Practical Exercises in Elementary Meteorology Part 3

[Ill.u.s.tration: FIG. 9.]

Make observations with the wet-bulb thermometer or the sling psychrometer as a part of your regular daily weather record. Note the temperatures indicated by the wet and dry bulbs, and, by means of the table in Chapter XXVI, obtain the _dew-point_ and the _relative humidity_ of the air at each observation. Enter these data in your record book, in a column headed "Humidity," and subdivided into two columns, one for the dew-point and one for the relative humidity.

[Ill.u.s.tration: FIG. 10.]

By means of observations with the psychrometer you will be able to answer such questions as the following:--

Does the relative humidity vary from day to day? Has it any relation to the direction of the wind? To the state of the sky? To precipitation? Does it show any _regular_ variations during the course of a day? How does a high degree of relative humidity affect you in cold weather? In hot weather? Between what limits of percentages does the relative humidity vary? Do the changes come gradually or suddenly? Are these changes related in any way to the changes in the other weather elements? How do the sensible temperatures vary? In what weather conditions do the sensible temperatures differ most from the air temperatures? In what seasons?

Compare the sensible temperatures obtained by your own observations with the sensible temperatures at various stations of the Weather Bureau, as given on the daily weather map. Are there any fairly regular differences between the sensible temperatures observed at your own station and the Weather Bureau stations?

=Standard Mercurial Barometer.=--A simple form of barometer has been described in Chapter II. The ordinary standard mercurial barometer used by the Weather Bureau (Fig. 10) has the gla.s.s tube containing the mercury surrounded by a thin bra.s.s covering, through which openings are cut, near the top, on the front and back, exposing to view the gla.s.s tube and the top of the mercury column. On one side of this opening there is a strip of metal, graduated to inches and tenths or twentieths, by means of which the height of the barometer is determined. This strip, for barometers used at or near sea level, is about 4 inches long, the variations in pressure under normal conditions not exceeding that amount. In addition to this fixed scale there is a small scale, also graduated, which may be moved up and down the opening in the enclosing bra.s.s case by means of a milled head outside and a small rack and pinion inside the bra.s.s case. This movable scale, known as the _vernier_ from the name of its inventor, Vernier, is an ingenious device, by means of which more accurate readings of the barometer can be made than is possible with the ordinary fixed scale. A _vernier_ graduated into twenty-five parts enables the observer to make readings accurately to the one-thousandth part of an inch. On the front of the barometer there is a small thermometer, known as the _attached thermometer_. The bulb of this thermometer, concealed within the metal casing of the barometer, is nearly in contact with the gla.s.s tube containing the mercury. The air, upon whose weight the height of the mercury column depends, gains access to the top of the cistern through leather joints, by which the cistern is joined to the gla.s.s tube.

Mercurial barometers of the Weather Bureau pattern are best hung in a barometer box, fastened securely against the wall of a room, where there is a good light on the instrument and where the temperature is as constant as possible.

In all accurate work certain corrections have to be applied to barometer readings to make them strictly comparable. These are: (1) _correction for alt.i.tude_; (2) _correction for temperature_; and (3) _correction for lat.i.tude_. The first is necessary because of the fact that the weight of the air decreases upwards, and a barometer reading on a hill or a mountain is not comparable with one at sea level unless the former has been corrected by the addition of the weight of the column of air between the hill or mountain and sea level. The correction for temperature is rendered necessary by the fact that with increasing temperature the mercury in the barometer tube expands more than the metallic scale, because mercury is more sensitive to heat, and unless some allowance is made for this fact, barometer readings made at high temperatures will show somewhat too high a pressure. The readings of the attached thermometer give the temperature of the mercury and are used in making the corrections for temperature. As gravity varies from a maximum value at the poles to a minimum value at the equator, barometer readings made at different lat.i.tudes are _corrected for lat.i.tude_, which means that they are reduced to lat.i.tude 45, midway between 0 and 90. The correction is +0.08? at the poles and -0.08? at the equator. Tables for use in correcting barometer readings for alt.i.tude and for temperature are given in Chapter XXVI.

[Ill.u.s.tration: FIG. 11.]

=Thermograph and Barograph.=--Two instruments of much interest are the self-recording thermometer, or _thermograph_, and the self-recording barometer, or _barograph_, manufactured by Richard Brothers of Paris. In the thermograph (Fig. 11) there is a bra.s.s cylinder around which a sheet of paper is wound, this paper being divided into two-hour intervals of time and into s.p.a.ces representing differences of 5 or 10 of temperature.

The cylinder revolves once in a week, being driven by clock-work contained within it. The thermometer consists of a flat, bent, hollow bra.s.s tube containing alcohol, one end of the tube being fastened to the metallic frame seen at the right of the figure, and the other end being free to move. With rising temperature, the liquid in the tube expanding more than the metallic casing, by reason of its greater sensitiveness to heat, tends to straighten the tube, while with falling temperature the elasticity of the tube turns it into a sharper curve. These movements of the free end of the tube are carried through a train of levers and thus magnified. At the end of the last lever is a metallic pen filled with ink, which rests lightly against the paper on the revolving drum. A rise or fall in temperature is thus recorded by a rise or fall of the pen on the record sheet, and a continuous curve of temperature is secured. The pen of the thermograph should be frequently adjusted to make the reading of the instrument accord with that of a standard mercurial thermometer, and care should be taken to have the clock keep good time. These adjustments can readily be made by means of a screw and a regulator, respectively. The thermograph should be exposed in the instrument shelter with the other thermometers. The sheets should be changed, the clock wound, and the pen filled once a week, preferably on Monday, at 8 A.M., or at noon.

The continuous records written by a thermograph are a valuable addition to the fragmentary observations which are the result of eye readings of the ordinary thermometer. From the former any omitted thermometer readings may be supplied. The interest of thermograph records may be seen in the following figure (Fig. 12), in which curves traced under different conditions are reproduced. Curve _a_ ill.u.s.trates a period of clear warming weather at Nashua, N. H., April 27-30, 1889. Curve _b_ was traced during a spell of cloudy weather at Nashua, accompanying the pa.s.sage of a West India hurricane, Sept. 13-16, 1889. Curve _c_ ill.u.s.trates the change from a time of moderate winter weather to a cold spell (Nashua, Feb.

22-25, 1889). Curve _d_ exhibits a steady fall of temperature from the night of one day over the next noon to the following night, during the approach of a winter cold spell (Nashua, Jan. 19-21, 1889). Curve _e_ shows a reverse condition, viz., a continuous rise of temperature through a night from noon to noon (Nashua, Dec. 16-17, 1888). Curve _f_ shows the occurrence of a high temperature at night, caused by warm southerly winds, followed by cold westerly winds (Cambridge, Ma.s.s., Nov. 30-Dec. 1, 1890).

Curve _g_ ill.u.s.trates the sudden rise of temperature due to the coming of a hot, dry wind (chinook) at Fort a.s.siniboine, Mont. (Jan. 19, 1892). A study of such records leads to the discovery of many important facts, which would be completely lost sight of without a continuous record.

[Ill.u.s.tration: FIG. 12.]

The =barograph= (Fig. 13) is very similar to the thermograph in general appearance. The essential portion of this instrument consists of a series of six or eight hollow sh.e.l.ls of corrugated metal screwed one over the other in a vertical column. These sh.e.l.ls are exhausted of air, and form, in reality, an aneroid barometer which is six or eight times as sensitive as the ordinary single-chamber aneroid. The springs for distending the sh.e.l.ls are inside. The base of the column being fixed, the upper end rises and falls with the variations in pressure. The movements of the sh.e.l.ls are magnified by being carried through a series of levers, and, as in the thermograph, the motion is finally given to a pen at the end of the long lever. The compensation for temperature is the same as in the ordinary aneroid. A small quant.i.ty of air is left in one of the sh.e.l.ls to counteract, by its own expansion at increased temperature, the tendency of the barometer to register too low on account of the weakening of the springs. The barograph may be placed upon a shelf in the schoolroom, where it can remain free from disturbance, and yet where the record may be clearly seen. The general care of the barograph is the same as that of the thermograph. Brief instructions concerning the care and adjustments of these instruments are sent out by the makers with each instrument.

Frequent comparison with a mercurial barometer is necessary, the adjustment of the barograph being made by turning a screw, underneath the column of sh.e.l.ls, on the lower side of the wooden case.

[Ill.u.s.tration: FIG. 13.]

Barograph records are fully as interesting as those made by the thermograph. The week's record traced on the writer's barograph during a winter voyage from Punta Arenas, Strait of Magellan, to Corral, Chile, Aug. 2-9, 1897, gives a striking picture of the rapid and marked changes of pressure during seven days in the South Pacific Ocean (Fig. 14).

[Ill.u.s.tration: FIG. 14.]

The following figure (Fig. 15) presents samples of barograph curves traced at Harvard College Observatory, Cambridge, Ma.s.s., during Feb. 22-28, 1887, and May 17-23, 1887. The February curve ill.u.s.trates well the large and irregular fluctuations in pressure, characteristic of our winter months; while the May curve shows clearly the more even quality of the pressure changes in our summer.

The =anemometer= shown in Fig. 16 is the most generally used of instruments designed to measure wind velocity. It is known as the Robinson cup anemometer, and consists of four hollow hemispherical cups upon arms crossed at right angles, and all facing the same way around the circle.

The cross-arms are fixed upon a vertical axis having an endless screw at its lower end. When the cups move around, the endless screw turns two dials which register the number of miles traveled by the wind. The Weather Bureau pattern of anemometer has the dials mounted concentrically, the outer dial having 100, and the inner, 99 divisions. The revolutions of the outer dial are recorded on the inner one, and in making an observation of the number of miles traveled by the wind, the hundreds and tens of miles are taken from the inner dial, and the miles and tenths from the outer one. Take from the inner scale the hundreds and tens of miles contained between the zero of that scale and the zero of the outer one.

Take on the outer scale the miles and tenths of miles contained between the zero of that scale and the index point of the instrument. The sum of these readings is the reading of the instrument at the time of the observation.

[Ill.u.s.tration: FIG. 15.]

[Ill.u.s.tration: FIG. 16.]

Wind velocities are recorded in miles per hour. The velocity of the wind at any particular moment is found by noting the number of miles and tenths of miles recorded by the index before and after an interval of one minute, or of five minutes, and multiplying this rate by 60 or by 12 as the case may be. This gives the number of miles an hour that the wind is blowing at the time of observation.

Records of wind velocity (in miles per hour) are to be made at each regular observation hour, and are to be entered in the proper column of the table in your record book. The total wind movement in each 24 hours is to be observed once a day, always at the same hour, and is to be entered in its proper column in the record book.

The total wind movement for 24 hours is obtained as follows: Subtract the reading of the anemometer at 12 noon (or 8 A.M., or any other hour) of the preceding day from the reading taken at 12 noon or the corresponding hour of the current day, and the difference will be the total movement of the wind. When the reading of the anemometer is less than the reading of the preceding day, 990 miles should be added to it; and the remainder, after subtracting the reading of the preceding day, will be the total wind movement for the 24 hours. Thus: To-day's reading = 91 miles; yesterday's reading = 950 miles. Hence 91 + 990 = 1081 miles, 1081 - 950 = 131 miles = total wind movement for the current day.

By means of an electrical attachment the anemometer may be arranged so as to record continuously on a cylinder rotating by clock-work, a pen making a mark on the paper for every mile traveled by the wind. The anemometer should be exposed on top of a building where there is as little obstruction as possible by tall chimneys, higher buildings, and the like.

The =nephoscope= (Greek: _cloud observer_) is an instrument used in determining the directions of movement of clouds. These directions, if determined by ordinary eye observation of the clouds as they drift across the sky, are apt to be quite inaccurate. The best method of observing directions of cloud movement is to note the path of the reflection of the cloud in a horizontal mirror, the observer looking at this reflection through an eyepiece which remains fixed during the operation. Such a horizontal mirror, adapted to measure the direction of motion of clouds, is known as a _nephoscope_. A form of nephoscope devised by Mr. H. H.

Clayton, of Blue Hill Observatory, Hyde Park, Ma.s.s., is shown in Fig. 17.

[Ill.u.s.tration: FIG. 17.]

This instrument consists of a circular mirror, 13 inches in diameter, sunk in a narrow circular wooden frame, on top of which is fastened a bra.s.s circle, S.W.N.E., divided to 5 of arc. Inside of this fixed circle is a movable bra.s.s one, to which is attached a bra.s.s arc, _BD_, rising above the mirror and bearing a movable eyepiece, _C_. This arc forms the quadrant of a circle whose center is the center of the mirror, and is divided to 5 of arc. Its top is held vertically over the center of the mirror by two rods fastened to the movable circle. The center of the mirror _A_ is marked by cross lines on the reflecting surface, the gla.s.s of which is thin. In order to determine the motion of a cloud, the movable circle and tripod are revolved until the arc _BD_ is in the vertical plane formed by the cloud, the center of the mirror, and the eye. The eyepiece _C_ is then s.h.i.+fted until some point of the cloud image, as seen through the eyepiece, is projected on the intersection of the cross lines on the gla.s.s. The cloud image soon changes its position, and while the eye is still held at the eyepiece, a small index is placed on the part of the cloud image which previously appeared on the center of the mirror. If now a ruler be placed on the index and the center of the mirror and extended backward, its intersection with the divided scale will give the direction from which the cloud came to the nearest degree, if all the measurements have been accurately made. The height of the cloud above the horizon is found by reading the position of the eyepiece on the divided quadrant.

The nephoscope may be placed on a table, out of doors in fine weather, or close to a window from which the clouds to be observed can be seen. The instrument must be properly oriented, so that the four points marked N., E., S., and W. on the frame shall correspond to the four chief compa.s.s directions. The zero (0) of the movable bra.s.s scale is usually put at the S. Hence, if a cloud is found moving from exactly SW., the angular measurement of its direction of motion will be 45. If a cloud is moving from due E., the angular measurement of its direction of motion will be 270.

When the sky is completely overcast with a uniform layer of cloud, it is usually impossible to determine any direction of movement, because of the difficulty of selecting and keeping in view, on the mirror, some particular point of cloud.

Observations with the nephoscope may be made as often as is desired, and should be entered in an appropriate column in the record book.

=Tabulation of Observations.=--A convenient form of table which may be used in the complete instrumental observations is given on the next page.

The number of columns and their arrangement may, of course, be varied to suit the number and the nature of the records.

=Summary of Observations.=--In the preceding chapter we have seen how to obtain the mean monthly temperature from the daily observations, the frequency of the different wind directions for each month, and the total monthly precipitation. The addition of the new instruments, the maximum and minimum thermometers, the psychrometer, the anemometer, and the nephoscope, enables us to obtain the following additional data in our monthly summaries.

_Temperature._--The _mean monthly temperature_ may be obtained from the maximum and minimum temperatures as follows: Add together all the daily maximum and minimum temperatures for a month. Divide this sum by the total number of readings you have made of each thermometer (_i.e._, one reading of the maximum and one of the minimum each day, making two readings a day), and the result will be the _mean monthly temperature_ derived from the maximum and minimum temperature. This is a more accurate mean temperature than the one noted in the summary of the preceding chapter.

Add together all the maximum temperatures noted during one month. Divide this sum by the number of observations, and the result gives the _mean maximum temperature_ for the month.

TABLE FOR METEOROLOGICAL RECORD.

DATE. +--------------------------------------+--------------------------------+ HOUR. +--------------------------------------+--------------------------------+ PRESSURE (in inches). +--------------------------------------+----------------+---------------+ DRY BULB. +--------------------------------------+----------------+ WET BULB. TEMPERATURE. +--------------------------------------+----------------+ MAX. +--------------------------------------+----------------+ MIN. +--------------------------------------+----------------+---------------+ DEW-POINT. +--------------------------------------+----------------+ RELATIVE HUMIDITY. HUMIDITY. +--------------------------------------+----------------+---------------+ DIRECTION. +--------------------------------------+----------------+ VELOCITY WIND. (miles per hr.). +--------------------------------------+----------------+ TOTAL MILES PER DAY. +--------------------------------------+----------------+---------------+ KIND. +--------------------------------------+----------------+ AMOUNT (in tenths). +--------------------------------------+----------------+ DIRECTION OF CLOUDS. MOVEMENT. +--------------------------------------+----------------+ ANGULAR ALt.i.tUDE +--------------------------------------+----------------+---------------+ TIME OF BEGINNING AND ENDING. +--------------------------------------+----------------+ KIND. PRECIPITATION. +--------------------------------------+----------------+ AMOUNT. +--------------------------------------+----------------+---------------+ REMARKS.

A similar operation applied to the minimum temperatures gives the _mean minimum temperature_ for the month.

In meteorological summaries it is customary also to include the _absolute maximum_ and the _absolute minimum_ temperatures, _i.e._, the highest and lowest single readings of the thermometer made during each month. These can easily be determined by simple inspection of your record book. Note also the dates on which the absolute maximum and the absolute minimum occurred.

The _absolute monthly range_ of temperature is the difference, in degrees, between the absolute maximum and the absolute minimum.

_Humidity._--The _mean relative_ humidity is obtained by adding together all the different percentages of relative humidity obtained during the month, and dividing this sum by the whole number of observations of this weather element.

_Wind._--The _mean velocity_ of the wind corresponding to the different wind directions is readily obtained by adding together all the different velocities (in miles per hour) observed in winds from the different directions, and dividing these sums by the number of cases. The wind summaries will thus give the frequency of the different directions during each month, and the corresponding mean velocities.

The _maximum hourly wind velocity_ is obtained by inspection of the velocity column.

The _total monthly wind movement_ is readily deduced from the daily records in the twelfth column of the table on p. 44.

_State of the Sky._--In connection with the more advanced records described in this chapter, the observations of cloudiness should record the number of _tenths_ of the sky cloudy, as closely as the amount can be estimated by eye, instead of indicating the state of the sky as _cloudy_, _fair_, etc. A detailed record of cloudiness in tenths gives opportunity to determine the _mean cloudiness_ for each month, by averaging, as in the case of the other means already described.

If nephoscope observations are made, the monthly summary may include the _mean direction of cloud movement_ for each month. This is obtained by adding together all the different angular measurements of directions of cloud movement, and dividing by the whole number of such observations.

By means of your monthly summaries compare one month with another. Notice how the means and the extremes of the different weather elements are related; how they vary from month to month. Are there any _progressive_ changes in temperature, cloudiness, precipitation, etc., from month to month? What are the changes? Summarize, in a short written statement, the meteorological characteristics of each month as shown by your tables.