HUMIDITY
59. Instruments. As a direct factor, humidity is intimately connected with water-content in determining the structure and distribution of plants. The one is in control of water loss; the other regulates water supply. Humidity as a climatic factor undergoes greater fluctuation in the same habitat, and the efficient difference is correspondingly greater. Accordingly, simple instruments are less valuable than automatic ones, since a continuous record is essential to a proper understanding of the real influence of humidity. As is the rule, however, the use of simple instruments, when they can be referred to an ecographic basis, greatly extends the field which can be studied. In investigation, both psychrometer and psychrograph have their proper place. In the consideration of simple instruments for obtaining humidity values, an arbitrary distinction is made between psychrometers and hygrometers. The former consist of a wet and a dry bulb thermometer, while the latter make use of a hygroscopic awn, hair, or other object.
Psychrometers
60. Kinds. There are three kinds of psychrometer, the sling, the cog, and the stationary. All consist of a wet bulb and a dry bulb thermometer set in a case; the first two are designed to be moved or whirled in the air. The same principle is applied in each, viz., that evaporation produces a decrease in temperature proportional to the amount of moisture in the air. The dry bulb thermometer is an ordinary thermometer, while the wet bulb is covered with a cloth that can be moistened. The former indicates the normal temperature of the air, the latter gives the reduced temperature due to evaporation. The relative humidity of the air is ascertained by means of the proper tables, from two terms, i. e., the air temperature and the amount of reduction shown by the wet bulb. The sling and the cog psychrometers alone are in general use. The stationary form has been found to be unreliable, because the moisture, as it evaporates from the wet bulb, is not removed, and, in consequence, hinders evaporation to the proper degree.
Fig. 5. Sling psychrometer.
61. The sling psychrometer. The standard form of this is shown in the illustration, and is the one used by the Weather Bureau. This instrument can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of a metal frame to which are firmly attached two accurately standardized thermometers, reading usually from –30° to 130°. The frame is attached at the uppermost end to a handle in such fashion that it swings freely. The wet bulb thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth for the wet bulb should be always of the same texture and quality; the standard used by the Weather Bureau can be obtained from the instrument makers. A slight difference in texture makes no appreciable error, but the results obtained with different instruments and by different observers will be more trustworthy and comparable if the same cloth be used in all cases. The jacket for the wet bulb may be sewed in the form of a close-fitting bag, which soon shrinks and clings tightly. It may be made in the field by wrapping the cloth so that the edges just overlap, and tying it tightly above and below the bulb. In either case, a single layer of cloth alone must be used. The cloth becomes soiled or thin after a few months’ constant use and should be replaced. It is a wise precaution to carry a small piece of psychrometer cloth in the field outfit.
Fig. 6. Cog psychrometer.
62. Readings. All observations should be made facing the wind, and the observer should move one or two steps during the reading to prevent the possibility of error. The cloth of the wet bulb is moistened with water by means of a brush, or, much better, it is dipped directly into a bottle of water. Distilled water is preferable, as it contains no dissolved material to accumulate in the cloth. Tap-water and the water of streams may be used without appreciable error, if the cloth is changed somewhat more frequently. The temperature of the water is practically negligible under ordinary conditions. Readings can be made more quickly, however, when the temperature is not too far from that of the air. The psychrometer is held firmly and swung rapidly through the air when the space is not too confined. Where there is danger of breakage, it is swung back and forth through a short arc, pendulum-fashion. As the reading must be made when the mercury of the wet bulb reaches the lowest point, the instrument is stopped from time to time and the position of the column noted. The lowest point is often indicated by the tendency of the mercury to remain stationary; as a rule it can be noted with certainty when the next glance shows a rise in the column. In following the movement, and especially in noting the final reading, great care must be taken to make the latter before the mercury begins to rise. For this reason it is desirable to shade the psychrometer with the body when looking at it, and to take pains not to breathe upon the bulbs nor to bring them too near the body. At the moment when the wet bulb registers the lowest point, the dry bulb should be read and the results recorded.
63. Cog psychrometer. This instrument, commonly called the “egg-beater” psychrometer, has been devised to obviate certain disadvantages of the sling psychrometer in field work, and has entirely supplanted the latter in the writer’s own studies. It is smaller, more compact, and the danger of breaking in carriage or in use is almost nil. It has the great advantage of making it possible to take readings in a layer of air less than two inches in thickness, and in any position. Fairly accurate results can even be obtained from transpiring leaves. The instrument can readily be made by a good mechanic, at a cost for materials of $1.75, which is less than half the price for the sling form. A single drawback exists in the use of short, Centigrade thermometers, inasmuch as tables of relative humidity are usually expressed in Fahrenheit. It is a simple matter, however, to convert Centigrade degrees into Fahrenheit, mentally, or the difficulty may be avoided by the conversion table shown on page [47], or by constructing a Centigrade series of humidity tables. The fact that the wet and dry bulbs revolve in the same path has raised a doubt concerning the accuracy of the results obtained with this instrument. Repeated comparisons with the sling psychrometer have not only removed this doubt completely, but have also proved that the standardization of the thermometers has been efficient.
64. Construction and use. A convenient form of egg-beater is the Lyon (Albany, New York), in which the revolving plates can be readily removed, leaving the axis and the frame. The thermometers used are of the short Centigrade type. They are 4½ inches long and read from –5° to 50°. Eimer and Amend, 205 Third Ave., New York city, furnish them at 75 cents each. The thermometers are carefully standardized and compared, and then grouped in pairs that read together. Each pair is used to construct a particular psychrometer. Each thermometer is strongly wired to one side of the frame, pieces of felt being used to protect the tube and increase the contact. The frame is also bent at the base angles to permit free circulation of air about the thermometer bulbs. The bulb of one thermometer is covered with the proper cloth, and the psychrometer is finished. Since the frame revolves with the thermometers, it is necessary to pour the water on the wet bulb, or to employ a pipette or brush. The thermometer bulbs are placed in the layer to be studied, and the frame rotated at an even rate and with moderate rapidity. The observation is further made as in the case of the sling psychrometer. As the circle of rotation is less than three inches in diameter, and the layer less than an inch, in place of nearly three feet for the sling form, the instrument should not be moved at all for extremely localized readings, but it must be moved considerably, a foot or more, if it is desirable to obtain a more general reading.
65. Hygrometers. While there are instruments designed to indicate the humidity by means of a hygroscopic substance, not one of them seems to be of sufficient accuracy for use in ecological study. The difficulty is that the hygroscopic reaction is inconstant, rather than that the instruments are not sufficiently sensitive. A number of hygrometers have been tested, and in all the error has been found to be great, varying usually from 10–20 per cent. In the middle of the scale they sometimes read more accurately, but toward either extreme they are very inexact. It seems probable that an accurate hygrometer can be constructed only after the model of the Draper psychrograph. Its weight and bulk would make it an impossible instrument for field trips, and the expense of one would provide a dozen psychrometers. In consequence, it does not seem too sweeping to say that no hygrometer can furnish trustworthy results. Of simple instruments for humidity, the psychrometer alone can be trusted to give reliable readings. Crova’s hygrometer, used by Hesselmann, is not a hygrometer in the sense indicated. As it is much less convenient to handle and to operate than the cog psychrometer, it is not necessary to describe it.
Psychrographs
Fig. 7. Draper psychrograph.
66. The Draper psychrograph. A year’s trial of the Draper psychrograph in field and planthouse has left little question of its accuracy and its great usefulness. Essentially, it consists of a band of fine catgut strings, which are sensitive to changes in the moisture-content of the air. The variations in the length of the band are communicated to a long pointer carrying an inking pen. The latter traces the record in per cent of relative humidity on a graduated paper disk, which is practically the face of an eight-day clock. The whole is enclosed in a metal case with a glass front. A glance at the illustration will show the general structure of the instrument. Continued psychrometric tests demonstrate that the margin of error is well within the efficient difference for humidity, which is taken to be 5 per cent. In the field tests of the past summer, two psychrographs placed side by side in the same habitat did not vary 1 per cent from each other. The same instruments when in different habitats did not deviate more than 1 per cent from the psychrometric values, except when the air approached saturation. For humidities above 90 per cent, the deviation is considerable, but as these are temporary and incident upon rainfall, the error is not serious. For humidities varying from 10–85 per cent, the psychrograph is practically as accurate as the psychrometer. Per cents below 10 are rare, and no tests have been made for them.
Fig. 8. Instrument shelter, showing thermograph and psychrograph in position.
67. Placing the instrument. The psychrograph should be located in a place where the circulation of the air is typical of the station observed. A satisfactory shelter will screen the instrument from sun and rain, and at the same time permit the air to pass freely through the perforations of the metal case. The form shown in figure 8 meets both of these conditions. A desirable modification is effected by fastening a strip about the cover of such depth as to prevent the sun’s rays from striking the case except when the sun is near the horizon. A cross block is fastened on the post of the shelter after being exactly leveled. The psychrograph rests upon this block, which is three feet above the ground in order to avoid the influence of radiation. The instrument is held in position by slipping the eye over a small-headed nail driven obliquely. It does not hang from the latter, but must rest firmly upon the cross block. The post is set to a depth that prevents oscillation in the wind, which is liable to obscure the record. In shallow mountain soils stability is attained by fastening a broad board at the base of the post before setting it. When two or more psychrographs are established in different habitats, great pains are taken to set them up in exactly the same way. The shelters are alike, the height above the soil the same, and the instruments all face the south.
68. Regulating and operating the instrument. When two or more psychrographs are to be used in series, they must be compared with each other in the same spot for several days until they run exactly together with respect to per cent of humidity and to time. During this comparison they are checked by the psychrometer and so regulated that they register the proper humidity. When a single instrument is used alone as the basis to which simple readings may be referred, all regulating may well be done after the instrument is in position. This is a simple process; it is accomplished by obtaining the relative humidity beneath the shelter and at the proper height by a psychrometer. The pen hand is then moved to the proper line on the disk by means of the screws at its base. These are reached by removing the lettered glass face. The thumbscrew on the side opposite the direction in which the pen is to move is released, and the opposite screw simultaneously tightened, until the pen remains upon the proper line. Experience has proved that the record sheet should be correctly labeled and dated before being placed on the disk. In the press of field duties, records labeled after removal are liable to be confused. It is likewise a great saving of time to write the date of the month in the margin of each segment. Care is taken to place the sheet on the disk in the same position each time; this can easily be done by seeing that the sharp point on the disk penetrates the same spot on the paper. A single drop of ink in the pen will usually give the most satisfactory line. A thin line is read most accurately. If the pen point is too fine, however, the ink does not flow readily, and the point should be slightly blunted by means of a file. More often the line is too broad and the pen must be carefully pointed. Occasionally the pen does not touch the sheet, and it becomes necessary to bend the hand slightly. This is a frequent difficulty if the records are folded or wrinkled, and consequently the sheets should always be kept flat.
69. The weekly visit. Psychrographs must be visited, checked, rewound, and inked every week. Whenever possible this should be done regularly at a specified day and hour. This is especially desirable if the same record sheet is used for more than one week. Time and energy are saved by a fixed order for the various tasks to be done at each visit. After opening the instrument the disk is removed, and the clock wound, and, if need be, regulated. The record sheet is replaced, the disk again put on the clock arbor, and the pen replenished with a drop of ink. A psychrometer reading is made, and the results in terms of relative humidity noted at the proper place on the disk sheet. If the psychrograph vary more than 1 per cent, it is adjusted to read accurately. In practice it has been found a great convenience to keep each record sheet in position for three weeks, and the time may easily be extended to four. In this event, the pen is carefully cleaned with blotting paper at each visit, and is then refilled with an ink of different color. To prevent confusion, the three different colored inks are always used in the same order, red for the first week, blue for the second, and green for the third. The advantages of this plan are obvious: fewer records are used and less time is spent in changing them. The records of several weeks are side by side instead of on separate sheets, and in working over the season’s results, it is necessary to handle but a third as many sheets.
The Draper psychrograph is made by the Draper Manufacturing Company, 152 Front St., New York city. The price is $30. A few record sheets and a bottle of red ink are furnished with it. Additional records can be obtained at 3 cents each. The inks are 25–50 cents per bottle, depending upon the color.
Humidity Readings and Records
70. The time of readings. If simple instruments alone are used for determining humidity, readings are practically without value unless made simultaneously through several stations, or successively at one. When it is possible to combine these, and to make psychrometer readings at different habitats for each hour of the day, or at the same hour for several days, the series is of very great value. Single readings are unreliable on account of the hourly and daily variations of humidity, but when these changes are recorded by a psychrograph, such readings at once become of use, whether made in the same habitat with the recording instrument or elsewhere. In the latter case, one reading will tell little about the normal humidity of the habitat, but several make a close estimate possible. When a series of psychrographs is in use, accurate observations can be made to advantage anywhere at any time. As a rule, however, it has been found most convenient to make simple readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as these hours afford much evidence in regard to the daily range. A good time also is that at which the temperature maximum occurs each day, but this is movable and in the press of field work can rarely be taken advantage of. A very fair idea of the daily mean humidity is obtainable by averaging the readings made at the hours already indicated. The comparison of single readings with the psychrograph record should not be made at a time when a rapid change is occurring, as the automatic instrument does not respond immediately. Such a condition is usually represented by a sudden rain, and is naturally not a satisfactory time for single readings in any event.
Fig. 9. Atmometer.
71. Place and height. As stated above, the psychrograph is placed three feet above the surface of the ground in making readings for the comparison of stations. In low, herbaceous formations, the instrument is usually placed within a few inches of the soil in order to record the humidity of the air in which the plants are growing. In forest formations, the moisture often varies considerably in the different layers. This variation is easily determined by simultaneous psychrometer readings in the several layers, or, if occasion warrants, a series of psychrographs may be used. In field work the rule has been to make observations with the psychrometer at 6 feet, 3 feet, and the surface of the soil, but the reading at the height of 3 feet is ordinarily sufficient. Humidity varies so easily that several readings in different parts of one formation are often desirable. In comparing different formations, the readings should be made in corresponding situations, for example, in the densest portion of each.
72. Check instruments. Humidity is so readily affected by temperature, wind, and pressure, that a knowledge of these factors is essential to an understanding of its fluctuations. Pressure, disregarding daily variation, is taken account of in the tables for ascertaining relative humidity, and is determined once for all when the altitude of a station has been carefully established. The temperature is obtained directly from the dry bulb reading. Its value is fundamental, as the amount of moisture in a given space is directly affected by it; like pressure, it also is taken account of in the formula. The movement of the air has an immediate influence upon moisture by mixing the air of different habitats and layers. So far as the plant is concerned, it has practically the effect of increasing or decreasing the humidity by the removal of the air above it. Thus, while the anemometer can furnish no direct evidence as to the amount of variation, it is of aid in explaining the reason for it. Likewise, the rate of evaporation as indicated by a series of atmometers, affords a ready method of estimating the comparative effect of humidity in different habitats. Potometers and other instruments for measuring transpiration throw much light upon humidity values. Since they are concerned with the response of the plant to humidity, they are considered in the following chapter.
73. Humidity tables. To ascertain the relative humidity, the difference between the wet and dry bulb readings is obtained. This, with the dry bulb temperature, is referred to the tables, where the corresponding humidity is found. A variation in temperature has less effect than a variation in the difference; in consequence, the dry bulb reading is expressed in the nearest unit, and the difference reckoned to the nearest .5. The humidity varies with the air pressure. Hence, the altitude must be determined for the base station, and for all others that show much change in elevation. Within the ordinary range of growing-period temperatures, the effect of pressure is not great. For all ordinary cases, it suffices to compute tables for pressures of 30, 29, 27, 25, and 23 inches. The following table indicates the decrease in pressure which is due to altitude.
| ALTITUDE | PRESSURE | ||
|---|---|---|---|
| Feet | Meters | Inches | Centimeters |
| 0 | 0 | 30 | 76 |
| 910 | 277 | 29 | 73.5 |
| 1850 | 574 | 28 | 71 |
| 2820 | 860 | 27 | 68.5 |
| 3820 | 1165 | 26 | 66 |
| 4850 | 1477 | 25 | 63.5 |
| 5910 | 1792 | 24 | 61 |
| 7010 | 2138 | 23 | 58.5 |
| 8150 | 2485 | 22 | 56 |
| 9330 | 2845 | 21 | 53.5 |
| 10550 | 3217 | 20 | 51 |
| 13170 | 4016 | 18 | 46 |
| 16000 | 4880 | 16 | 41 |
The fluctuations of pressure due to weather are usually so slight that their influence may be disregarded. An excellent series of tables of relative humidity is found in Marvin’s Psychrometric Tables, published by the U. S. Weather Bureau, and to be obtained from the Division of Publications, Washington, D. C., for 10 cents. A convenient field form is made by removing the portion containing the tables of relative humidity, and binding it in stiff oilcloth.
Fig. 10. Conversion scale for temperatures.
74. Sums, means, and curves. An approximate humidity sum can be obtained by adding the absolute humidities for each of the twenty-four hours, and expressing the results in grains per cubic foot. It is possible to establish a general ratio between this sum and the transpiration sum of the plant, but its value is not great at present. Means of absolute and of relative humidity are readily determinable from the psychrograph records; the latter are the most useful. The mean of relative humidity for the twenty-four hours of a day is the average of the twenty-four hour humidities. From these means the seasonal mean is computed in the same manner. A close approximation, usually within 1 degree, may be obtained in either case by averaging the maximum and minimum for the period concerned. Various kinds of curves are of value in representing variation in humidity. Obviously, these must be derived from the psychrograph, or from the psychrometer when the series is sufficiently complete. The level curve indicates the variation in different stations at the same time. These may be combined in a series for the comparison of readings made at various heights in the stations. The day or point curve shows the fluctuations during the day of one point, and the station curve the variation at different heights in the same station. The curves of successive days or of different stations may of course be combined on the same sheet for comparison. Level and station curves based upon mean relative humidities are especially valuable.
75. Records. A field form is obviously unnecessary for the psychrograph. The record sheets constitute both a field and permanent record. The altitude and other constant features of the station and the list of species, etc., are entered on the back of the first record sheet, or, better, they are noted in the permanent formation record. For psychrometer readings, whether single or in series, the following record form is employed:
| Date | Hour | Formation | Station | Altitude | Community | Height of reading | Dry bulb | Wet bulb | Diff. | Rel. Hum. | Base Hum. | Abs. Hum. | NOTES | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sky | Rain | Wind | |||||||||||||
| 15/8/’04 | 6:20 A.M. | Spruce | Brook bank | 2500 m | Mertensiare | 1 ft. | 51° | 46° | 5 | 72% | 63% | 2.9 | Clear | 0 | 0 |
| „ | „ | Half gravel | Hiawatha | „ | Asterare | „ | 56° | 49° | 7 | 64% | 63% | 3.0 | „ | 0 | 0 |
| „ | 6:45 P.M. | Spruce | Brook bank | „ | Mertensiare | „ | 54° | 52° | 2 | 89% | 69% | 4.2 | „ | 2 cc. | 0 |
| „ | „ | Half gravel | Hiawatha | „ | Asterare | „ | 56° | 52° | 4 | 79% | 69% | 4.0 | „ | 2 cc. | 0 |
On page [47] is given a table for the conversion of Centigrade into Fahrenheit temperatures. This may be done mentally by means of the formula F = C/5 × 9 + 32°.