CHAPTER X.

The Deflection of the Wind from the Gradient: Ferrel’s Law.—The law of the deflection of the wind prevailingly to the right of the gradient is known asFerrel’s Law, after William Ferrel, a noted American meteorologist, who died in 1891. The operation of this law has already been seen in the spiral circulation of the winds around the cyclone and the anticyclone, as shown on the maps of our series. In the case of the cyclone the gradient is directed inward towards the center; in the case of the anticyclone the gradient is directed outward from the center. In both cases the right-handed deflection results in a spiral whirl, inward in the cyclone, outward in the anticyclone. The operation of this law is further seen in the case of theNortheast Trade Winds. These winds blow from about Lat. 30° N. towards the equator, with wonderful regularity, especially over the oceans. Instead of following the gradient and blowing as north winds, these trades turn to the right of the gradient and becomenortheastwinds, whence their name. From about Lat. 30° N. towards the North Pole there is another great flow of winds over the earth’s surface. These winds do not flow due north, as south winds. They turn to the right, as do the trades, and become southwest or west-southwest winds, being known as thePrevailing Westerlies. Ferrel’s Law thus operates in the larger case of the general circulation of the earth’s atmosphere, as well as in the smaller case of the local winds on our weather maps.

Fig. 47.

CORRELATION OF THE VELOCITY OF THE WIND ANDTHE PRESSURE.

Prepare a scale of latitude degrees, as explained in Chapter V. Select some station on the weather map at which there is a wind arrow, and at which you wish to study the relation of wind velocity and pressure. Find the rate of pressure change per degree as explained in Chapter VII. Note also the velocity, in miles per hour, of the wind at the station. Repeat theoperation 100 or more times, selecting stations in different parts of the United States. It is well, however, to include in one investigation either interior stations alone (i.e., more than 100 miles from the coast) or coast stations alone, as the wind velocities are often considerably affected by proximity to the ocean. And, if coast stations are selected, either onshore or offshore winds should alone be included in one exercise. The investigation may, therefore, be carried out so as to embrace the following different sets of operations:—

Enter your results in a table similar to the one here given:—

Table II.—Correlation of Wind Velocity and BarometricGradient.

For interior (or coast) stations, with onshore (or offshore) winds, in the United States during the month (or months) of

Rates of Pressure Change per Latitude Degree∞-2020-1010-55- 31⁄231⁄2- 21⁄221⁄2-2etc.Distances between Isobars in Latitude Degrees01⁄21⁄2-11-22-33-44-5etc.Wind Velocities (miles per hour)SumsCasesMeans

The wind velocity for each station is to be entered in the column at whose top is the rate of pressure change found for that station. Thus, if for any station the rate of pressure change is 31⁄2(i.e., .03 inch in one latitude degree), and the wind velocity at that station is 17 miles an hour, enter the 17 in the fourth and fifth columns of the table. When you find that the rate of pressure change for any station falls into two columns of the table, as,e.g., 10, or 5, or 31⁄3, then enter the corresponding wind velocity in both those columns.

In the space markedSumswrite the sum-total of all the wind velocities in each column. TheCasesare the number of separate observations you have in each column. TheMeansdenote the average or mean wind velocities found in each column, and are obtained by dividing the sums by the cases.

Study the results of your table carefully. Deduce from your own results a general rule for wind velocities as related to barometric gradients.

The dependence of wind velocities on the pressure gradientis a fact of great importance in meteorology. The ship captain at sea knows that a rapid fall of his barometer means a rapid rate of pressure change, and foretells high winds. He therefore makes his preparations accordingly, by shortening sail and by making everything fast. The isobaric charts of the globe for January and July show that the pressure gradients are stronger (i.e., the rate of pressure change is more rapid) over the Northern Hemisphere in January than in July. This fact would lead us to expect that the velocities of the general winds over the Northern Hemisphere should be higher in winter than in summer, and so they are. Observations of the movements of clouds made at Blue Hill Observatory, Hyde Park, Mass., show that the whole atmosphere, up to the highest cloud level, moves almost twice as fast in winter as in summer. In the higher latitudes of the Southern Hemisphere, where the barometric gradients are prevailingly much stronger than in the Northern, the wind velocities are also prevailingly higher than they are north of the equator. The prevailing westerly winds of the Southern Hemisphere, south of latitude of 30° S., blow with high velocitiesnearly all the time, especially during the winter months (June, July, August). These winds are so strong from the westward that vessels trying to round Cape Horn from the east often occupy weeks beating against head gales, which continually blow them back on their course.

FORM AND DIMENSIONS OF CYCLONES AND ANTICYCLONES.

A.Cyclones.—Provide yourself with a sheet of tracing paper about half as large as the daily weather map. Draw a straight line across the middle of it; mark a dot at the center of the line, the letterNat one end, and the letterSat the other. Place the tracing paper over a weather map on which there is a fairly well enclosed center of low pressure (low), having the dot at the center of thelow, and the line parallel to the nearest meridian, the end markedNbeing towards the top of the map. When thus placed, the paper is said to beoriented. Trace off the isobars which are nearest the center. In most cases the 29.80-inch isobar furnishes a good limit, out to which the isobars may be traced. Continue this process, using different weather maps, until the lines on the tracing paper begin to become too confused for fairly easy seeing. Probably 15 or 20 separate areas of low pressure may be traced on to the paper. It is important to have all parts of the cyclonic areas represented on your tracing. If most of the isobars you have traced are on the southern side of cyclones central over the Lakes or lower St. Lawrence, so that the isobars on the northern sides are incomplete, select for your further tracings weather maps on which the cyclonic centers are in the central or southern portions of the United States, and therefore have their northern isobars fully drawn.

When your tracing is finished you have acomposite portraitof the isobars around several areas of low pressure. Now studythe results carefully. Draw a heavy pencil or an ink line on the tracing paper, in such a way as to enclose the average area outlined by the isobars. This average area will naturally be of smaller dimensions than the outer isobars on the tracing paper, and of larger dimensions than the inner isobars, and its form will follow the general trend indicated by the majority of the isobars, without reproducing any exceptional shapes.

Write out a careful description of the averageform,dimensions[measured by a scale of miles or of latitude degrees (70 miles = 1 degree about)] andgradientsof these areas of low pressure, noting any tendency to elongate in a particular direction; any portions of the composite where the gradients are especially strong, weak, etc.

B.Anticyclones.—This investigation is carried out in precisely the same manner as the preceding one, except that anticyclones (highs) are now studied instead of cyclones. The isobars may be traced off as far away from the center as the 30.20-inch line in most cases. When, however, the pressure at the center is exceptionally high, it will not be necessary to trace off lower isobars than those for 30.30, or 30.40, or sometimes 30.50 inches.

Loomis’s Results as to Form and Dimensions of Cyclones and Anticyclones.—One of the leading American meteorologists, Loomis, who was for many years a professor in Yale University, made an extended study of the form and dimensions of areas of low and high pressure as they appear on our daily weather maps. In the cases of areas of low pressure which he examined, the average form of the areas was elliptical, the longer diameter being nearly twice as long as the shorter (to be exact the ratio was 1.94 : 1). The average direction of the longer diameter he found to be about NE. (N. 36° E.), and the length of the longer diameter often 1600 miles. In the case of areas of high pressure, Loomis also found an elliptical form predominating; the longer diameter being about twice as long as the shorter (ratio 1.91 : 1), and the direction of trend about NE. (N. 44° E.). These characteristics hold, in general, for the cyclonicand anticyclonic areas of Europe also. The cyclones of the tropics differ considerably from those of temperate latitudes in being nearly circular in form.

CORRELATION OF CYCLONES AND ANTICYCLONES WITH THEIRWIND CIRCULATION.

A.Cyclones.—Something as to the control of pressure over the circulation of the wind has been seen in the preliminary exercises on the daily weather maps. We now proceed to investigate this correlation further by means of the composite portrait method. This method is a device to bring out more clearly the general systems of the winds by throwing together on to one sheet a large number of wind arrows in their proper position with reference to the controlling center of low pressure. In this way we have many more observations to help us in our investigation than if we used only those which are given on one weather map, and the circulation can be much more clearly made out.

Provide yourself with a sheet of tracing paper, prepared as described in Chapter XI. Place the paper over an area of low pressure on some weather map, with the dot at the center of thelow, and having the paper properly oriented, as already explained. Trace off all the wind arrows around the center of low pressure, making the lengths of these arrows roughly proportionate (by eye) to the velocity of the wind, according to some scale previously determined upon. Include on your tracing all the wind arrows reported at stations whose lines of pressure-decrease converge towards the low pressure center. Repeat this operation, using other centers of low pressure on other maps, until the number of arrows on the tracing paper is so great that the composite begins to become confused. Be careful always to center and orient your tracing paper properly.Select the weather maps from which you take your wind arrows so that the composite shall properly represent winds in all parts of the cyclonic area.

Deduce a general rule for the circulation and velocity of the wind in a cyclonic area, as shown on your tracing, and write it out.

B.Anticyclones.—This exercise is done in precisely the same way as the preceding one, except that anticyclones and their winds are studied instead of cyclones.

Deduce a general rule for the circulation and velocity of the wind in an anticyclonic area, as shown on your tracing, and write it out.

The control of the wind circulation by areas of low and high pressureis one of the most important laws in meteorology. Buys-Ballot, a Dutch meteorologist, first called attention to the importance of this law in Europe, and it has ever since been known by his name. Buys-Ballot’s Law is generally stated as follows:Stand with your back to the wind, and the barometer will be lower on your left hand than on your right.[4]This statement, as will be seen, covers both cyclonic and anticyclonic systems. The circulations shown on your tracings hold everywhere in the Northern Hemisphere, not only around the areas of low and high pressure seen on the United States weather maps, but around those which are found in Europe and Asia, and over the oceans as well. Mention has already been made, in the chapter on isobars (VII), of the occurrence of immense cyclonic and anticyclonic areas, covering the greater portion of a continent or an ocean, and lasting for months at a time. These great cyclones and anticyclones have the same systems of winds around them that the smaller areas, with similar characteristics, have on our weather maps. A further extension of what has just been learned will show that if in any region there comes a change from low pressure to high pressure, orvice versa, the system of winds in that region will also change. Many such changes of pressures and winds actually occur in different parts of the world, and are of great importance in controlling the climate. The best-known and the most-marked of all these changes occurs in the case ofIndia. During the winter, an anticyclonic area of high pressure is central over the continent of Asia. The winds blow out from it on all sides, thus causing general northeasterly winds over the greater portion of India. These winds are prevailingly dry and clear, and the weather during the time they blow is fine. India then has its dry season. As the summer comes on, the pressure over Asia changes, becoming low; a cyclonic area replaces the winter anticyclone, and inflowing winds take the place of the outflowing ones of the winter. The summer winds cross India from a general southwesterly direction, come from over the ocean, and are moist and rainy. India then has its rainy season. These seasonal winds are known asMonsoons, a name derived from the Arabic and meaningseasonal.

[4]In the Northern Hemisphere.

Fig. 48.

The accompanying figure (Fig. 48) is taken from thePilot Chart of the North Atlantic Ocean, published by the Hydrographic Office of the United States Navy for the use of seamen. It shows the wind circulation around the center of a cyclone which is moving northward along the Atlantic Coast of the United States. The long arrow indicates the path of movement; the shorter arrows indicate the directions of the winds. By means of such a diagram as this a captain is able to calculate, with a considerable degree of accuracy, the position of the center of the cyclone, and can often avoid the violent winds near that center by sailing away from it, or by “lying to,” as it is called, and waiting until the center passes by him at a safe distance. These cyclones which come up the eastern coast of the United States at certain seasons are usually violent, and often do considerable damage to shipping. The Weather Bureau gives all the warning possible of the coming of thesehurricanes, as they are called, by displayinghurricane signalsalong the coast, and by issuing telegraphic warnings to newspapers. In this way ship captains, knowing of the approach of gales dangerous to navigation, may keep their vessels in port until all danger is past. Millions of dollars’ worth of property and hundreds of lives have thus been saved.

CORRELATION OF THE DIRECTION OF THE WIND AND THETEMPERATURE.

It is evident, from even the most general observation of the weather elements, that the temperature experienced at any place is very largely dependent upon the direction of the wind. Thus, for instance, in the United States, a wind from some northerly point is likely to bring a lower temperature than a southerly wind. To investigate this matter more closely, and to discover how the winds at any station during any month are related to the temperatures noted at that station, we proceed as follows:—

Select the Weather Bureau station at which you wish to study these conditions. Note the direction of the wind and the temperature at that station on the first day of any month. Prepare a table similar to the following one.

Table III.—Correlation of the Direction of the Windand the Temperature.

At ..................... during the Month of ........Wind DirectionsN.NE.E.SE.S.SW.W.NW.TemperaturesSumsTotalCasesTotalMeansMean

Enter the temperature at 8A.M.on the first day of the month in a column of the table under the proper wind direction. Thus, if the wind is NE., and the temperature 42°, enter 42 in the second column of the table. Repeat the observation for the same station, and for all the other days of the month, recording the temperatures in each case in their appropriate columns in the table. Omit all cases in which the wind islight, because winds of low velocities are apt to be considerably affected by local influences. When the observations for the whole month have been entered in the table, add up all the temperatures in each column (sums). Find the mean temperature (means) observed with each wind direction by dividing the sums by the number of observations in each column (cases). Add all the sums together; divide by the total number of cases, and the result will be the mean temperature[5]for the month at the station. The general effect of the different wind directions upon the temperature is shown by a comparison of the means derived from each column with the mean for the month.

[5]Derived from the 8A.M.observations. This does not give the true mean temperature.

Fig. 49.

A graphic representation of the results of this investigation will help to emphasize the lesson. Draw, as in the accompanying figure (Fig. 49), eight lines from a central point, each line to represent one of the eight wind directions. About the central point describe a circle, the length of whose radius shall correspond to the mean temperature of the month, measured on some convenient scale. Thus, if the meantemperature of the month is 55° and a scale of half an inch is taken to correspond to 10° of temperature, the radius of the circle must be five and a half times half an inch, or 23⁄4inches. Next lay off on the eight wind lines the mean temperatures corresponding to the eight different wind directions, using the same scale (1⁄2in. = 10°) as in the previous case. Join the points thus laid off by a heavy line, as shown in Fig. 49. The figure, when completed, gives at a glance a general idea of the control exercised by the winds over the temperatures at the station selected. Where the heavy line crosses a wind lineinsidethe circle it shows that the average temperature accompanying the corresponding wind direction is below the mean. When the heavy line crosses any wind lineoutsidethe circle, it shows that the average temperature accompanying the corresponding wind direction is above the mean. Such a figure is known as awind rose.

The cold wave and the siroccoare two winds which exercise marked controls over the temperature at stations in the central and eastern United States. Thecold wavehas already been described in Chapter V. It is a characteristic feature of our winter weather. It blows down from our Northwestern States or from the Canadian Northwest, on the western side of a cyclone. It usually causes sudden and marked falls in temperature, sometimes amounting to as much as 50° in 24 hours. Thesiroccois a southerly or southwesterly wind. It also blows into a cyclone, but on its southern or southeastern side. Coming from warmer latitudes, and from over warm ocean waters, the sirocco is usually a warm wind, in marked contrast to the cold wave. In winter, in the Mississippi Valley and on the Atlantic Coast, the sirocco is usually accompanied by warm, damp, cloudy, and snowy or rainy weather. The high temperatures accompanying it (they may be as high as 50° or 60° even in midwinter) are very disagreeable. Our warm houses and our winter clothing become oppressive and we long for the bright, crisp, cold weather brought by thecold wave. In summer when a sirocco blows we have our hottest spells. Then sunstrokes and prostrations by the heat are most common, and our highest temperatures arerecorded. The wordsirocco(fromSyriacus=Syrian) was first used as the name of a warm southerly wind in Italy. The cause and the characteristics of the Italian sirocco and of the American sirocco are similar, and the name may therefore be applied to our wind as well as to the Italian one. In the Southern Hemisphere, at Buenos Ayres, Argentine Republic, there is a similar contrast between two different winds. Thepamperois similar in many respects to our cold wave. It is a dry, cool, and refreshing wind, blowing over the vast level stretches of the Argentine pampas from the southwest. Thenorteis a warm, damp, depressing northerly wind corresponding to our sirocco.

CORRELATION OF CYCLONES AND ANTICYCLONES AND THEIRTEMPERATURES.

A.Cyclones.—It follows from the two preceding exercises that some fairly definite distribution of temperature, depending upon the wind direction, should exist around areas of low and high pressure. Try to predict, on the basis of the results obtained in Chapters XII and XIII, what this relation of temperatures and cyclones and anticyclones is. Then work out the relation independently of your prediction, by studying actual cases obtained from the weather maps, as follows:—

Fig. 50.

Prepare a sheet of tracing paper as shown in Fig. 50. The diameter of the circle should be sufficiently large to include within the circle the average area covered by a cyclone on the weather maps. Place the tracing paper, properly divided in accordance with the figure, over a well-defined area of low pressure on a weather map, centering and orienting it carefully.Take the temperature at the station which lies nearest the center of the figure as the standard. Notice the temperatures at all the other stations which fall within the limits of the circle, and mark down at the proper places on the tracing paper, the + or - departures of these local temperatures from the standard temperature. Thus, if the standard is 37°, and a station has a temperature of 46°, enter +9° at the proper place on your tracing paper. Again, if a certain station has 24°, enter -13° at the proper place on the paper. Continue this process until your paper has all of its divisions well filled. It is best to select all the maps used in this investigation from the same month, for in that case the data are more comparable than if different months are taken. When a sufficient number of examples has been obtained, find the average departure (+ or -) of the temperatures in each division of the tracing from the central standard temperature. Express these averages graphically by means of awind rose, as in the last exercise.

Another Method.—The above correlation may be investigated by means of another method, as follows:—

Prepare a piece of tracing paper by drawing an N. and S. line upon it, and placing a dot at the center of the line. Lay the paper over an area of low pressure on any weather map, centering and orienting it properly, as in the previous exercises. Trace off the isotherms which are near the center of low pressure. Repeat this process with several maps, selecting different ones from those used in the first part of this exercise. Formulate a rule for the observed distribution of temperature, and determine the reasons for this distribution. Note carefully any effects of the cyclone upon the temperature gradient.

B.Anticyclones.—The correlation of anticyclones with their temperatures is studied in precisely the same way as the preceding correlation. Both methods suggested in the case of cyclones should be used in the case of anticyclones. When your results have been obtained, formulate a general rule for the observeddistribution of temperature in anticyclones, and determine the reasons for this distribution.

Find from your composites the average temperature of cyclones and of anticyclones, and compare these averages.

The unsymmetrical distribution of temperature around cyclones, which is made clear by the foregoing exercises, is very characteristic of these storms in our latitudes, and especially in the eastern United States. That this has an important effect upon weather changes is evident, and will be further noted in the chapter onWeather Forecasting. The cyclones which begin over the oceans near the equator at certain seasons, and thence travel to higher latitudes,—tropical cyclones, so called,—differ markedly from our cyclones in respect to the distribution of temperature around them. The temperatures on all sides of tropical cyclones are usually remarkably uniform, the isotherms coinciding fairly closely with the isobars. The reason for this is to be found in the remarkable uniformity of the temperature and humidity conditions over the surrounding ocean surface, from which the inflowing winds come. In the case of our own cyclones, in the eastern United States, the warm southerly wind, or sirocco, in front of the center has very different characteristics from those of the cold northwesterly wind, orcold wave, in the rear, as has become evident through the preceding exercise. These winds, therefore, naturally show their effects in the distribution of the temperatures in different parts of the cyclonic area.

CORRELATION OF THE DIRECTION OF THE WIND AND THEWEATHER.

Select a file of daily weather maps for some month. Commencing with the first map in the set, observe the weather and the direction of the wind at a considerable number of stations in the same general region (as,e.g., the Lake region, the lower Mississippi Valley, the Pacific Coast, etc.). Enter each case in a table, similar to Table IV below, by making a check in thecolumn under the appropriate wind direction and on a line with the appropriate type of weather.

Table IV.—Correlation of the Direction of the Windand the Weather.

At ________ during the Month of ________.N.NE.E.SE.S.SW.W.NW.TotalsPercentagesClearDJFairFKCloudyGLRain and SnowHMTotalsAPercentagesBCetc.

Count every observation ofrainorsnow also as cloudy, for it usually rains or snows only when the sky is cloudy. Continue your observations on all the maps for the month you have chosen. Then count up the whole number of cases ofclearweather you have found with north winds, and write down this sum in the first space, in the column reserved for N. winds. Do the same withfairandcloudyweather. Add up and enter at the bottom of the column in the space markedTotalsthe whole number of observations ofclear,fair, andcloudyweather you have observed with N. winds. Then find what percentage of the weather with N. winds wasclear, and enter this percentage next to the sum ofclearweather observations, in the first division in the column headed N. Do the same forfair,cloudy, andrainyorsnowyweather, deriving the percentages of rain or snow from the total ofclearandfairandcloudy. Repeatthis process of summarizing in every column. Your results will then show the percentages of the different kinds of weather noted with the different wind directions.

The lower division of the table and the last two columns on the right are to be used for a general summary of the whole investigation. By adding together all the totals ofclear,fair, andcloudyweather observed with all the eight wind directions you obtain the whole number of observations you have made. Enter this in the space marked A, at the right of the table. From this grand total and the total number of observations in each column you may find (in percentages) the relative frequencies of the different wind directions. These should be entered under the totals at the bottom of each column, in the spaces markedPercentages(spaces B, C, etc.). The total number of observations ofclear,fair,cloud, andrainorsnow, noted with all the wind directions, are to be entered in spaces D, F, G, and H, at the right of the table. From these totals, and from the grand total in space A, we can determine the relative frequency (in percentages) of each kind of weather during the month. These results should be entered in spaces J, K, L, and M.

Study these results carefully. Formulate them in a brief written statement. Express graphically the following:—

A.The percentages of frequency of the different wind directions during the month.

B.The percentages of the different kinds of weather noted during the whole month for all wind directions.

Fig. 51.

A wind rose, indicating the percentages of frequency of different winds during a month, ora year, or several years, may be constructed as shown in Fig. 51.

A certain convenient scale is adopted as representing a frequency of 10%, and a circle is drawn with this unit as a radius. A second circle, with a radius twice as long, represents a frequency of 20%, and a third circle, with a radius three times as long, represents a frequency of 30%. Additional circles may be added if necessary. Distances corresponding to the different percentages of frequency of the eight wind directions are then laid off along the eight radii of the circles, and the points thus fixed are joined by a line.

The results asked for under questionBmay be plotted as a weather rose on a diagram similar to that above figured. In this case the percentages of frequency of the different varieties of weather (clear,fair,cloudy,stormy) may be indicated on the same figure by using different kinds of lines. Thus, asolidline may be employed to representclearweather; abrokenline forfair; abroken and dottedline forcloudy; and adottedline forstormyweather.

CORRELATION OF CYCLONES AND ANTICYCLONES AND THEWEATHER.

A.Cyclones.—Prepare a piece of tracing paper as shown in Fig. 52, making the diameter of the outer circle about 1000 miles[6]and of the inner circle 500 miles. Place this diagram over a cyclone on any weather map, centering and orienting it carefully. Trace off the weather signs (indicatingclear,fair,cloudy,rainorsnow) around the cyclonic center from the map on to the tracing paper, taking only observations which are not more than halfway from the cyclonic center to the neighboringanticyclonic center. Repeat this process with successive weather maps until the diagram is well filled in all its different divisions.

[6]Use the scale of miles given on the weather map.

A.Draw a line on the tracing paper enclosing the average area ofcloud(includingrain and snow), and a second line enclosing the average area ofprecipitation(rainorsnow).

B.Determine the percentages ofclear,fair,cloudy, andstormyobservations for each division of the tracing paper,i.e., (a) for the eight sectors of the large circle; (b) for the whole of the small circle; and (c) for the portion of the diagram between the circumference of the inner circle and the circumference of the outer circle.

Fig. 52.

C.Write out in general terms a description of the weather distribution in cyclones as illustrated by your own investigation.

B.Anticyclones.—This exercise is done in the same way as the preceding one, except that anticyclones are substituted for cyclones.

The association of fair weather with anticyclones and of stormy weather with cyclonesis one of the most important lessons learned from a study of the weather maps. The great areas of high and low pressure control our weather. On land, where daily weather maps are so easily accessible, a glance at the map serves in most cases to give a fairly accurate idea of the position and extent of cyclones and anticyclones, and hence also of the distribution of weather. At sea, on the other hand, the navigator has no daily weather maps to refer to, and his knowledge of the weather conditions which he may expect must be gained from his own observations alone. Of these local observations, the pressure readings are by far the most important. A falling barometer usually means the approach of a cyclone, with wind, or storm, or both. A risingbarometer, on the other hand, is usually an indication of the fine weather associated with an anticyclone. The unsymmetrical distribution of weather, characteristic of our cyclones in the United States, and also of most cyclones in the Temperate Zone, is associated with their unsymmetrical form, and the unsymmetrical distribution of their temperature already studied. Tropical cyclones have a wonderfully uniform distribution of weather on all sides of their centers, just as they have a symmetrical form and an even temperature distribution all around them.

PROGRESSION OF CYCLONES AND ANTICYCLONES.

So far no definite study has been made of the changes in the positions of cyclones and anticyclones. If these areas of stormy and fair weather always occupied the same geographic positions, the different portions of the country would always have the same kinds of weather. A knowledge of the movements of the areas of low and high pressure makes weather forecasting possible.

A.Cyclones.—Select a set of daily weather maps for a month. Turn to the first map of the series. Note the position of the center of low pressure, and indicate this position on a blank weather map of the United States by marking down a small circle at the proper place. If there are two or more areas of low pressure on the map, indicate the position of each one of them in a similar way. Turn to the second map of the series, and again enter on the blank map the position of the center of low pressure. Connect the two positions of each center by a line. This line may be called thetrackof the low pressure center. Continue this process through the whole set of maps, connecting all the new positions with the last positions of their respective centers. Mark each position with the appropriate date in small, neat figures. When completed your map will show at a glance the tracks followed by all the cyclones whichtraveled across the United States during the month you selected. Study these tracks carefully. Notice whether there is any prevailing direction in which the cyclones move, and whether they show any preference for particular paths across the country. Can you frame a general rule for the prevailing direction and path of movement? Are there any cases which do not accord with the rule? If so, describe them. In what position, with reference to the cyclonic tracks during the month you are studying, is the region in which you are now living?

Next determine thevelocitieswith which these cyclones moved. Prepare a scale of latitude degrees, as described in Chapter V, or of miles, as given at the bottom of the weather map. Measure the distances, in miles, between the successive positions of all the cyclonic centers. Divide these distances by 24 in order to obtain the velocity inmiles per hour. What is the highest velocity per hour with which any cyclone moved during the month? What is the lowest? What is the mean, or average, velocity?

Study the tracks and velocities of cyclones in a similar way during several other months. Compare the positions of the tracks, and the velocities of progression, in summer and winter.

B.Anticyclones.—Study the tracks and velocities of anticyclones in precisely the same manner. Compare the results derived from your investigations in the two cases.

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