Practical Exercises in Elementary Meteorology

CHAPTER V

In this chapter a series of six consecutive weather maps is taken as the basis of the work. The study of the weather elements on such a series of maps gives a far clearer understanding of the distribution of these elements and of their relation one with another, than if a far larger number of single maps are studied which do not follow one another in regular sequence. Teachers should add to the map-drawing exercises by giving their classes the data from other sets of maps selected from the school files. Summer maps as well as winter ones should be used, in order that too much emphasis may not be laid on winter conditions. The search for the various cities in which Weather Bureau stations are established, involved in the work of entering the data on the blank maps, furnishes excellent practice in geography. This exercise may be varied, and practice in the location of the different States may be given, if the teacher reads out to the class the temperatures in different parts of the various States, as, e.g., central Arizona, 34°; southwestern Tennessee, 30°; northern Nevada, 38°, etc. As the State names are not given on the Weather Bureau maps, such a method as this will give a very desirable familiarity with the relative positions of the States of the Union. If the school possesses a blackboard outline map of the United States, it may be a good plan, if the class is small, to have one of the pupils enter the temperatures and draw the isotherms on the board before the class, and to let the others correct him if they see that he is going wrong. As to irregularities in the isotherms, which may cause trouble, the officials of the Weather Bureau vary somewhat among themselves in dealing with such cases, and no definite rules can be laid down to fit all occasions. It is best to select maps with few irregularities at first, and in time experience will show how the exceptional cases may be treated.

By the scheme of coloring isothermal maps, suggested in this chapter, a valuable series of permanent illustrations of noteworthy weather types for class use can readily be prepared at very slight cost. For the purposes of these colored illustrations it is best to use the large scale Washington blank weather maps, as suggested in the preceding chapter, and to have each map mounted on heavy cardboard after it has been colored. By means of this mounting the maps are prevented from tearing, and can be kept smooth and in good condition.

The scheme of coloring may be varied to suit the fancy of the scholars, for the preparation of these permanent illustrations may well be intrusted to those of the class who are especially interested in the work, and who are skillful in their use of the paint brush. The work is really very simple and needs only ordinary care. As soon as the drawing of isotherms and the coloring of isothermal charts have been sufficiently studied, the teacher should hang up the daily weather map each day (or a blank map with the isotherms for the day drawn on it), and should call attention to the temperature changes from day to day. In this way the facts of actual temperature changes experienced by the class will be associated with the larger temperature changes shown on the weather maps.

The sections on temperature gradients may be postponed, if the teacher deems it advisable, until other matters of a simpler nature are passed. The idea of rates of change is not an easy one for students to grasp, and it is far better to postpone the consideration of this subject than to involve the class in any confusion at this stage. It is a mistake, however, to omit these sections altogether, as a clear conception of the principle of rates is a valuable part of a student’s mental equipment. It is a good plan, in the exercises on lines of temperature decrease, to have maps prepared with faint isotherms and heavy lines of temperature decrease, in order to emphasize the idea of change of temperature, in contrast to the idea of constancy of temperature expressed by the isotherms.

CHAPTER VI

In the work on the wind charts it is essential to proceed very slowly, in order that the best results may be obtained by the pupils. Some of the aberrant wind courses, which complicate the discovery of the cyclonic and anticyclonic spirals, may well be omitted in the case of the younger classes, and considerable assistance in facilitating these discoveries may be given by suggestions as to adding intermediate dotted wind arrows, in sympathy with the observed wind directions. The anticyclonic systems are always much more difficult to discover than the cyclonic, and care should be taken to assist the class in this matter as much as seems necessary.

The questions in the text are merely suggestive, and are by no means as numerous as it would be well to make them in the class. The discovery of the spirals will probably be made by degrees. The concise formulation of the facts discovered will furnish excellent basis for exercises in writing.

It is interesting to note that the discussion as to whether the winds blow circularly around, or radially in towards, centers of low pressure, which usually comes up in every class in meteorology at some stage in the study, was carried on in a very animated way about the middle of the present century in this country. Two noted American meteorologists, Redfield and Espy, and their respective followers, took opposite sides in this controversy, Redfield maintaining at the start that the winds moved in circles, and Espy maintaining that they followed radial courses. The truth lay between the two.

CHAPTER VII

The study of atmospheric pressure is not easy, because the pupils cannot perceive the changes in pressure from day to day by their unaided senses. Especially difficult does this study become if the class has not already had some practice in making barometer readings. When this observational work has not preceded the consideration of the isobaric lines of the daily weather maps, the teacher should introduce the subject of pressure very carefully. The experiment with the Torricellian tube will show the class something of the reality of atmospheric pressure, and the variations in this pressure from day to day can readily be made apparent by means of a few barometer readings, if time cannot be spared for a regular and continued series of barometer observations. A word may be said as to the correction of barometric readings for local influences, in order to make these readings comparable, if this matter has not been previously met with, but care should be taken not to confuse the younger pupils too much with explanations at this stage of the work. It may be well to omit this point unless it is brought up by some pupil. The questions asked in the text are merely suggestive. They may be added to and varied at the discretion of the teacher.

Lines of pressure-decrease should be drawn on all isobaric charts studied in the class, as they are highly instructive. When the isobars are near together, these lines of pressure-decrease may be drawn heavier, to indicate a steeper gradient. The convergence of these lines towards regions of low pressure, and their divergence from regions of high pressure, seen on every map on which these gradients are drawn, emphasizes an important lesson. Before measuring rates of pressure-decrease by means of a scale, considerable practice should be given in the study, by means of the eye, of the rapidity or slowness of decrease of pressure, as shown by the heavier or lighter lines of pressure-decrease. When the broad facts of differing rates are comprehended, then the actual measurement of these rates is a comparatively easy matter. In any case, however, an appreciation of these rates of change is not always readily gained, even by older scholars in the high school. It is, therefore, of prime importance to proceed very slowly indeed at this point, and to have every step fully understood before another step is taken.

The instructions in the text for measuring rates of pressure-decrease are, that these rates shall be recorded as so many hundredths of an inch of change of pressure in one latitude-degree. This is done for the sake of simplicity. If this rate is expressed in hundredths of an inch of pressure in a quarter of a latitude-degree of distance, the numerical value is the same as if expressed in millimeters of pressure per latitude-degree of distance.

CHAPTER VIII

The fact of the prevalence of different kinds of weather over the country at the same time is of great importance. It should be strongly emphasized by the teacher in the course of the discussion of the maps of weather distribution. Additional exercises of the same sort may be given to advantage, by letting the class plot and study the weather signs taken from any current weather map. Instructive lessons may be taught by talking over, in the class, the different ways in which people all over the country are affected by the character of the weather that happens to prevail where they are.

CHAPTERS IX-XVIII

The correlation exercises will, as a whole, teach few entirely new facts to the brighter scholars who have faithfully completed the preceding work in observations and in the construction and study of the daily weather maps. These exercises do, however, lead to detailed examination and to the careful working out of the relations which may have been previously noticed in a general way only. They give the repeated illustration which is necessary in order to impress firmly on the mind the lesson that the weather map has to teach.

It is a good plan to let different scholars work out the problems for different months. The results reached in each case should be discussed in the class, and thus each member may have the double advantage of working out his own problem, and of profiting by the work done by his fellows. Throughout these exercises care should be taken to have weather maps of all months studied. The exercise on the correlation of the velocity of the wind with the pressure cannot be undertaken unless the work on temperature and pressure gradients (Chapters V and VII) has been completed.

CHAPTERS XX-XXV

It is not expected that any one scholar can accomplish all that is here outlined. Examples may be selected from the list, as opportunity offers, so that each scholar shall become familiar with several problems.

Few of the problems suggested call for continuous routine observation at fixed hours. They require, on the other hand, an intelligent examination of ordinary weather phenomena, with special reference to discovering their explanation. In most of the problems a small number of observations will suffice. Under the supervision of the teacher, different problems may be assigned to the several members of a class; or several scholars may work on different parts of the same problem, exchanging records in order to save time. All the scholars should have a general knowledge of the results which have been obtained from the observations made by the other members of their class. The teacher will use his discretion in arranging the order of the problems, and in selecting those that are best suited to the season in which the work is done, to the locality in which the school is situated, and to the facilities and apparatus at command. Although the variety of accessible problems is less in city schools than in country schools, much may be done in the city as well as in the country.

The opportunities for carrying out such observational work vary so much in different schools that it is impossible to give specific instructions, which shall be available in all cases. Some general suggestions are therefore given, which the teacher may supplement by more detailed instructions framed to fit the particular circumstances of each case.

A review of the headings of the different problems shows that a very general correlation exists among them, whereby the subjects of every heading are associated with those of nearly every other. In other words, every weather element is treated as a function of several other elements. It follows from this that the variety of work here outlined is more apparent than real, and that many problems which appear from their wording to be entirely new are in large part rearrangements of problems previously encountered.

APPENDIX B