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.
[Illustration: FIG. 45.]
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.
[Illustration: FIG. 46.]
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.
[Illustration: FIG. 47.
FORTIN BAROMETER.
_a_—Attached thermometer. _b_—Screw of vernier. _c_—Screw for setting
level of cistern.]
_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_.
[Illustration: FIG. 48.
SCALE OF BAROMETER (TO RIGHT) AND OF VERNIER (TO LEFT).]
=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).
[Illustration: FIG. 49.
NEGRETTI AND ZAMBRA’S MAXIMUM THERMOMETER.]
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.
[Illustration: FIG. 50.
MINIMUM THERMOMETER.]
=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.
[Illustration: FIG. 51.
STEVENSON’S THERMOMETER SCREEN.]
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.
[Illustration: FIG. 52.
SYMONS’ EARTH THERMOMETER.]
[Illustration: FIG. 53.
DRY AND WET BULB THERMOMETERS.]
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.
[Illustration: FIG. 54.
SNOWDON PATTERN RAIN-GAUGE.
A. Copper Upper Part of Gauge. B. Funnel. C. Bottle. To the right is
shown the glass measure inverted.]
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.
[Illustration: FIG. 55.
CAMPBELL-STOKES SUNSHINE RECORDER.]
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.