CHAPTER XXXV.

METEOROLOGICAL OBSERVATIONS.

The Royal Meteorological Society recognises stations for the making and recording of observations of three kinds: (1) _Second Order Stations_, at which observations are taken twice daily at 9 a.m. and 9 p.m.; (2) _Climatological Stations_, at which the observations are taken once daily, at 9 a.m.; (3) Stations at which _one or more elements only_, _e.g._ rainfall, _are observed_. All instruments used should have been previously verified at Kew Observatory, so that the corrections for index error may be known.

The =Barometer= used should be of a standard kind. Five chief kinds of barometer are in use, only the last two of which are sufficiently accurate for scientific purposes.

1. _The Dial or wheel barometer_ consists of a bent tube A B, the open end of which supports an ivory float B. This, as it rises and falls with the mercury, by means of the rack C turns a wheel, in the axle of which a needle is fixed. The needle turns in one direction, or the other as the mercury rises or falls (Fig. 45); the dial is divided by comparing it with a standard barometer. As the ordinary variations of the barometer are from 28 to 31 inches, the circumference of the wheel is made exactly 1½ inches, and thus the float B will rise or fall 1½ inches for a rise or fall of 3 inches in the barometer.

2. The ordinary _syphon barometer_ (Fig. 46) consists of a bent tube attached to a piece of wood, and furnished with a screw _v_. The atmospheric pressure acts on the mercury at _d_, and the difference between the level of the mercury in the two arms of the syphon is the height of the barometer. To find the true height of the barometer the screw is turned till the shorter column stands at _a_ opposite zero.

3. The _aneroid barometer_ is made by exhausting the air from a small round metal box. This box is closed by a flexible lid of metal which, being elastic, yields to changes in the atmospheric pressure. To the lower end of the lid a spring is attached which runs downwards to the floor of the box and resists the atmospheric pressure. The movements thus produced by variations in pressure are magnified by a rack and pinion, and so communicated to a long index which moves over a graduated scale.

The standard Kew and Fortin barometers are both cistern barometers, the mercury in the inverted tube communicating with the mercury in a cistern below.

4. The _Kew pattern barometer_ has a closed cistern below, the area of which being accurately known, the inches on the scale are not real inches, but inches of pressure, _i.e._ inches so shortened as to compensate for the rise of the mercury in the cistern. This compensation is necessary inasmuch as changes in atmospheric pressure affect the level of the mercury in the cistern as well as of that in the tube.

5. In the _Fortin barometer_ (Fig. 47) the cistern has a pliable base of leather, which can be raised or lowered by means of a screw. The upper part of the cistern is made of glass, a piece of ivory indicating the zero of the scale. Before taking a reading, the level of the mercury must always be set exactly to this point by means of the screw. The Fortin is the most sensitive form of barometer, and the adjustment required in order to take a reading is easily performed.

To ensure more accurate reading of the barometer, a secondary scale or =vernier= is used, which slides upon the principal scale. This vernier is so graduated that 25 of its divisions correspond to 24 of the divisions on the fixed scale. The fixed scale is divided into inches, tenths (·1), and half-tenths (·05). Each division of the movable scale or vernier is therefore shorter than each division of the scale by 1∕25 of ·05, _i.e._ ·002 inch. Consequently the vernier shows differences of two thousands of an inch.

_Method of reading Fortin’s Barometer._—First note the reading of the attached thermometer; next turn the screw at the bottom of the cistern, so that the ivory point just touches the surface of the mercury. Next adjust the vernier by means of the rack and pinion at the side of the barometer (Fig. 47) so as to bring its two lower edges exactly on a level with the convex surface of the mercury. In reading the barometer, first read off the division next below the lower edge of the vernier. In Fig. 48 this is 29·05. Then the true reading is 29·05 _plus_ the vernier indication. Next look along the vernier until one of its lines is found to agree with a line on the scale. In Fig. 48 this is at the fourth division on the vernier. But each of the figures marked on the vernier counts as 1∕100 (·01), and each intermediate division as 2∕1000 (·002); hence the reading of the vernier will be ·008 inch, and the reading of the barometer 29·05 + ·008 = 29·058 inch. If two lines on the vernier are in equally near agreement with two on the scale, the intermediate value should be adopted.

Certain _corrections_ are required in the actual reading for (1) index error; (2) temperature; and (3) height above sea-level.

The _index error_ is found by comparison with a recognised standard at Kew. Correction for _temperature_ is required. Every barometer has a thermometer attached, and the reading is reduced to the standard temperature of 32° F, by means of tables such as are given on page 32 of Marriott’s _Hints to Meteorological Observers_.

The height of the _cistern_ of the barometer above _sea-level_ should always be exactly obtained.

The correction necessary to reduce observations to sea-level (_i.e._ mean half-tide level at Liverpool), depends on the temperature and pressure of the air, as well as on the altitude. The data for this correction are given in Table III. of Marriott’s _Hints_.

=Thermometers.=—The =maximum thermometer= may be on Negretti and Zambra’s, or on Phillips’ principle. In the former (Fig. 49) the bore of the tube is reduced in section near the bulb (A) in such a way that while the expanding mercury forces its way into the tube, the column of mercury breaks off on contraction, so that its upper limit shows the highest temperature that has been reached. The thermometer is set by holding it bulb downwards and shaking to make the mercurial column continuous. It is mounted in the screen horizontally (Fig. 51).

The =minimum thermometer= chiefly used is Rutherford’s. It contains spirit in which is an immersed index (A, Fig. 50). With a falling temperature the spirit draws the index along with it; but on rising again, the spirit passes the index, leaving it at the lowest point to which it has been drawn. Thus the end farthest from the bulb registers the minimum temperature. The instrument is set by raising the bulb and allowing the index to slide to the end of the column of spirit. The thermometer must be firmly fixed and mounted quite horizontally.

=Thermometer Screen.=—The above thermometers, as well as the dry and wet bulb thermometers are mounted in a Stevenson’s screen (Fig. 51). This is a doubled-louvred box through which the air can pass freely, but the sun cannot enter. The horizontal position of the maximum and minimum and the vertical position of the dry and wet bulb thermometers are shown in Fig. 51.

Three additional thermometers are usually included in a well-organised meteorological station.

A =minimum thermometer= placed on the grass gives the lowest temperature on the grass, which is often considerably lower than that of the neighbouring gravel walk. This record is chiefly useful for agricultural purposes.

The =earth thermometer= chiefly used is shown in Fig. 52. It consists of a sluggish thermometer mounted in a short weighted stick attached to a strong chain, and of a stout iron pipe which is drawn out at the bottom to a point and driven into the earth, usually to a depth of 4 feet.

=Solar radiation= is measured by black-bulb and light-bulb thermometers in _vacuo_, which are mounted on a post 4 feet above the ground and record the maximum temperature.

=Humidity= in the air is measured by direct or indirect hygrometers. Of the former Dines’, Daniell’s, and Regnault’s are the best known, but as they are not used in observations acknowledged by the Royal Meteorological Society, the reader may be referred to their description in books on physics. The indirect hygrometer which is universally employed in this country is that furnished by the =dry and wet bulb thermometers=. In frosty weather they require much attention, and then a Saussure’s hair hygrometer may be used as supplementary. The general arrangement of the dry and wet bulb thermometers is shown in Fig. 53.

The wet bulb is covered with a single layer of soft muslin, while a noose of six to eight strands of darning cotton connects the neck of the wet bulb with a covered water receptacle 2 to 3 inches distant, below and at its side. This receptacle is kept filled with rain-water.

From the readings of the dry and wet bulb thermometers three deductions can be made:

1. The temperature of the dew point.

2. The elastic force of aqueous vapour.

3. The relative humidity.

The =dew point temperature= is that temperature at which the outside air at the time the observation is taken will deposit the moisture contained in it. It is the temperature at which the air is saturated with moisture. It is calculated from the readings of the wet and dry bulb thermometers

(_a_) by Glaisher’s tables; (_b_) by Apjohn’s formula.

Glaisher’s tables are based on a series of numbers called Greenwich or Glaisher’s factors, which he determined by comparison between observations made with the dry and wet bulb thermometers and with Daniell’s hygrometer. The formula for using the factors is as follows:—

d = D - {(D - W) × f}

where d = dew point, D = dry bulb temperatures, W = wet bulb temperature, and f = factor.

The following examples are from Glaisher’s table of factors.

┌───────────────────┬───────┐ │READING OF DRY BULB│FACTOR.│ │THERMOMETER FAHR. │ │ ├───────────────────┼───────┤ │ 55° │ 1·96 │ │ 56° │ 1·94 │ │ 57° │ 1·92 │ │ 58° │ 1·90 │ │ 59° │ 1·89 │ │ 60° │ 1·88 │ └───────────────────┴───────┘

Thus, if D = 60°, W = 55°, then dew point = 60 - {(60 - 55) 1·96} = 50°·2.

The dew-point may also be obtained by Apjohn’s formula; which for a pressure of about 30 inches is F = f-(D-W)/87

D being dry and W wet bulb temperature,

F elastic force of vapour corresponding to dew-point, and

f, elastic force corresponding to wet bulb temperature (ascertained from a table of tensions).

The _elastic force of aqueous vapour_, _i.e._ the amount of barometric pressure due to the vapour present in the air is dependent upon the temperature of the dew-point. It is given for every tenth of a degree of temperature in Table VI. (p. 42) of Marriott’s _Hints_.

The _relative humidity_ is a term expressing the percentage of saturation of the air with water vapour. It is obtained from Table VI. (above) as follows:—

Elastic force of water vapour at the temperature of the Relative } dew-point } = —————————————————————————————————————————————————————————— Humidity } Elastic force of water vapour at the temperature of the air (_i.e._ the dry-bulb reading.)

Thus elastic force with dry bulb = 55° is ·433 in.} in the table. „ „ dew-point = 46°·5 is ·317 in.}

·317/·433 = ·73.

If saturation = 100, relative humidity is 73.

In Table VII. of Marriott’s _Hints_, a table is given which enables the relative humidity to be found by mere inspection. Thus if the dry bulb temperature is 58°·5, wet-bulb 51°·7, and the difference 6°·8, the relative humidity given in the table is 62.

The =Rain-Gauge= is best made of copper in the shape of a circular funnel, usually 5 or 8 inches in diameter, leading into a bottle underneath. It must always be set in an open situation away from trees, walls, and buildings. According to Scott no object ought to subtend a greater angle with the horizon than 20° in any direction from the gauge. The rain is measured by pouring the contents of the bottle into a glass measure, which is graduated to represent tenths and hundredths of an inch on the area of the gauge, the measure holding half an inch of rain on this area. Snow is melted before being measured.

Observations of =wind= should include its direction and force. The direction is observed by means of a well-oiled and freely exposed vane. There are 32 points to the compass, but a reading to eight points suffices. The force of the wind should be estimated by Beaufort’s scale, from 0 to 12. Thus:—

FORCE. MILES PER HOUR. 0. _Calm_ 3 1. _Light air_ 8 2. _Light breeze_ 13 3. _Gentle_ 18 4. _Moderate_ 23 5. _Fresh_ 28 6. _Strong_ 34 7. _Moderate gale_ 40 8. _Fresh_ 48 9. _Strong_ 56 10. _Whole_ 65 11. _Storm_ 57 12. _Hurricane_ 90

_Robinson’s anemometer_ is also employed, but it is not altogether trustworthy.

=Sunshine= is recorded by the Campbell-Stokes burning recorder, and the Jordan photographic recorder. Of these the former is the more easily worked and gives more uniform results. It consists of a sphere of glass 4 inches in diameter, supported on a pedestal in a metal zodiacal frame (Fig. 55). The setting of the recorder should be due south, level from east to west, and with the axis of the ring inclined to the horizon at an angle equal to the latitude of the place, and so that the image of the sun, when the sun is due south, shall fall on the meridian line marked on the ring. The sun burns away or chars the surface of the cards inserted in the proper groove, and so gives a record of the duration of bright sunshine.

The amount of =Cloud= should be estimated daily, according to a scale ranging from 0 to 10, _i.e._ clear sky up to completely overcast. The form of cloud should also be stated, as cirrus, cirro-cumulus, cirro-stratus, cumulus, cumulo-stratus, stratus, and nimbus.