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CHAPTER XXVI.
METEOROLOGICAL TABLES

The tables which follow are those which are now in use by the United States Weather Bureau. They were first published in the Instructions for Voluntary Observers issued in 1892, and were reprinted in 1897. The following instructions will be found of service in the use of the tables:—

Table I.—Dew-Point

The figures in heavy type, arranged in vertical columns at each side of the page, are the air temperatures in degrees Fahrenheit, as recorded by the dry-bulb thermometer. The figures in heavy type, running across the page, denote the differences, in degrees and tenths of degrees, between the dry-bulb and wet-bulb readings, or, technically, the depression of the wet-bulb thermometer. The figures in the vertical columns denote the dew-points. Make your observation of the wet and dry-bulb thermometers and note the difference between the two readings. Find, in the vertical columns of heavy type, the temperature corresponding to your dry-bulb reading, or the nearest temperature to that. Then look along the horizontal lines of figures in heavy type for the figure which corresponds exactly, or most nearly, with the difference between your wet and dry-bulb readings. Look down the vertical column under this latter figure until you reach the horizontal line corresponding to your dry-bulb reading. At this point the figures in the vertical column give the dew-point of the air at the time of your observation.

Example: Air Temperature (dry bulb), 47°; Wet Bulb, 44°; Difference, 3°. On page 148 will be found the table containing both 47° (dry bulb) and 3° (depression of the dew-point). In the twenty-eighth line of this table and in the seventh column will be found the dew-point, viz., 41°.

Example: Air Temperature, 61.5°; Wet Bulb, 55.5°; Difference, 6°.

In this case 61.5° is not found in the vertical columns of dry-bulb readings, but 61° and 62° are found. The dew-point, with a difference between wet and dry-bulb readings of 6°, for an air temperature of 61°, is 50°; for an air temperature of 62°, it is 52°. Evidently, then, for an air temperature of 61.5° the dew-point will be 51°, i.e., halfway between 50° and 52°. This method of determining dew-points at air temperatures or with depressions of the wet-bulb thermometer which are not given exactly in the tables, is known as interpolation.

Example: Air Temperature, 93°; Wet Bulb, 90.5°; Difference, 2.5°. Our table gives no dew-points for wet-bulb depressions of 2.5°, with air temperature 93°, but we find (on page 152) that for air temperature 93° and depression of wet bulb of 2°, the dew-point is 91°, while for a wet-bulb depression of 3°, the dew-point is 89°. By the method of interpolation we can readily determine the dew-point in the special case under consideration as 90°, i.e., halfway between 89° and 91°.

Table II.—Relative Humidity

The general plan of this table is the same as that of Table I. The figures in the vertical columns are the relative humidities (in percentages) corresponding to the different readings of the wet and dry-bulb thermometers.

Table III.—Reduction of Barometer to 32°

The figures in heavy type, arranged in vertical columns at the left of the page, refer to the temperature in degrees Fahrenheit, as indicated by the attached thermometer. The figures in heavy type, running across the top of the page, are the barometer readings in inches and tenths. Make a reading of the attached thermometer and of the barometer. Find in the vertical column the temperature corresponding to the reading of the attached thermometer, and in the horizontal line of heavy figures the reading corresponding to the height of the barometer. The decimal in the vertical column, under the appropriate barometer reading, and in the same horizontal line with the appropriate thermometer reading, is to be subtracted from the height of the barometer as observed, thus correcting the reading to freezing. When the attached thermometer reads below 28°, the correction is additive.

Example: Attached Thermometer, 69°; Barometer, 30.00 inches; Correction, -.110; Corrected reading, 29.890 inches.

Example: Attached Thermometer, 73°; Barometer, 29.75 inches; Correction = ?

We do not find any column corresponding to a barometer reading of 29.75 inches. We do find, however, that with a barometer reading of 29.50, and an attached thermometer reading of 73, the correction is -.118 inch, and with a barometer reading of 30.00, the correction is -.120. By interpolating, as in the case of the humidity table above, we find the correction for a barometer reading of 29.75 inches, and an attached thermometer reading of 73°. The correction is -.119, and the corrected reading is 29.75 – .119 = 29.63 inches.

Table IV.—Reduction of Barometer to Sea Level

The figures in heavy type, in the left-hand vertical columns, are the heights, in feet, of the barometer above sea level. The figures in heavy type at the top of the columns, running across the page, are the readings of the ordinary thermometer. The numbers of inches and hundredths of inches to be subtracted from the barometer reading (corrected for temperature by Table III), for the different heights above sea level, are given in the vertical columns.

The altitude above sea level of the city or town at which the observation is made should be ascertained as accurately as possible from some recognized authority, as, e.g., from a railroad survey; from Government measurements, or from some engineer’s office. The correction to be made is determined by a simple inspection of the table or by the method of interpolation.

Example: Altitude of Barometer above sea level, 840 feet; Temperature of the air, 40°; Correction, +.931 inch.

Example: Altitude of Barometer above sea level, 205 feet; Temperature of the air, 45°; Correction = ?

Here 205 feet and 45° are neither of them found in the table. Hence a double interpolation is necessary. For 200 feet and 40° the correction is +.224 inch. For 200 feet and 50° the correction is +.220 inch. Hence for 200 feet and 45° the correction is +.222 inch. For 210 feet and 40° the correction is +.235 inch. For 210 feet and 50° the correction is +.231 inch. Hence for 210 feet and 45° the correction is +.233 inch. Now for 205 feet we should have a correction midway between +.235 inch and +.233 inch or +.234 inch.

Table I.—Temperature of the Dew-Point, in Degrees Fahrenheit.

Table II.—Relative Humidity, per cent.

Table II.—Relative Humidity, Per cent.

Table II.—Relative Humidity, per cent.

Table III.—Reduction of Barometer Reading to 32°.

Table III.—Reduction of Barometer Reading to 32°.—Continued.

Table IV.—Table for Reducing Observations of the Barometer to

Sea Level, Correction Additive.

Table IV.—For Reducing Observations of the Barometer to Sea

Level.—Continued.

Table IV.—For Reducing Observations of the Barometer to Sea

Level.—Continued.

APPENDIX A

SUGGESTIONS TO TEACHERS

It is the object of this book to lead the student to the independent discovery of the most important facts in our ordinary weather conditions, and of the interrelations of the different weather elements. This practical study having taught something as to the real nature of atmospheric phenomena by actual observation, rapid and substantial progress may be made in the knowledge of the distribution and of the explanation of similar phenomena in other parts of the world, as derived through a study of the text-books. By means of this combination of the two kinds of study, the inductive and the didactic, the advantages of both may be preserved, and the slow progress of the first method and the unsound progress of the second may be avoided. This book is not a text-book, and it therefore does not attempt to give explanations of various phenomena discovered by the class. Explanations will, of course, be called for by the scholars, in increasing number as the work progresses, and the larger relations of the study become apparent. It is best, if possible, to leave the more complicated matters (such as the cause of the deflection of the wind from the gradient, of cyclones and anticyclones, etc.) until the subjects can be taken up in detail and fully explained, for instance in the later years of the high school course. It is not advisable to raise such complicated questions in the grammar school work if they can be avoided. The teacher who has a fairly good knowledge of one comprehensive modern text-book of meteorology, such as Davis’s Elementary Meteorology, will find himself sufficiently well equipped to answer the questions put by the class.

The value of the work outlined in this little book can be much increased if the larger applications of the lessons here learned are strongly emphasized. Suggestions along this line have been made in fine print throughout the text, but the examples given may be further extended to the great advantage of the student. Careful attention ought to be given to the formulating and writing out of the generalizations reached by the class, for in these written summaries the results are preserved in compact form.

CHAPTER I

The work outlined in this chapter is adapted to the lower grades in the grammar school. It is assumed that the pupils have already had some preliminary training in the simplest non-instrumental weather observations, such as can readily be made during the primary school years. For the convenience of teachers who may desire it, a brief outline of work suited to the primary school grades is here given. It is desirable that even older scholars be given some such training as this before they take up the exercises of Chapter II.

The central idea in this elementary work is to train the children in intelligent weather observation, so that they may come to appreciate what our typical weather changes are; that they may recognize the types as they recur, and may see how each example differs from, or accords with, those that have preceded it. We are all so directly affected by the weather conditions prevailing at any time that even the youngest children are forced, unconsciously to be sure, to take some notice of these changes. The work of the teacher is, therefore, simply to direct attention to what is already seen.

When the children come to school on some snowy winter day, with a northeast wind, chilling and damp, attention may be called to the need of overshoes and overcoats, to the piling up of the snow in deep drifts at certain places near the school or in the town, while in other places the ground is left bare; to the ease with which snowballs may be made, and to other facts which will very readily suggest themselves. A day or two after such a storm, when the sun is shining bright in a cloudless sky, when there is no wind and the air is dry, cold, and crisp, the contrasts between these two weather types should be brought out. Instead of snow we now have sunshine: instead of a damp, chilling northeaster we now have a calm and the air is dry; snowballs cannot easily be made in the early morning because the snow is frozen hard and is too dry, but towards noon, if the temperature rise high enough, there may be thawing on the tops or sides of the snowdrifts, and there the snow becomes soft enough for snowballing. Another weather type, often noted during our winter in the central and eastern United States, and strongly contrasted with both of the preceding conditions, is that which brings us a warm, damp, southerly wind, frequently accompanied by heavy rains. As these damp winds blow over snow-covered surfaces they become foggy and the ground is said to “smoke”; the heavy rain rapidly melts the snow; slush and mud make bad walking; rivers and brooks rise rapidly, perhaps overflowing their banks; low-lying places become filled with standing water. These and other features should all be brought out by the teacher, not by telling the class of them directly, but by judicious questioning, and they should be contrasted with the conditions which may immediately follow, when the storm has cleared off, and when the low temperatures brought by a cold wave, with its dry northwest wind, have resulted in freezing lakes, rivers, and brooks, and when skating and sliding may be indulged in. Early summer weather conditions, with their characteristic warm spells, cumulus clouds, thunderstorms, and (near the coast) sea breezes, furnish another long list of typical changes that should be just as carefully noted and described as the more striking winter characteristics. Autumn types add further to the list, which might be extended almost indefinitely.

One whole year of the grammar school course may well be given to the observations suggested in Chapter I, provided that there is no need of hastening on to the more advanced work. The advantage of extending the course over a whole school year is great, because such extension gives opportunity for becoming familiar with late summer, autumn, winter, spring, and early summer weather types, and this is far better than attempting to crowd all the work into one short season. The interest of a class can easily be kept up throughout a school year by means of a progressive system of observations. It is best to vary the observations from time to time, and to arrange them so that, beginning with the more simple, they shall gradually become more complete and more advanced as the year goes on. Thus, starting with temperature observations alone, these may be continued for one or two weeks before they are supplemented by records of wind direction and velocity. After some practice in the observation of these two weather elements (say during one month), data as to the state of the sky may be added. Cloud observations themselves may well be graded during successive weeks, so that, beginning with the simplest notes concerning amounts of cloudiness, the pupils shall gradually advance to the point of observing, and perhaps even of sketching, the common cloud forms and their changes. Thus an important step will have been taken towards appreciating the need of a standard cloud classification, which may be given later.

The addition of records of precipitation completes the list of simple non-instrumental weather observations, and these records, as well as the cloud records, can easily be graded, so that, during successive weeks, every week’s work shall be different from that of every other week. In this progression from the simpler to the more complicated observations lies the secret of making the work attractive. Nothing will sooner check interest in the study than the necessity of making exactly the same observations day after day and week after week throughout the year. A graded course of non-instrumental observation, such as is suggested, gives a very practical general knowledge of our common weather types and changes, and of the relations of one weather element to another. The questions asked under the different headings in this chapter are designed to awaken the interest of the scholars, and to call their attention to the more important points of diurnal, cyclonic, and seasonal changes in weather elements. The teacher will readily think of other questions which may be suggested for the consideration of the class.

Although the non-instrumental records are of little value for future reference, as compared with the instrumental observations, they should nevertheless be systematically preserved by the class in their record books. After discussion of the daily observations made by the different scholars, or by one of their number, the records may be written upon one of the blackboards reserved for this purpose. At the close of the day, or the next morning, the blackboard notes should be entered in a record book kept in the schoolroom. The teacher may guide in the discussion of the observations; may suggest points overlooked by the scholars; may draw comparisons between the weather conditions of other weeks and of other days. This talking over of the observations is most important, as it never fails to bring out much of interest.

CHAPTER II

This work may usually be begun in the early years of the grammar school course, as soon as the non-instrumental observations have been satisfactorily completed. The scheme of progressive observations already suggested may be followed to advantage in the instrumental work as well as in the non-instrumental. It is often a good plan to have a different scholar assigned to the task of taking the observations every day, or it may be more advisable to divide the work, making one responsible for the temperature observations, another for the precipitation, etc. It is well to have the daily instrumental weather records written upon the blackboard in the schoolroom, as already suggested in the case of the non-instrumental observations. At the end of each day the blackboard data should be entered in a permanent record book by some one of the scholars, and some ingenuity can be exercised in devising the best scheme for keeping this record. The record book should be carefully preserved in the schoolroom, where it may be referred to by the scholars of future years when any unusually severe storm, or a spell of excessively hot or dry weather, or a remarkable cold wave occurs, in order that comparison with past occurrences of a similar kind may be made. It is well to have the record book of large size, and to have each day’s record entered across two full pages. On the left-hand page the temperature, pressure, rainfall, wind direction and velocity, etc., may be entered, each observation in its proper column, the number of columns being increased according to the increasing number of observations. The right-hand page may be left for “Remarks.” These “Remarks” should include notes of any meteorological phenomena which did not find a place in the columns reserved for the regular observations, e.g., occurrence of hail, or frozen rain; damage by lightning, winds, or floods; freezing up of rivers or brooks; interruption of railroad or street-car traffic by snow, etc., and, in general, explanatory comments on the weather conditions. Instructive lessons may be taught as to the relation of the local weather conditions which prevail in the vicinity of the school, and those of other portions of the country, by comments on newspaper despatches concerning gales along the coast or on the lakes, and resulting damage to shipping; of snow blockades and stalled trains; of severe thunderstorms and tornadoes; of hot waves and sunstrokes, or of cold waves and the destruction of crops or fruits by the frost. The scholars should be encouraged to bring into the class any comments on such phenomena as may be of interest in the work. Such of these newspaper clippings as are of the most value may be pasted in the space reserved for the “Remarks,” where they may be referred to by succeeding classes; and in this space also may be pasted at the end of each week the barograph and thermograph sheets, if these instruments are in use at the school.

CHAPTER III

These observations may usually be profitably undertaken in the later grammar and in the high school years. The instruments described, while all desirable, are by no means all necessary, and no teacher should postpone the establishment of a course in observational meteorology for the reason that a complete set of first-class instruments cannot be secured at the start.

If the school is provided with a psychrometer, there will be no need of the ordinary thermometer, because the psychrometer gives the true air temperature. It is well, however, to have both stationary wet and dry-bulb thermometers, in the shelter, for ordinary school use, and also a sling psychrometer for use in the meteorological field work which forms an important part of the more advanced instrumental work in meteorology. The sling psychrometer may, of course, be used simply as an ordinary sling thermometer.

The simple form of mercurial barometer, without vernier and without attached thermometer, described in Chapter II, will be found the best barometer for general school use. The standard barometer, described in this chapter, is too expensive and too complicated to come into extended use in our schools. Full instructions concerning the care, the reading, and the corrections of the standard mercurial barometer are published by the Weather Bureau, and to these instructions teachers who have such an instrument are referred. (See Appendix B.)

The form of table given at the end of this chapter is intended merely as a suggestion, and not as a rigid scheme to be adopted in every school. In using the instruments here described, practice with the maximum and minimum thermometers (in addition to the simpler work of Chapter II) may be given before any attempt is made to have the class use the psychrometer. And in using the psychrometer one week may well be given to the determination of the dew-point alone, before the wet and dry-bulb readings are employed to determine the relative humidity. Absolute humidity, which is not referred to in this chapter, may, if the teacher deem it advisable, be added as another weather element for study. A refinement in the notes on the state of the sky is suggested, viz., that cloudiness should be recorded in tenths of the sky covered by clouds. This is an advance over the earlier, less accurate cloud observations, and is in line with such a progressive scheme as has been recommended. This book is not intended to present a rigid scheme of observational work in meteorology, alike for all schools, but rather to make suggestions for the guidance of teachers in laying out such a course as may fit their own cases.

Under the heading Summary of Observations only a few of the most important climatic elements have been noted. The list may easily be extended by the addition of such data as the following: For temperature, mean diurnal range; mean diurnal variability (the mean of the differences between the successive daily means). For humidity, monthly mean absolute humidity. For precipitation, the maximum daily precipitation; the number of rainy and snowy days in every month, the number of clear, fair, and cloudy days in every month; the mean frequency of rainfall in every month (number of rainy days divided by the total number of days); the number of days with thunderstorms, etc.

It is important that the monthly summaries should be discussed in the class, and that the scholars should give verbal statements as to the numerical results which they have obtained. In this way the work will have a living interest, which the mere compilation of summaries does not possess.