Practical Exercises in Elementary Meteorology Part 9

TABLE IV.--CORRELATION OF THE DIRECTION OF THE WIND AND THE WEATHER.

At ________ during the Month of ________.

+-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ N. NE. E. SE. S. SW. W. NW. TOTALS PERCENTAGES +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ Clear D J +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ Fair F K +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ Cloudy G L +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ Rain and Snow H M +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ Totals A +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+ Percentages B C etc. +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+

Count every observation of _rain_ or _snow 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 of _clear_ weather you have found with north winds, and write down this sum in the first s.p.a.ce, in the column reserved for N. winds. Do the same with _fair_ and _cloudy_ weather. Add up and enter at the bottom of the column in the s.p.a.ce marked _Totals_ the whole number of observations of _clear_, _fair_, and _cloudy_ weather you have observed with N. winds. Then find what percentage of the weather with N. winds was _clear_, and enter this percentage next to the sum of _clear_ weather observations, in the first division in the column headed N. Do the same for _fair_, _cloudy_, and _rainy_ or _snowy_ weather, deriving the percentages of rain or snow from the total of _clear_ and _fair_ and _cloudy_. Repeat this 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 of _clear_, _fair_, and _cloudy_ weather observed with all the eight wind directions you obtain the whole number of observations you have made. Enter this in the s.p.a.ce 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 s.p.a.ces marked _Percentages_ (s.p.a.ces B, C, etc.). The total number of observations of _clear_, _fair_, _cloud_, and _rain_ or _snow_, noted with all the wind directions, are to be entered in s.p.a.ces D, F, G, and H, at the right of the table. From these totals, and from the grand total in s.p.a.ce A, we can determine the relative frequency (in percentages) of each kind of weather during the month. These results should be entered in s.p.a.ces 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.

[Ill.u.s.tration: FIG. 51.]

A wind rose, indicating the percentages of frequency of different winds during a month, or a 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 question _B_ may 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, a _solid_ line may be employed to represent _clear_ weather; a _broken_ line for _fair_; a _broken and dotted_ line for _cloudy_; and a _dotted_ line for _stormy_ weather.

CHAPTER XVI.

CORRELATION OF CYCLONES AND ANTICYCLONES AND THE WEATHER.

_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 (indicating _clear_, _fair_, _cloudy_, _rain_ or _snow_) 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 neighboring anticyclonic center. Repeat this process with successive weather maps until the diagram is well filled in all its different divisions.

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

_A._ Draw a line on the tracing paper enclosing the average area of _cloud_ (including _rain and snow_), and a second line enclosing the average area of _precipitation_ (_rain_ or _snow_).

_B._ Determine the percentages of _clear_, _fair_, _cloudy_, and _stormy_ observations 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 circ.u.mference of the inner circle and the circ.u.mference of the outer circle.

[Ill.u.s.tration: FIG. 52.]

_C._ Write out in general terms a description of the weather distribution in cyclones as ill.u.s.trated by your own investigation.

_B._ =Anticyclones.=--This exercise is done in the same way as the preceding one, except that anticyclones are subst.i.tuted for cyclones.

=The a.s.sociation of fair weather with anticyclones and of stormy weather with cyclones= is 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 rising barometer, on the other hand, is usually an indication of the fine weather a.s.sociated 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 a.s.sociated 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.

CHAPTER XVII.

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 the _track_ of 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 which traveled 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 the _velocities_ with which these cyclones moved. Prepare a scale of lat.i.tude 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 in _miles 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.

=Cyclonic Tracks and the Prevailing Westerly Winds.=--The correspondence in the direction of movement of most of our cyclones and of the prevailing westerly winds of the Northern Hemisphere (see Chapter IX) will readily be noted. Our weather maps show us the atmospheric conditions over the United States alone, and we can therefore trace the progression of our cyclones over but a limited area of the Northern Hemisphere. An examination of the daily weather maps of the North Atlantic and North Pacific Oceans, which are based on observations made on board s.h.i.+ps, and of the weather maps of other countries, shows us that these atmospheric disturbances which appear on our maps may often be traced for long distances across oceans and lands, and that they in reality form a great procession across the northern portion of this hemisphere, and towards and around the North Pole.

=The average velocity of cyclones in the United States= was carefully determined by Loomis. His observations show that the mean hourly velocity of cyclones for the entire year is 28.4 miles, the maximum (34.2 miles) coming in February, and the minimum (22.6 miles) in August. Over the North Atlantic Ocean the hourly velocity is 18 miles; in Europe, 16.7 miles. The greater velocity of cyclonic movement in the United States in winter recalls what was said at the close of Chapter X concerning the steeper barometric gradient and the more rapid movement of the whole atmosphere over the Northern Hemisphere in winter.

CHAPTER XVIII.

SEQUENCE OF LOCAL WEATHER CHANGES.

The next, and last, step in our study of the correlation of the various weather elements concerns the sequence of weather changes at a station before, during, and after the pa.s.sage of a cyclone and of an anticyclone.

_A._ =Cyclones.=--I. Select some station which the weather maps show to have been directly on the track of a well-developed cyclone, _i.e._, to have been pa.s.sed over by the center of the cyclone. Note the weather conditions at this station, before, during, and after the pa.s.sage of the storm. Tabulate your observations according to the following scheme:--

TABLE V.

Weather Changes at ________________________ during ______________.

+-------+----------+---------+-----------+----------+---------+----------------+ DATES PRESSURE TEMPER- WIND WIND WEATHER DIREC. AND DIST. ATURE DIRECTION VELOCITY OF STORM CENTER +-------+----------+---------+-----------+----------+---------+----------------+ +-------+----------+---------+-----------+----------+---------+----------------+

In the last column of the table enter the direction and the distance of the cyclonic center from the station, at each observation.

II. Select a station which was north of the track of a cyclone, and tabulate (in a separate table) the weather conditions at that station before, during, and after the pa.s.sage of the center.

III. Do the same for a station which was south of the track of a cyclone.

Repeat these observations for several stations.

_B._ =Anticyclones.=--Make a similar series of observations for the pa.s.sage of an anticyclone centrally over, north and south of, several stations.

Study the sequence of the weather changes shown in the various tables.

Deduce a general rule for these changes and write it out.