inasmuch as life in its various forms depends on the process whereby ocean waters are taken into the air and are dropped upon the land as rain or as snow, the study of the water vapor content of the air is of vital importance to humanity. Before the waters of the sea can be poured over the land, several distinct processes take place: Evaporation, diffusion, condensation, and precipitation.
Evaporation.—It is assumed that the molecules of a volume of water are in constant motion among themselves. Some of the molecules at the surface are in such rapid motion that they bombard themselves into the air, thereby becoming a part of it. This loss to the water goes on at all ordinary temperatures and even at very low temperatures. At 212° F (100° C) the pressure, or tension of the vapor is as great as that of the air, and the water is said to boil.
In meteorology, evaporation is a term applied practically to the net loss of water, or other liquid exposed to the air. Free water surfaces, soil and vegetation have each their problems; meteorology is concerned chiefly with evaporation from a free surface of water. Diffusion of the water vapor derived from the ocean, and from bodies of fresh water, is so universal that in no part of the earth is the air free from water vapor.
Various conditions affect evaporation. Under ordinary conditions of light winds and moderately dry air the rate of evaporation is proportional to the surface.[1] It is also directly proportional to the difference in the readings of the dry bulb and the wet bulb of a sling psychrometer. Evaporation increases very rapidly with a dry wind[2] and more slowly as the relative humidity of the air increases. It increases rapidly with rising temperature and decreases with falling temperature. It increases inversely with barometric pressure. The rate of evaporation of sea water is about 95 per cent that of fresh water, all other conditions being the same.
Condensation.—The process whereby water vapor changes to a liquid form is condensation. Condensation may occur as a result of mechanical processes, such as pressure and artificial cooling; in the free air, however, it results from cooling by contact, cooling by mixture, or cooling by expansion—practically adiabatic cooling. More definitely: warm air resting on the ground, or on the sea, may be cooled by contact therewith, until some of its moisture is condensed. An area of warm, moist air may be invaded by a cold wind and the mixing process may cool the vapor to the temperature of condensation. A body of air warmed above the temperature of the surrounding air is pushed upward. Its expansion causes adiabatic cooling and if the temperature falls below that of saturation, condensation of the water vapor occurs. Practically all the cases of condensation with which weather science has to do result from one or another of the causes named. The condensation resulting from contact causes dew and frost; that which results from mixing causes fog and cloud; that resulting from adiabatic cooling—that is, updraught—causes rain and snow. There are occasional exceptions to the fore-going, especially where superficial turbulence of the air is involved; in the main, however, these processes of condensation are fundamental in weather science.
Dust Motes and Condensation.—The invisible, floating dust motes of the air and many of the gaseous products of combustion are important factors in condensation. Each droplet of cloud or fog condenses upon a dust mote or upon a hygroscopic gas product.[3] In general, the dust motes which cool most quickly are regarded as the “most favorable” nuclei. Were it not for this feature of condensation, gentle rains would become sporadic cloudbursts. The measurement of the dust content of the air is not yet a part of the scope of weather observations, but the importance of it is universally recognized.
Conditions of Condensation and Precipitation.—In another chapter the relation of temperature to the amount of water vapor has been discussed. The absolute water vapor content of the air is the gross amount of water it contains. This is usually estimated in grains per cubic foot or in milligrams per cubic decimeter. Between the twenty-fifth and fiftieth parallels of latitude the amount of water per cubic foot averages roughly from 1 to 3 grains in winter and from 5 to 7 grains in summer—north to south. The proportion varies, however. Sea winds are wet winds; land winds are usually dry. The higher the temperature, the greater the possible absolute content of water vapor.
Condensation does not begin until the temperature of the air has reached the degree below which only a certain proportion of vapor can exist—that is, below the temperature of saturation, or dew-point. Any excess is condensed and appears in one or another of the forms noted.
Relative Humidity.—The water vapor content of the air which is not condensed is so important to life and to human comfort that its measurement is an essential part of Weather Bureau observations. The higher the temperature of the air, the greater the amount of water vapor it may contain—about 4 times as much at 70° F (21° C) as at 32 F (0° C), and 10 times as much at 100° F (38° C); hence the term relative humidity. This is expressed in terms of the per cent of water vapor necessary to saturation. Thus, if the relative humidity is 50 per cent, half the vapor necessary for saturation at the observed temperature is present.
Ordinarily, the humidity is highest in early morning, when the temperature is lowest; it is usually lowest at the warmest part of the day. On dewy and frosty mornings the humidity at ground level is 100 per cent; a few feet above ground it is probably at 96 per cent; during the hottest part of the day it may be as low as 30 per cent, or even lower; on cloudy days it may not vary materially during the day. During foggy weather it is practically 100 per cent.[4] During summer rainstorms it is approximately 95 per cent.
The relative humidity of the air has a profound effect upon public health. General Greely noted the fact that, during prolonged spells of very dry air when the per cent of humidity fell materially below the normal, a notable increase in the death rate followed. Dr. Ellsworth Huntington has shown that the same result is true of the death rate in hospitals.
Humanity, both the conscious and the sub-conscious self, is sensitive to changes in temperature, noting a difference even of 1 degree Fahrenheit. The conscious self rarely notices changes in humidity between 35 per cent and 85 per cent. The subconscious self is far more sensitive; it rebels against a condition of humidity materially higher than 75 per cent or lower than 40 per cent when the temperature of the air is that of comfort. There is a noticeable difference to the feelings between indoor and out-of-door air. Indoors, a humidity of 25 per cent is extremely uncomfortable; out of doors it is hardly perceptible.
During the winter season when buildings are artificially heated, the humidity of living-rooms is not often above 40 per cent; usually it is lower than 35 per cent; in school rooms it may be less than 25 per cent. Dr. C.-E. A. Winslow has pointed out the effect upon the health of the pupils of air so deficient in moisture.[5] P. R. Jameson, using empiric but very practical standards of measurement—that is, comfort or discomfort—has tabulated the results of several thousand tests:
| Rel. Hum. 75% | 55° F | very cold | |
| 65° F | chilly | ||
| 75° F | comfortable | ||
| Rel. Hum. 50% | 35° F | very cold | |
| 50° F | chilly | ||
| 65° F | comfortable | ||
| Rel. Hum. 30% | 55° F | very cold | |
| 65° F | chilly | ||
| 75° F | comfortable |
That is, with the relative humidity at 50 per cent, the temperature of comfort is 10 degrees lower than with a very dry or a moist air. These conclusions do not differ from those of Dr. Huntington.
Many manufacturers have installed humidifiers within their factories in order to provide wholesome air to their employees and a correct atmosphere for the economical production of their output. Exhaust ducts carry the air from the work rooms to the humidifier where it is screened, washed, and returned to the various rooms with but little loss of temperature. The saving in fuel very soon pays the cost of a humidifying plant.
Forms of Condensation.—The condensation of the water vapor of the air takes place in many forms—fog, cloud, dew, frost, rain, snow, and hail. The “sweating” of walls, and the film of moisture that forms on the outside of a vessel filled with iced water are also examples of condensation. In any case the cause is the same; the temperature of the air falls below the temperature of saturation and the excess of water vapor is condensed in one or another of the forms noted. The formation of fog and cloud are considered in the following chapter; hail is a feature of thunder-storms.
Dew.—Dew consists of the moisture condensed on such surfaces as radiate their warmth after sundown. If the chilling of the air next to such surfaces carries its temperature below that of saturation—that is, the “dew-point”—the excess is deposited in the form of minute droplets. Not infrequently so much moisture is deposited that foliage and grass become very wet. Vegetation radiates its heat rapidly, and therefore dew is apt to form copiously thereon. At night the temperature of the air two or three inches from the ground may be as much as 5 degrees lower than at a height of 6 feet. Dew therefore may form on grassy surfaces when none occurs on objects materially above ground.
Falling temperature at night is the rule; nevertheless, dew does not always form. The temperature may not go down to the dew-point; the absolute humidity may be very low; wind may keep the air stirring so that the air next the ground may not remain long enough to be cooled to the dew-point; low clouds may prevent the radiation of ground warmth; a “lid” also may prevent radiation. For the foregoing reasons the problems concerning the formation of dew are of much importance.
Frost.—If the temperature of the air is below freezing, the water vapor will be deposited in the form of minute crystals of ice, which reflect the light in such a manner that a silvery appearance results—the hoar frost of popular tradition. In some instances the moisture is doubtless deposited as dew, which afterwards is frozen. Sometimes, too, partly melted frost or slowly freezing dew forms a glazed and semi-transparent coating—the rime of tradition. From the nature of the case, rime is more hurtful to vegetation than is hoar frost.
When rain freezes as it falls on leaves, stalks, and twigs the ice varnish is the traditional black frost. Strictly speaking, it is not frost at all. It is a freezing which involves the surface of the vegetation. The superficial juices of the plant are frozen, to the extent that the cells of the plant are ruptured.
Hoar frost injures tender plants but does not necessarily kill them. Black frost, on the other hand, is apt to kill tender plants and to injure many hardy plants. A temperature as low as 25°, without frost, may be as fatal to tender plants as a black frost.
Warnings of late spring and early fall frosts are sent out from Weather Bureau stations. Close observation, however, will enable one to foretell a possible frost by watching the temperature and humidity. When the air is still, the humidity high, and the sky clear, frost may be expected if the temperature at sunset is 40° or lower; indeed, under such conditions the temperature is likely to fall to the freezing point by 2 o’clock on the following morning and to remain below freezing for a short time after sunrise.
The greater likelihood of frost in low spots, such as valley floors, as compared with the higher levels of the adjacent slopes, is an important factor in fruit farming and, in fruit-growing regions, pretty accurate surveys have been made of the lands likely to be visited by killing frosts. Nevertheless, by far the greater area of tender crops is within the region of killing frosts; hence the necessity of making use of all available knowledge in the matter.[6]
The growing season. The figures show the number of days between spring and fall frosts.
Weather Bureau records make a distinction between light frosts and killing frosts, the latter being so called because of their destructive effects. Ground frosts, as a rule, are not killing; the freezing temperature does not extend more than a few inches above the grass. If the freezing temperature extends so high that frost covers the roofs of buildings, the frost is apt to be killing. Records of the dates of the latest killing spring frost and the earliest killing fall frost are highly important from the fact that the number of days intervening constitutes the growing season.
In the latitude of the Great Lakes the growing season is from 110 days to 150 days; in the latitude of Illinois and Missouri it is from 150 days to 200 days; in the belt extending from the northern boundary of Tennessee to the Gulf it is from 200 to 300 days. Florida and Texas, south of the twenty-seventh parallel, are very rarely visited by killing frosts.
- ↑ In still air over a circular area evaporation increases as the square root of the area; with a horizontal wind it varies approximately in theory, at least, as the three-fourth power of the area. If the rate be calculated in direct proportion to the area, the result will not be materially incorrect.
- ↑ The rate varies approximately as the square root of the wind velocity, and as the cube of the square root of the diameter of a circular container.
- ↑ There are certain cases of super-saturation to which this statement is an exception; indeed, condensation is still a field for investigation.
- ↑ During the prevalence of a “dry fog” the humidity may be not higher than 85 per cent.
- ↑ Dr. Winslow has noted that the air of schoolrooms in winter is as dry as that of a desert. As a matter of fact, the air of the Gila Desert, Arizona, is rarely so dry as that of a schoolroom at 9 o’clock.
- ↑ Bulletin V of the U. S. Weather Bureau publications is a summary of observations collected from more than one thousand stations and sub-stations. The information is graphically charted on maps which show the dates of late spring and early fall frosts, the average dates of killing frosts, and the number of days between spring and fall frosts. The following paragraphs apply pretty generally to all parts of the United States: Frost becomes more severe as one goes from hillside to low spots, such as hollows and stream valleys. It is more severe on the grass than at shrub heights. It may form on the grass when the temperature 3 or 4 feet above ground is several degrees above the freezing-point. If the temperature at sunset is not lower than 40° F (5° C) and the sky is overcast, frost is not likely to occur. But if the sky is clear and the wind is at calm, frost is likely. With a brisk wind and a sky either clear or cloudy, frost is not likely to occur unless the temperature falls materially below freezing. If the air is moist at sunset and the temperature is 40° F or lower, frost is likely to occur even with a light wind; but if fog occurs, enough latent heat may be set free to prevent frost. A low-lying fog is a blanket which retards radiation, not only from grass and shrubbery, but from the ground itself.