saturated with moisture, and the amount of aqueous vapour held in
suspension is very variable. This fact has important bearings on many
branches of industry, as also on the hygienic qualities of the
atmosphere. The consideration that a certain amount of moisture in the
air is necessary to the continuance of health will suggest the
importance of maintaining that due proportion in the atmosphere of sick
rooms, where the artificial heat so injudiciously used, often disturbs
the healthful hygrometric condition of the air. Mr. GLAISHER is of
opinion that the medical profession should enforce, as far as lies in
their power, the use of this simple and effective instrument, which
gives indications so important to the comfort of the patient.
The _amount_ of moisture in the air is _estimated_ by the use of
instruments called Hygrometers, which may be thus classified:—
1. Hygrometers of Absorption.—Made with hair, oatbeard, catgut,
seaweed, grass, chloride of calcium.
2. Hygrometers of Condensation.—Regnault’s, Daniell’s, Leslie’s,
Dyne’s.
3. Hygrometers of Evaporation.—Mason’s Psychrometer, or Wet and Dry
Bulb Thermometers.
By an ingenious application of the affinity of the oatbeard for
moisture, Damp Detectors are constructed for tourists, commercial
travellers, &c., to test moisture and avoid the consequences of
sleeping in damp beds. They are strongly gilt, and resemble in size and
shape a lady’s watch.
[Illustration: 38.
Damp Detector.
Scale about 2/3.]
In Saussaure’s Hygrometer the frame is of brass, and the scale of the
same metal silvered. It has an attached thermometer, and the
indications are the result of the contraction and expansion of a
prepared human hair, consequent upon its absorbing or yielding
moisture. The scale is divided on the arc of a circle, and an index
needle, working on an enlarged arc, multiples the indications.
Regnault’s Hygrometer (Fig. 39) consists of a thin and highly polished
silver tube or bottle, into the neck of which is inserted a delicate
thermometer. The bottle has a lateral tubular opening, to which is
attached a flexible tube with an ivory mouthpiece.
Ether is poured into the silver tube in sufficient quantity to cover
the bulb of the thermometer. The ether is then agitated by breathing
through the flexible tube until, by the rapid evaporation thus
produced, a condensation of moisture takes place, readily observable on
the bright polished silver surface, and the temperature indicated by
the thermometer at that moment is the dew-point.
[Illustration: 39.
Regnault’s Hygrometer. Scale
about 1/10.]
Daniell’s Hygrometer, or Dew-point Thermometer (Fig. 40), consists of a
glass tube, bent twice at right angles, each extremity terminating in a
bulb about 1-1/2 inch in diameter, supported on a brass stand, to which
a thermometer is attached to indicate the temperature of the
surrounding air. The lower bulb is of blackened glass, to facilitate
the observation of the dew-point; it is about three parts filled with
pure ether, and contains a very delicate thermometer. The upper bulb at
the extremity of the short stem is transparent, but covered with thin
muslin, upon which, when an observation is made, pure ether is slowly
dropped. The evaporation rapidly lowers the temperature, until a moment
arrives at which dew condenses on the black bulb. A quick eye is
necessary to note _this_ and the temperature shown by the thermometer
_simultaneously_, the latter showing the degree at which the atmosphere
is saturated with moisture _at the time of observation_. To avoid
error, it is usual to note the temperature at which the dew disappears,
and take the mean of the two temperatures.
[Illustration: 40.
Daniell’s Hygrometer.
Scale about 1/5.]
Dyne’s Hygrometer, for showing the dew-point by direct observation, by
means of iced water and black glass, enables the observer to dispense
with the use of ether, and shows the dew-point with great distinctness.
[Illustration: 41.
Mason’s Hygrometer.
Scale about 1/6.]
The hygrometer in most general use is the wet and dry bulb thermometer,
and for which Mr. Glaisher has calculated an elaborate set of tables, a
brief abstract of which sufficient for general purposes is subjoined.
For finding the Degree of Humidity of the Air from Observations of
a Dry Bulb and a Wet Bulb Thermometer, sometimes called Mason’s
Psychrometer.
+-------------+-----------------------------------------+
| |DIFFERENCE BETWEEN DRY BULB AND WET BULB |
| | READINGS. |
|TEMPERATURE +------+------+------+------+------+------+
| BY THE | | | | | | |
| DRY BULB | 2° | 4° | 6° | 8° | 10° | 12° |
|THERMOMETER. | | | | | | |
| +------+------+------+------+------+------+
| | DEGREE OF HUMIDITY. |
+-------------+------+------+------+------+------+------+
| 34° | 79 | 63 | 50 | ... | ... | ... |
| 36 | 82 | 66 | 53 | ... | ... | ... |
| 38 | 83 | 68 | 56 | 45 | ... | ... |
| 40 | 84 | 70 | 58 | 47 | ... | ... |
| 42 | 84 | 71 | 59 | 49 | ... | ... |
| 44 | 85 | 72 | 60 | 50 | ... | ... |
| 46 | 86 | 73 | 61 | 51 | ... | ... |
| 48 | 86 | 73 | 62 | 52 | 44 | ... |
| 50 | 86 | 74 | 63 | 53 | 45 | ... |
| 52 | 86 | 74 | 64 | 54 | 46 | ... |
| 54 | 86 | 74 | 64 | 55 | 47 | ... |
| 56 | 87 | 75 | 65 | 56 | 48 | ... |
| 58 | 87 | 76 | 66 | 57 | 49 | ... |
| 60 | 88 | 76 | 66 | 58 | 50 | 43 |
| 62 | 88 | 77 | 67 | 58 | 50 | 44 |
| 64 | 88 | 77 | 67 | 59 | 51 | 45 |
| 66 | 88 | 78 | 68 | 60 | 52 | 45 |
| 68 | 88 | 78 | 68 | 60 | 52 | 46 |
| 70 | 88 | 78 | 69 | 61 | 53 | 47 |
| 72 | 89 | 79 | 69 | 61 | 54 | 48 |
| 74 | 89 | 79 | 70 | 62 | 55 | 48 |
| 76 | 89 | 79 | 71 | 63 | 55 | 49 |
| 78 | 89 | 79 | 71 | 63 | 56 | 50 |
| 80 | 90 | 80 | 71 | 63 | 56 | 50 |
| 82 | 90 | 80 | 72 | 64 | 57 | 51 |
| 84 | 90 | 80 | 72 | 64 | 57 | 51 |
| 86 | 90 | 80 | 72 | 64 | 58 | 52 |
+-------------+------+------+------+------+------+------+
The total quantity of aqueous vapour which at any temperature can be
diffused in the air being represented by 100, the percentage of vapour
actually present will be found in the table _opposite_ the temperature
of the dry thermometer, and _under_ the difference between the dry bulb
and wet bulb temperatures. The degree of humidity for intermediate
temperatures and differences to those given in the table can be easily
estimated. Thus dry bulb 51°, wet bulb 46°, give 69 for the degree of
humidity.
The instrument, as shown at page 48, consists of two thermometers
attached to a support, which may be either slate or wood. The bulb of
one of the thermometers has some thin muslin tied over it, and is kept
moist by the capillary action of a thread dipping into a cistern of
water placed underneath. It will be obvious that the amount of
evaporation will be in proportion to the dryness of the air, and that
the differences of temperature indicated by the two thermometers will
be greatest when the atmosphere is dry, and least when the air is damp.
[Illustration: 42.
Board of Trade Thermometer
Screen. Scale about 1/9.]
HYGROMETER PRECAUTIONS.
Hygrometers should be exposed in the shade free from air-currents.
The covering of the wet bulb must be very thin.
The supply of water must be carefully regulated.
The bulb must be constantly _moist_, yet not _too wet_.
The supply of water must be ample in dry weather.
In damp weather water must not drip from the wet bulb.
Water reservoir should be as far as possible from the dry bulb.
Dry bulb must never receive moisture from any source.
Use distilled, rain, or softest water procurable, for wet bulb.
When lime deposits from use of hard water change muslin and worsted.
Replenish reservoir after, or long before, taking an observation.
Well wash muslin and worsted before using.
Also wash occasionally while in use.
Change muslin twice a month or according to condition.
Dust and blacks must not be allowed to accumulate on muslin.
⎧ When wet bulb is frozen, wet with ice-cold water by brush.
⎪ The water will first freeze, then cool to air-temperature.
⎨ After which wet bulb falls a trifle lower than dry one.
⎩ Then temperature of evaporation may be noted.
⎧ In thick fog wet bulb reads _above_ dry bulb.
⎪ In cold calm weather, wet bulb reads _above_ dry bulb.
⎨ This is owing to the air being perfectly saturated.
⎪ Covered bulb cannot therefore show temperature as well as uncovered.
⎩ In such cases both readings are assumed to be identical.
It is important that the instrument should be protected not only from
the sun’s direct rays, from rain and snow, but also from wind, the
currents of which would, by increasing evaporation, cause the wet bulb
thermometer to indicate a temperature not strictly due to the
hygrometric condition of the atmosphere. For this purpose Thermometer
Screens are employed. Illustrations of two forms are shown at Figs. 42
and 43; they should be placed facing the north at a distance of four
feet from the ground. Fig. 42 shows the form adopted by the Board of
Trade, for marine service, while Fig. 43 shows Mr. Stevenson’s
double-louvred screen with perforated bottom, which ensures free
ingress and egress of air, the exclusion of snow and rain, and the
direct rays of the sun. Professor Wild recommends overlapping segments
of sheet zinc for the construction of these screens, as possessing the
advantage over wood of becoming sooner in _thermic equilibrium_ with
the surrounding air, and thus preventing radiation. Stevenson’s Screen
should be erected on legs four feet high, and should stand over grass
on open ground. It should not be under the shadow of trees, nor within
twenty feet of any wall.
[Illustration: 43.
Stevenson’s Thermometer Screen.
Scale about 1/10.]
CLOUDS.
The important office performed by clouds in the economy of nature
entitles them to extended consideration. A cloud may be defined as
“water-dust,” since aqueous vapour diffused through the air is
invisible until the temperature is sufficiently lowered to produce
condensation; no satisfactory explanation, however, has yet been given
of the mode of suspension of this water-dust, nor why it remains
suspended in opposition to gravitation. It is tolerably certain that
electricity is not without its influence, though the apparently
_stationary_ character of some clouds is deceptive, for while there may
be no apparent motion in the mass the particles constituting the mass
are undergoing continuous renewal, which justifies the assertion of
Espy that every cloud is either a forming or dissolving cloud.
Aeronauts in ascending from the earth pass through many successive
alternations of cloud-strata and clear air which owe their existence to
the varying temperature and degrees of humidity of the atmospheric
currents so superposed.
Luke Howard in his Askesian Lectures, 1802, divides clouds into three
primary modifications: cumulus, stratus, and cirrus, with intermediate
forms resulting from combinations of the primaries, viz.,
cirro-cumulus, cirro-stratus, cumulo-stratus, and cumulo-cirro-stratus
or nimbus. This nomenclature is now universally adopted.
[Illustration: 44.
Cirrus.]
CIRRUS, or mare’s tail cloud, appears as parallel, flexuous, or
diverging streaks or fibres, partly straight. It is the lightest and
the highest of all clouds, being seldom less than three miles, and
often ten miles, above the earth, and shows the greatest variety of
form. On account of its great height it is assumed to consist of minute
snowflakes or crystals of ice, the refractions and reflections from
which produce the halos, coronæ, and mock suns and moons which occur
chiefly in this cloud and its derivatives. It retains its varied
outlines longer than any other cloud; at sunrise it is the first to
welcome the sun’s rays, and at sunset the last to part with them. It is
the most useful of all clouds for weather warnings.
1. _Serene, settled weather may be expected_ when groups and
threads of cirri are seen during a gentle wind after severe
weather.
2. _A change to wet may be expected_ when, after continued fair
weather, filaments, or bands of cirri (_apparently_
stationary), with converging ends, travel across the sky.
3. _Rain or snow, and windy, variable weather may be expected_ when
cirri with fine tails vary much in a few hours.
4. _Continued wet weather may be undoubtedly expected_ when
horizontal sheets of cirri fall quickly and pass into the
cirro-stratus.
5. _A storm of wind and rain may be expected within forty-eight
hours_ when fine threads of cirri seem brushed backward from
the south-west.
[Illustration: 45.
Cumulus.]
CUMULUS.—This modification of cloud is most frequently seen on bright
summer days, and is appropriately called “the day cloud” and “the
summer cloud.” It is formed only in the daytime, in summer calms, and
results from the rise of vapours from rivers, lakes, and marshes into
the colder regions of the air, the lower portions of which are readily
saturable. They are characterized by a horizontal base, from which they
rise in dense conical and hemispherical masses rivalling mountains in
their magnitude.
Their formation is due to the convection of heat from the earth’s
surface, which renders the lower atmospheric strata capable of holding
a larger amount of aqueous vapour and simultaneously establishes an
upward current, which reaching the colder regions of the air brings
about the condensation of the aqueous vapour into the elegant and
ever-beautiful forms admired alike “by saint, by savage, and by sage.”
These begin as mere specks, which enlarge until the sky is nearly
covered in the afternoon, and towards sunset they generally disappear,
their tops becoming cirri when the air is dry.
1. _Fine, calm, warm weather may be expected_ when cumuli are of
moderate size and of pleasing form and colour.
2. _Cold, tempestuous, rainy weather may be expected_ when cumuli
cover the sky, rolling over each other in dense, dark, and
abrupt masses.
3. _Thunder may be expected_ when cumuli of hemispherical form are
characterized by an extreme silvery whiteness.
4. _Rain may be expected_ when cumuli increase in number towards
evening, sinking at the same time into the lower portions of
the air.
[Illustration: 46.
Stratus.]
STRATUS.—As its name implies, this is a horizontal sheet of cloud
formed near the earth at night (whence it has been called “the night
cloud”) by the condensation of moist air from rivers, lakes, and
marshes, or damp ground which has lost its day-heat by radiation,
especially in calm clear evenings, after warm days. It appears as a
white mist near, and sometimes touching, the earth. It attains its
maximum density about midnight, but is dissipated by the rays of the
morning sun. Its formation, watched from a height over a large city, is
highly interesting, and is attributed by Sir John Herschel to the soot
suspended over such localities, each particle of which acts as “an
insulated radiant, collects dew on itself, and sinks down rapidly as a
heavy body.” Still more interesting is it to observe from a similar
elevation the dissipation of this cloud when the sun has attained such
an altitude that its rays fall on the upper surface of the stratus
cloud, which then heaves like the billows of the ocean, while the whole
mass seems to rise spontaneously from the earth, and speedily vanishes
“into air, into thin air.”
1. _The finest and most serene weather may be expected_ when
stratus clouds present the appearances just described.
CIRRO-CUMULUS, or “mackerel sky,” is a well-known form of cloud
occurring in small roundish masses, looking like flocks of sheep at
rest, and often at great heights. It is seldom seen in winter.
1. _Increased heat may be expected_ when cirro-cumuli appear.
2. _A storm or thunder may be expected_ when cirro-cumuli occur
mingled with cumulo-stratus in very dense, round, and close
masses.
3. _Warm wet weather, and a thaw, may be expected_ when
cirro-cumuli occur in winter.
CIRRO-STRATUS “appears to result from the subsidence of the fibres of
cirrus to a horizontal position, at the same time approaching
laterally. The form and relative position when seen in the distance
frequently give the idea of shoals of fish.” It is called “the vane
cloud” and “mackerel-backed sky.”
1. _Rain, snow, and storm may be expected_ when _cirro-stratus_ is
seen alone or mingled with cirro-cumulus, especially if the
cirro-cumulus passes away.
2. _Fair weather may be expected_ when from a mixture of
cirro-stratus and cirro-cumulus the former disappears, leaving
the latter in possession of the sky.
3. _Thunder and heat_ are generally attended by waved cirro-stratus.
CUMULO-STRATUS.—This form of cloud results from the mingling of the
cumulus and cirro-stratus; it appears sometimes as a thick bank of
cloud with overhanging masses. The cloud known as “_distinct_”
cumulo-stratus appears as a cumulus surrounded by small fleecy clouds.
1. _Thunder may be expected_ when “distinct” cumulo-stratus appear.
2. _Sudden atmospheric changes may be expected_ when cumulo-stratus
appear.
[Illustration: 47.
Nimbus.]
NIMBUS, OR CUMULO-CIRRO-STRATUS.—The name of this cloud at once
suggests that it is produced by a combination of the three primary
forms of cloud. The _nimbus_ is popularly known as “the rain cloud.” It
is really a system of clouds, having its origin chiefly in the tendency
of the _cumulo-stratus_ to spread, overcast the sky, and settle down to
a dense horizontal black or grey sheet, above which spreads the
_cirrus_, and from below which rain begins to fall.
1. _A cessation of rain may be expected_ when the grey lower
portion of _nimbus_ begins to break up.
2. _A thunderstorm may be expected_ when the _nimbus_ character of
the cloud is very perfect.
3. _Very copious showers may be expected_ when the _cirri_
projected from the top of the rain-cloud are very numerous.
AMOUNT OF CLOUDS.—Any record of the proportion of sky covered by cloud
should be made on a scale of 0 to 10. A clear sky is registered 0, and
a sky wholly obscured as 10, any intermediate condition being
represented by 5—7, or other figures deemed appropriate by the
observer. The _kind_ of cloud should be noted, as also the direction in
which it is driven by the wind, whether in the upper or lower strata of
the air. This operation may be assisted by an ingenious arrangement,
exhibited by Mr. Goddard in 1862, and called a “cloud reflector,”
obtainable at any optician’s. Observations at the Greenwich Observatory
establish the facts that the least amount of cloud exists during the
night, especially in May and June, and the greatest amount at midday,
and in winter; also that from November to February three-fourths of the
heavens are obscured by sun-repelling clouds.
HEIGHT OF CLOUDS.—Great diversity of opinion exists on this point. It
is asserted, on the one hand, that the region of clouds does not extend
beyond five miles above sea-level, but Glaisher has attained a height
of 36,960 feet, and from thence saw clouds floating at a great height
above him; and it is considered probable that cirri are often ten miles
above the earth.
VELOCITY OF CLOUDS.—This is of two kinds: 1st. Velocity of
Propagation; and 2nd. Velocity of Motion. The first occurs when at a
given altitude the dew-point is suddenly attained, when the sky on one
occasion was covered from the eastern to the western horizon at the
rate of 300 miles per hour. The second is dependent on the force of
atmospheric currents, which is much greater in the upper regions of the
air than in those nearer the earth. Accurate observations of the
shadows of clouds, borne across the fields on a summer’s day, warrant
the assertion that an apparently slow motion of clouds is equal to
eighty miles an hour, while a velocity of 120 miles is attained without
impressing the observer with the idea of rapidity.
On the subject of clouds Admiral Fitzroy says:—
May be Expected
-----
Fine weather When clouds are “soft-looking or delicate.”
Wind When clouds are hard-edged or oily-looking.
Less wind In proportion as the clouds look _softer_.
More wind The harder, more “greasy,” rolled, tufted, or
ragged the clouds look.
Rain When small-inky-looking clouds appear.
Wind _and_ rain When light scud clouds are seen driving across
heavy masses.
Wind only When light scud clouds are seen alone.
Change of wind When high upper clouds cross the sun, moon, or
stars in a direction different from that of the
lower clouds, or the wind then felt below.
Wind With tawny or copper-coloured clouds.
The following “Weather Warnings” may be gathered from the COLOUR OF THE
SKY:—
Whether clear or cloudy, a rosy sky at sunset presages fine weather; a
sickly greenish hue, wind and rain; a red sky in the morning, bad
weather, or much wind or rain; a grey sky in the morning, fine weather;
a high dawn (_i. e._, when the first indications of daylight are seen
above a bank of clouds), wind; a low dawn (_i. e._, when the day breaks
on or near the horizon), fair weather. Light, delicate, quiet tints or
colours, with soft, indefinite forms of clouds, indicate and accompany
fine weather; but gaudy or unusual hues, with hard, definitely outlined
clouds, foretell rain and probably strong wind. Also a bright yellow
sky at sunset presages wind; a pale yellow, wet; orange or
copper-coloured, wind and rain: and thus, by the prevalence of red,
yellow, green, grey, or other tints, the coming weather may be told
very nearly; indeed, if aided by instruments, almost exactly.
After fine, clear weather the first signs in a sky of a coming change
are usually light streaks, curls, wisps, or mottled patches of white
distant cloud, which increase and are followed by an overcasting of
murky vapour that grows into cloudiness. This appearance, more or less
oily or watery as wind or rain will prevail, is an infallible sign.
Usually, the higher and more distant such clouds seem to be, the more
gradual, but general, the coming change of weather will prove.
Misty clouds, forming or hanging on heights, show wind and rain coming,
if they remain, increase, or descend; if they rise or disperse, the
weather will improve or become fine.
May be Expected
-----
Fine weather When the sky is grey in the morning.
Wind With a high dawn.
Fair weather With a low dawn.
Wind When the sky at sunset is of a _bright_ yellow.
Rain When the sky at sunset is of a _pale_ yellow.
Wind and rain When the sky is orange or copper colour.
Fine weather When the sky has light, delicate, quiet tints and
soft, indefinite forms of clouds.
Rain and wind When the sky has gaudy, unusual hues, with hard,
definite outlined clouds.
Fair weather When sea-birds fly out early and far to seaward.
Stormy weather When sea-birds hang about the land, or fly inland.
Fair weather When dew is deposited. Its formation never
_begins_ under an overcast sky, or when there is
much wind.
Rain On what is called a good _hearing_ day.
Rain When remarkable clearness of atmosphere,
especially near the horizon, exists, distant
objects, objects, such as hills, being unusually
visible or well defined.
RAIN.
The atmosphere at a given temperature is capable of retaining only a
given quantity of aqueous vapour, invisibly diffused through it, at
which temperature it is said to be _saturated_. Should the temperature
from any cause be lowered, the aqueous vapour at once becomes visible
in the form of either cloud, dew, rain, snow, or hail. It has already
been shown that, although marshes and rivers, inland seas and lakes,
yield by evaporation watery vapours to the air, the ocean is the great
source of rain, whence it is lifted in vast quantities by the sun’s
radiant heat, to be subsequently condensed by passing into cooler
regions, or by contact with cold mountain peaks, falling to earth as a
fertilizing shower or a devastating flood.
Sir John Herschel accounts for the formation of raindrops by
saying:—“In whatever part of a cloud the original ascensional movement
of the vapour ceases, the elementary globules of which it consists
being abandoned to the action of gravity, begin to fall. The larger
globules fall fastest, and if (as must happen) they overtake the slower
ones, they incorporate, and the diameter being thereby increased, the
descent grows more rapid, and the encounters more frequent, till at
length the globule emerges from the lower surface of the cloud at the
‘vapour plane’ as a drop of rain, the size of the drops depending on
the thickness of the cloud stratum and its density.”