Handbook of Meteorology/Hygrometers
THE MEASUREMENT OF HUMIDITY: HYGROMETERS
The Water-vapor Content of the Air.—Water vapor exists in the troposphere, or lower shell of the air, at all times, the amount depending very largely on the temperature of the air. To the best of knowledge, little if any water vapor exists in the air of the stratosphere. The high cirrus clouds presumably mark the upper limit of the condensation of the water vapor of the air.
The maximum proportion of vapor—that is, the maximum quantity per unit of volume—depends on temperature; it is independent of the other constituents of the air. Were there no other constitutents, the atmosphere would be an atmosphere of water vapor, and the amount per unit of volume would be about the same as under existing conditions. It is best, therefore, to consider the water vapor content as an. independent factor so far as measurements are concerned.
When the air—or rather, the water vapor itself—is near the point of saturation it is moist to the senses. Hygroscopic substances, such as sugar, salt and many other substances, absorb moisture; sized paper and starched fabrics swell and become limp. These conditions begin to be noticeable when the water vapor of the air passes 85 per cent of the amount that may exist.
When the vapor content is 30 per cent, or less, of the amount required for saturation, the dryness becomes apparent to the senses, especially to the lips and throat. The gummed surface of stamps, labels and adhesives shrinks, causing the paper to curl. Doors warp and thin panels of wood shrink and split.
The moisture sensation of the air is its humidity. Before saturation is reached, the moisture is in the form of vapor; at the point of saturation it may appear in the air as fog, or cloud; when the ground temperature reaches the point of saturation condensation takes place in the form of dew; or, if below 32° F (0° C) in the form of frost.
The table, p. 280, shows the amount of moisture at different temperatures which may be present mingled with the air. Thus, at 30° F a little less than 2 grains per cubic foot can be present before condensation begins; while at 70° F there may be nearly 8 grains. In other words, if 2 grains per cubic foot were present when the temperature was 30° F, condensation would be taking place; while, if the temperature were 70°, the air would be very dry, because only one-quarter of the moisture required for saturation is present. Sensible moisture, therefore, is relative, requiring measurement of temperature and absolute humidity at the same time.
The measurement of absolute humidity by direct methods is not ordinarily required in weather observations. It may be determined by aspirating a measured quantity of air very slowly through a hygroscopic substance, weighing the substance before and after. It may be determined more easily, however, by ascertaining the relative humidity.
The Measurement of Humidity; Wet-Dry-Bulb Hygrometers.—Various methods of determining the humidity of the air have been devised. Some of them are merely hygroscopes. Thus, a slip of paper moistened with a solution containing gelatine and cobaltic chloride is pink in moist air and blue in dry air. A better hygroscope is the toy chalet from which a woman emerges in dry weather, while a man with an umbrella stands in the door during damp weather. The manikins are suspended by a short piece of catgut which, twisting in the one case and untwisting in the other, because of changing moisture content, indicates roughly the changes in the humidity of the air.
A curled piece of vegetable fiber, one end fastened to a base, the other carrying an index hand, has become popular as a hygroscope. It has practically no value for quantitative determinations, but is not without value in indicating conditions of moisture not at once apparent to the senses but at the same time necessary to bodily comfort.
For the quantitative measurement of humidity hygrometers are now practically reduced to two types, dry-wet-bulb thermometers, and hair hygrometers. The United States Weather Bureau provides several kinds of the first-named type.
The Mason hygrometer, nearly two hundred years old in principle, has been a standard in all countries for many years. It is made in various forms, but the essential features do not 
Mason Hygrometer. Weather Bureau pattern. vary. The instrument consists of two thermometers mounted on the same base. One measures the temperature of the free air; the bulb of the other is covered with a single thickness of thin bolting silk or muslin, the lower end of which is in a small vessel of water attached to the baseboard. Capillary attraction keeps the fabric wet and evaporation is almost always taking place.
The evaporation of the water chills the bulb and the wet-bulb thermometer therefore registers a lower temperature. The more rapidly the evaporation takes place, the less is the moisture content of the air; and the percentage may be determined by the difference of the readings of the two thermometers. Tabulated determinations accompany the hygrometer, and the percentage of moisture already calculated is found from tables contained in the Weather Bureau Circular of Instruction.
The hygro-autometer is a very convenient form of the Mason hygrometer. The tables are carried on a roll attached to the hygrometer. Thumbscrews turn the rolls until the difference between air temperature and wet-bulb temperature appears in the space at the top; the per cent of humidity is opposite the air temperature reading. The hygro-autometer is a most excellent hygrometer for auditoriums and for household use.
The hygrodeik is a form of hygrometer in which the tabulated matter is shown on a card ruled with ordinates and co-ordinates for the convenience of reading. An index fastened by a hinge joint at the top carries also a sliding point. By the adjustment of these the relative humidity is read from the tabulated figures. The experience of nearly a century has shown the usefulness of this instrument for indoor purposes.
Unless “coaxed” by fanning, dry -wet-bulb calculations are subject to error, the nature of which is obvious. The use of an ordinary fan—or, better, an electric fan—on the bulbs will give much more accurate readings.
The sling psychrometer obviates this difficulty. The two thermometers of this instrument are made fast to a metal strip which whirls upon a pivoted handle. The covered bulb is dipped in water of the same temperature as the air and whirled on the pivot until the temperature of the wet bulb ceases to lower. Ordinarily, about twenty seconds are required to obtain a correct reading.
The whirling table is now generally employed where systematic observations are made. The geared mechanism used in whirling the thermometers does not give more accurate results than the sling psychrometer, but it affords an easier method of stimulating evaporation, and the thermometers are not so likely to be broken.
When the humidity of the air is near the point of saturation, determinations made at the same time may vary several points; and, unless a sling psychrometer or a whirling apparatus is used, the determinations are pretty certain to vary. The thermometer scales of the best psychrometers are graduated to half-degrees and may be read to quarter-degrees. This conduces materially to accuracy.
The chief source of inaccuracy, however, is the covering of the wet-bulb thermometer. No matter what the material of which it is constructed may be, sooner or later it becomes hard and loses its capillarity. It is no longer of use and should be thrown away. If it shows signs of discoloration it should be thrown away also, for its usefulness is gone. Tubular wicks are now much used and are kept by dealers in meteorological instruments. If any doubt as to the cleanliness of a wick exists, it should be boiled briskly in water and, when dry, soaked in pure alcohol in order to remove any traces of grease. Under no circumstances should gasoline be used. In adjusting it to the bulb, every trace of oil, grease, or other substance should be removed from the fingers. If the capillarity of the wick is sluggish it is better to try another. In an emergency, if the fabric about the bulb is dry it may be wet with a camel’s hair brush dipped in water. Water containing any sort of impurity is apt to impair the capillarity of the wick or even destroy it. In “hard water” localities distilled water would better be used. In catching rain water it is well to bear in mind that the water falling during the first part of a shower may be very dirty.
When the moisture is close to saturation two or three successive determinations may be necessary for a satisfactory result. When the humidity is very low the difference between the dry- and the wet-bulb reading of several determinations may be considerable. In this case, too, the observer must use his judgment. A mean of several determinations is a fairly safe record.
When the water in the cup of the Mason hygrometer is frozen, care should be used in making the reading. If the upper end of the wick is dry—sometimes this is the case—the determination may be regarded with suspicion. It is better to wet the part around the bulb by means of a camel’s hair brush and wait a few minutes until the thermometer has settled to a fixed temperature before reading. A sling psychrometer gives a more accurate result in freezing weather, and its use is more convenient. The wick, or covering, may be wet with water at ordinary temperature, but the whirling must be continued until no further reduction of the wet-bulb temperature occurs.
The Hair Hygrometer.—Human hair freed from its natural oil and from grease of every sort, is highly sensitive to moisture. It may be made chemically clean by a bath, first in water with a mild soap and, after drying, in ether. After the ether bath it should not come in contact with bare hands. If one end of a clean hair—or a strand of several hairs—be made fast to a binding post and the other wound around the axle of a dial needle and kept taut by a spring, the lengthening and shortening of the hair by changing moisture may be made to indicate humidity with a fair degree of accuracy.
Commercial hair hygroscopes and hair hygrometers of various forms are now made by several manufacturers. Those of the best quality retain the name of the manufacturer; others are stamped with the name of the retailer. When new and clean, those of the best quality are about equal in accuracy to the psychrometer. Hair hygrometers usually deteriorate with continued use. The chief trouble comes from dust, and from gumming or fouling of bearings. Like any other delicate mechanism, careful cleaning and oiling will prolong the life of a hair hygrometer and preserve the accuracy of its registration. In the case of a commercial hygrometer, when once it has gone wrong, it is usually less expensive to purchase a new instrument than to repair an old one.
In spite of its shortcomings, the convenience of the hair hygrometer outweighs its disadvantages. For use in dwellings, school-rooms, textile establishments, candy factories and tobacco factories it is far better than the ordinary Mason type of instrument. The humidity is read instantly; computation from reference tables is not required.
The hygrograph is a hygrometer with recording mechanism like that of the thermograph. Drum, clock and record sheets are much the same in both, except that the record sheets of the hygrograph show per cent values in their horizontal rulings. The instrument is delicate and very sensitive to changes. Its records are not always trustworthy, but its errors are readily checked and adjustments are easily made. It should be sheltered so that by no possibility can rain or snow be driven upon it.
A hygrograph is usually a part of the equipment of each Weather Bureau station. In spite of the difficulties of transportation it is a very useful instrument in field stations. It is useful not only in noting the changes in relative humidity; it also may be an indication of change in absolute humidity.
The normal movement of the thermograph pen is upward from sunrise until 3 o’clock, then downward to the minimum of the next morning; the normal curve of the hygrograph is opposite in movement—downward from sunrise to 3 o’clock and then upward. These movements usually are so regular that experience enables one to read temperature approximately from the hygrogram sheet, and humidity from the thermogram. But when the temperature line is normal and the humidity line is abnormal, a change in the absolute humidity has occurred. Experience will teach the observer to look for the unusual in comparing the daily records, and also to interpret it.
The dew-point may be found without the use of the hygrometer by a very practical method. A thermometer, a polished tin cup—a “shaker” is better—and ice water are required. The cup must be absolutely free from grease. The cup is half-filled with water at about the temperature of the air. Ice water is added little by little and stirred with the thermometer until mist forms on the outside of the cup. The water is then at the temperature of the dew-point, and this is shown by the thermometer.
The Measurement of Evaporation; Evaporimeters.—The rate of evaporation depends on the amount of moisture in the air. If the humidity is low, evaporation is more rapid than when it is high. With the humidity above 95 per cent a piece of wet muslin in the open air may require more than an hour to dry; with the humidity at 100 per cent it does not dry at all. A high temperature also favors evaporation—chiefly from the fact that, with rising temperature, the relative humidity decreases without any change in absolute humidity. The rate of evaporation is therefore roughly an indication of the degree of humidity.
Evaporimeters vary in form but not in principle. In every case the evaporimeter is a device for measuring the depth of water lost by evaporation from an open surface. A very common form consists of a graduated glass tube filled with water, inverted in a vessel of water. A pin-hole aperture about half an inch from the lower end of the tube admits air to the top of the tube when evaporation lowers the water in the level of the pan. The level of the water in the pan is constant; the loss is in the tube. If the area of the surface of the container is 0.01 that of the section of the tube, a loss of I inch of water in the tube is equivalent to a loss of 0.01 inch by evaporation.
The Piche evaporimeter is a type of the best sort of instrument. A collar fastened around the tube at its mouth carries a disk which presses against and covers the mouth of the tube. A circular piece of filter paper, about twice the diameter of the tube, between the disk and the mouth of the tube allows a sufficient flow of water to keep the paper wet. By this device, loss of water by accident is avoided, and evaporation is recorded with a fair degree of accuracy. The Weather Bureau issues evaporimeters when they are deemed a necessary part of the equipment of a station. Field evaporimeters of different patterns, each for a specific use, are used at various Weather Bureau stations.
A bottle with straight sides may be inverted in a pan and used as an evaporimeter in an emergency. If the area of the section of the bottle be determined, an approximate rate of evaporation may be found. It is better to have the mouth of the bottle rest on circular pieces of blotting paper, several thicknesses being used. The flow of water through the blotting paper will be kept fairly steady by the admission of bubbles of air. It is hardly necessary to add that values obtained thus are only approximates.