Practical Exercises in Elementary Meteorology Part 7

Proceed similarly with the weather on the five remaining days, as noted in the table. Enter the weather symbols for each day on a separate blank map, enclosing and shading or coloring the areas of cloud and of snow as above suggested. In Figs. 40-45 the cloudy areas are indicated by single-line shading, and the snowy areas by double-line shading.

Now study carefully each weather chart with its corresponding temperature, wind, and pressure charts. Note whatever relations you can discover among the various meteorological elements on each day. Then compare the weather conditions on the successive maps. What changes do you note? How are these changes related to the changes of temperature; of wind; of pressure?

Write a summary of the results derived from your study of these four sets of charts.

[Ill.u.s.tration: FIG. 40.--Weather. First Day.]

[Ill.u.s.tration: FIG. 41.--Weather. Second Day.]

[Ill.u.s.tration: FIG. 42.--Weather. Third Day.]

[Ill.u.s.tration: FIG. 43.--Weather. Fourth Day.]

[Ill.u.s.tration: FIG. 44.--Weather. Fifth Day.]

[Ill.u.s.tration: FIG. 45.--Weather. Sixth Day.]

=The Weather of Temperate and Torrid Zones.=--The facts of the presence of clear weather in one region while snow is falling in another, and of the variability of our weather from day to day in different parts of the United States, are emphasized by these charts of weather conditions. This changeableness of weather is a marked characteristic of the greater portion of the Temperate Zones, especially in winter. The weather maps for successive days do not, as a rule, show a repet.i.tion of the same conditions over extended regions. In the Torrid Zone it is different. Over the greater part of that zone the regularity of the weather conditions is such that, day after day, for weeks and months, the same features are repeated. There monotony, here variety, is the dominant characteristic of the weather.

PART IV.--THE CORRELATIONS OF THE WEATHER ELEMENTS AND WEATHER FORECASTING.

CHAPTER IX.

CORRELATION OF THE DIRECTION OF THE WIND AND THE PRESSURE.

The study of the series of weather maps in Chapters V-VIII has made it clear that some fairly definite relation exists between the general flow of the winds and the distribution of pressure. We now wish to obtain some more definite result as to the relation of the direction of the wind and the pressure. In doing this it is convenient to refer the wind direction to the _barometric_ or _pressure gradient_ at the station at which the observation is made. The barometric gradient, it will be remembered, is the line along which there is the most rapid change of pressure, and lies at right angles to the isobars (Chapter VII).

[Ill.u.s.tration: FIG. 46.]

Take a small piece of tracing paper, about 3 inches square, and draw upon it a diagram similar to the one here shown. Select the station (between two isobars on any weather map) at which you intend to make your observation. Place the center of the tracing paper diagram over the station, with the dotted line along the barometric gradient, the minus end of the line being towards the area of low pressure. Observe into which of the four sectors (marked _right_, _left_, _with_, _against_) the wind arrow at the station points. Keep a record of the observation. Repeat the observation at least 100 times, using different stations, on the same map or on different maps. Tabulate your results according to the following scheme, noting in the first column the date of the map, in the second, third, fourth, and fifth columns the number of winds found blowing _with_, to the _right_ or _left_ of, and _against_, the gradient.

TABLE I.--CORRELATION OF THE DIRECTION OF THE WIND AND THE PRESSURE.

+------------+--------+---------+--------+---------+ DATES WITH RIGHT LEFT AGAINST +------------+--------+---------+--------+---------+ +------------+--------+---------+--------+---------+ Sums +------------+--------+---------+--------+---------+ Percentages +------------+--------+---------+--------+---------+

At the bottom of each column write down the number of cases in that column, and then determine the percentages which these cases are of the total number of observations. This is done by dividing the number of cases in each column by the sum-total of all the observations. When you have obtained the percentage of each kind of wind direction, you have a numerical result.

A graphical presentation of the results may be made by laying off radii corresponding in position to those which divide the sectors in Fig. 46, and whose lengths are proportionate to the percentages of the different wind directions in the table. Thus, for a percentage of 20, the radii may be made 1 inch long, for 40%, 2 inches, etc. When completed, the relative sizes of the sectors will show the relative frequencies of winds blowing in the four different directions with reference to the gradient, as is indicated in Fig. 47.

=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 as _Ferrel'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 the _Northeast 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 become _northeast_ winds, 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 the _Prevailing 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.

[Ill.u.s.tration: FIG. 47.]

CHAPTER X.

CORRELATION OF THE VELOCITY OF THE WIND AND THE PRESSURE.

Prepare a scale of lat.i.tude 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 the operation 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 onsh.o.r.e or offsh.o.r.e 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:--

_A._ Interior stations.

_B._ Coast stations with onsh.o.r.e winds.

_C._ Coast stations with offsh.o.r.e winds.

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

TABLE II.--CORRELATION OF WIND VELOCITY AND BAROMETRIC GRADIENT.

For interior (or coast) stations, with onsh.o.r.e (or offsh.o.r.e) winds, in the United States during the month (or months) of

+--------------------+-------+-------+-------+--------+--------+---------+------+ Rates of Pressure Change per Lat.i.tude 8-20 20-10 10-5 5- 3-1/2- 2-1/2 etc. Degree 3-1/2 2-1/2 -2 +--------------------+-------+-------+-------+--------+--------+---------+------+ Distances between Isobars in Lat.i.tude 0-1/2 1/2-1 1-2 2-3 3-4 4-5 etc. Degrees +--------------------+-------+-------+-------+--------+--------+---------+------+ +--------------------+-------+-------+-------+--------+--------+---------+------+ Wind Velocities (miles per hour) +--------------------+-------+-------+-------+--------+--------+---------+------+ Sums +--------------------+-------+-------+-------+--------+--------+---------+------+ Cases +--------------------+-------+-------+-------+--------+--------+---------+------+ Means +--------------------+-------+-------+-------+--------+--------+---------+------+

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 3-1/2 (_i.e._, .03 inch in one lat.i.tude 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 3-1/3, then enter the corresponding wind velocity in both those columns.

In the s.p.a.ce marked _Sums_ write the sum-total of all the wind velocities in each column. The _Cases_ are the number of separate observations you have in each column. The _Means_ denote 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 gradient= is 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, Ma.s.s., show that the whole atmosphere, up to the highest cloud level, moves almost twice as fast in winter as in summer. In the higher lat.i.tudes 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 lat.i.tude of 30 S., blow with high velocities nearly 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.

CHAPTER XI.

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 letter _N_ at one end, and the letter _S_ at 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 the _low_, and the line parallel to the nearest meridian, the end marked _N_ being towards the top of the map. When thus placed, the paper is said to be _oriented_. 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 a _composite portrait_ of the isobars around several areas of low pressure. Now study the 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 average _form_, _dimensions_ [measured by a scale of miles or of lat.i.tude degrees (70 miles = 1 degree about)] and _gradients_ of 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.