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°.