PHYSICAL GEOGRAPHY. CLIMATOLOGY.
WEATHER, CLIMATE, AND TEMPERATURE—ISOTHERMAL LINES—ISOBARS, WEATHER
FORECASTS, AND SIGNS OF THE SKY.
It is usually considered a sign of a paucity of ideas when one begins
a conversation about the “weather,” but there can be no doubt that
there is no more interesting question in social life at certain times
as to whether it will or will not rain. Our outdoor amusements are all
dependent upon weather, and a little cloud may throw a deep shadow
over all our pleasure if we neglect to bring out an umbrella, or to
carry a waterproof. We are never independent of what we term the
“capricious” climate, but in reality the laws of “the Weather,” though
so imperfectly understood, are fixed and invariable, and if we could
read the signs in the sky and learn the condition of the atmosphere, we
might leave the “prayers for rain” and “for fine weather” out of the
Church service, for then we should understand that unless miracles are
performed for us the laws of Nature can in no wise be altered.
Of late years weather forecasts (not prophecies) have come before us
in our newspapers after the manner instituted by the late Admiral
Fitzroy, whose name has become a household word in England. But at
the commencement of the Christian era and before that time the signs
of the heavens and the behaviour of animals and birds were noted with
reference to changes of weather. If we read Virgil we shall find
numerous references to these portents, and the translation usually
quoted will furnish us with information which must be as true nowadays
as it was in Virgil’s time, for wild animals do not change their
habits. Speaking of wet weather in the Georgics the poet wrote:—
“The wary crane foresees it first, and sails
Above the storm, and leaves the hollow vales;
The cow looks up, and from afar can find
The change of heaven, and sniffs it in the wind;
The swallow skims the river’s watery face,
The frogs renew the croaks of their loquacious race;
The careful ant her secret cell forsakes,
And draws her eggs along the narrow tracks;
Huge flocks of rising rooks forsake their food,
And, crying, seek the shelter of the wood.
* * * * *
The owls, that mark the setting sun, declare
A starlight evening and a morning fair.”
We might quote further selections respecting the signs in the heaven
and earth mentioned, but the foregoing verses will be sufficient to
illustrate our position, and to show us that weather forecasting is, at
any rate, as old as the Christian era.
The moon is generally supposed to influence the weather—a “Saturday’s
Moon” being particularly objectionable, or when she appears anew at
some hours after midnight thus—
“When first the moon appears, if then she shrouds
Her silver crescent, tipped with sable clouds,
Conclude she bodes a tempest on the main,
And brews for fields impetuous floods of rain.”
[Illustration: Fig. 732.—In the northern Seas.]
Weather permitting, we can go out and study the clouds as described
in the foregoing chapters, or consult the barometer, and see which
way the wind blows. The child will tell us that a high “glass” means
fine weather, and a low barometer indicates rain, but this is only
relatively true. A high glass may be falling, a low glass may be
rising. A _sudden_ fall or a _sudden_ rise are indicative of bad,
windy weather, or a short-lived fine period. The glass may rise with
a northerly wind, and rain will supervene, so careful observation is
necessary before one can obtain even a superficial knowledge of the
weather. (See subsequent observations on “Weather.”)
The Americans telegraph the results of their observations of coming
storms across the Continent, corrected by the signs noticed and
recorded by vessels arriving in New York. Thus they are frequently very
accurate; steady application and observation at Sandy Hook must give
them a great deal of useful information for the “forecasts.”
The word CLIMATE is derived from the Greek _klima_, a slope; and thus
at a glance we perceive how the aspect it presents to the rays of the
sun in the earth’s revolutions, must affect the “climate” of a country.
Of course the position of any portion, the elevation and locality
of the mountains, have also a share with the soil, winds, rains, and
sea-board, in determining the climate of any region. Many points have
already been touched upon in former chapters. Temperature, moisture,
and vegetation are the chief natural features which determine climate,
and we must find out the position of the land with reference to the sun
first, to ascertain the _climate_.
The more vertical the sun is the hotter the atmosphere, for the rays
strike directly upon the earth, which radiates the warmth received.
These heat rays are, as we know, invisible. The hottest portion of the
earth must be at the equator for the sun is overhead, and the rays beat
down directly upon the earth. The sun is also nearer than when at the
horizon, and less rays are absorbed by the atmosphere. The longer the
day the greater the heat.
[Illustration: Fig. 733.—In the southern steppes.]
Temperature is registered by observation of the thermometer, and the
distribution of heat is represented upon a chart across which lines
are drawn at places of equal temperature. These lines are called
“isothermal.” There are also terms to denote equal winter temperature
and the average summer heat—_isochimines_ and _isotheres_ respectively.
Temperature decreases as we ascend from, and increases as we descend
into, the earth. This fact proves that the air is not warmed by the
sun’s heat, but by radiation from the ground. As we ascend we reach the
line of perpetual snow, which varies in different parts of the globe.
In the tropics it extends from 15,000 to 18,000 feet; but it varies
even in places of the same latitude, according as the towns are inland
or on the coast, as in the Pyrenees and Caucasus, where there is a
difference of three thousand feet in the snow limit.
The line of the snow limit, as a rule, gets lower as we journey from
the equator to the poles. Exception will be found in the Himâlaya,
where the snow line is higher on the northern side, in consequence
of the existence of the Thibetan tableland, which causes a higher
temperature than that existing upon the abrupt southern slope.
Countries, therefore, though in the same latitude, may have different
climates according to the elevation of the land.
The proximity to the sea is another reason for climatic difference.
Water takes some time to become warm, but when it has once become so
it will not readily part with its heat. The Gulf Stream, with its warm
current beating along our shores, gives us a high temperature and
a moist climate—a very different condition to Newfoundland or Nova
Scotia, which are in much the same latitude as England and Ireland. By
the sea the climate is more uniform, and the extremes of heat and cold
are not so distant. We send invalids to the seaside to save them the
effects of such violent changes. Winters are milder and summers cooler
by the sea.
We can readily understand how such circumstances affect the vegetation,
and places which in winter may enjoy a mild and genial climate
(comparatively speaking), may have a cold summer. Ferns may flourish in
winter out of doors, but wheat will not ripen in the autumn owing to
the want of heat.
The winds also, and the soil and aspect of a region, all have a share
in determining its climate. Trees bring rain by evaporation, and a
wooded country is a blessing to its inhabitants, defending their
habitations from wind and avalanches in mountainous districts. But
the climatic conditions are altering. The ground is being more and
more cleared; the soil is more cultivated, and moisture is being more
eliminated from it. Therefore the air becomes warmer by the radiation
of the ground, and clouds are formed which keep the warm layers down
nearer the earth. Mountains, as we have seen, affect the rain-fall in
districts; and in Scandinavia—in Norway chiefly—the average rain-fall
is very high. The sheltering effects of mountains from east or
northerly winds also alter the climate, while clay or gravel soils are
cold or warm inasmuch as they absorb, or evaporate, moisture. Some
surfaces being different from others give out more heat.
In some mountainous districts we shall find every variety of climate
from the sea-level tropical heat to the rigours of the pole. The
greatest average temperature is north of the equator in Africa; the
lowest in the north, to the west of Greenland. Masses of land act in a
different manner to the oceans, and the former become heated and cooled
with equal rapidity, while the sea, as already mentioned, is slow to
lose its heat. Our land enjoys a mild and equable climate as a rule,
because it is surrounded by water, and the Gulf Stream warms it. The
European climate, taken altogether, may be considered the best on the
globe.
We will now pass on to a few observations concerning the weather, and
the means of determining it beforehand.
It is always a dangerous thing to act the part of a prophet, and the
uncertainty attending an uninspired foreteller’s predictions must in
time disparage him in the estimation of his hearers and disciples.
But there are signs in the sky which we can discern and render
valuable by the aid of instruments. We must have a reliable barometer
and thermometer, and keep a record of the average conditions of the
weather, if we wish to wear the mantle of the weather-prophet—a term
now, in America, applied (jokingly, no doubt) to people who are not
particular in their statements of facts.
But without entering upon any scientific discussion, we may state a few
plain rules which can be observed, besides the indications of a rising
or falling barometer. Having frequently studied the aspects of the
clouds, with the assistance of the hints from the wind-currents, we can
fairly _prognosticate_ or suggest _probable_ changes of weather.
[Illustration: Fig. 734.—Weather chart.]
We have already remarked upon the colours of the sunset, which are
attributable to the vapours in the atmosphere, and we say a red sky
foretells fine weather; a yellow sky changing into green means rain,
or rain and wind; on the other hand, when the red rays appear we may
anticipate fine weather, as the atmosphere is becoming less and less
moist.
A “low” dawn is known as a good sign; so when the first rays appear at,
or near the horizon, we may anticipate a fine day, as we may when the
morning is grey.
“Evening red and morning grey”
are almost unfailing tokens of fine weather.
Very often a yellow sunset means wind; a wild, crimson sky means a
gale. On the afternoon (Saturday) before the _Eurydice_ foundered off
the Isle of Wight, we particularly noted the sunset at Gravesend; and
it was evident (in our estimation) that a sudden storm was imminent,
and we remarked it to our companions. The sudden fall of the barometer,
and the appearance of the rising clouds early on that sad Sunday
afternoon, approaching in dark masses from the west and north-west,
spoke of rain and (possibly) snow. How true the forecast was the event
proved.
When clouds are soft and thin we expect fine weather; when they are
dark and hard, rain and wind. A ragged-edged and heavy cloud indicates
thunder and lightning, with squalls when we see dark clouds flying
rapidly across the mass of cumuli. A “mackerel” sky and “mares’ tails”
generally foretell wind, the direction and the upper currents being
noted. The longer the warning given by the heavens, the longer the bad
(or fine) weather will last; and the converse is also true.
“Evening grey and morning red,
Put on your hat, you’ll wet your head.”
The cirrus is a wispy cloud, and is often observed extending across
the sky on a fine afternoon. This may or may not indicate rain; it
generally points to wind. If its direction be northerly and west to
southerly and east, it is a good sign, but from west to east it is
a bad sign. The habits of birds and animals, and their anxiety for
shelter, “pigs running with pieces of straw in their mouths,” and the
low-flying swallow, are all signs of approaching rain and bad weather,
and the scintillation of stars betokens moisture in the atmosphere.
These are well-known appearances, but there are others regarding the
winds and currents of air which require the assistance of Admiral
Fitzroy’s book.
For instance, a falling barometer with rising temperature means
southerly winds and rain; in winter, with low temperature, snow.
But a rising barometer with northerly wind often means rain.
A rising glass after a low fall may, and often does, indicate _more_
wind from the north, and after that fine weather, if lower temperature
also supervene. If warm weather continue under the circumstances, the
wind may back and blow from the southward.
“The most dangerous shifts of wind happen soon after the barometer
rises from a very low point, or if the wind veers gradually shortly
after with a rising barometer.”
If the barometer rises with a southerly wind fine weather may be
expected, and if it falls with a northerly wind rain, hail, and snow
are imminent, for the rule is a fall for southerly, and a rise for
northerly winds.
A sudden fall with west wind indicates storm from northerly quarters
(N.E. to N.W.). An east gale veering southwards with falling glass
indicates a change of storm-direction to a point from N.W. or N.E.,
suddenly and violently, though a change might have been expected from
the appearance of the glass. A calm frequently occurs between these
disturbances.
A backing wind—that is, a wind going in a direction opposed to the
sun’s course (and with the earth)—is a bad sign after unsettled
weather. The wind is said to “veer” when it goes _with_ the sun.
The south-east wind, with clear sky, warm weather, and low clouds on
the horizon, is a sign of wet. A dry east wind means fine weather.
Heavy clouds in the north-west generally bring a thunderstorm. When
really distant objects look very near rain must be expected.
There are many exceptions to weather rules, and none can be laid down
as invariable. The ever-changing currents of air, and varying moisture
of the atmosphere, give rise to barometric changes, which should be
carefully noted. A little experience and close observation for one
year, with notes of signs, and indications of temperature, will assist
any one to tell the probable change that is approaching.
There are a great number of signs of weather which are observable in
the animal and even in the vegetable kingdom, as well as in the moon
and stars. Many flowers close their petals before rain comes on, and
the behaviour of domestic animals often foretells storm. Sheep huddling
together in a corner tell us the direction from which the tempest is
approaching; sea birds fly to shore, and land birds become restless.
The naturalist will observe the domestic animals which become
uncomfortable and sniff the air; the cat lies with her head down,
the brain lowest; and frequently washes her face, or scampers about
aimlessly. Spiders disappear, and worms come up to seek the expected
water. When fine weather is coming all nature appears glad, but leeches
sink into the water as far as they can.
The above are some of the domestic and common signs of coming rain,
and conversely for fine weather. A wailing wind, a cloudy mountain, a
greenish rainbow or too red a one, a pale moon with indistinct points,
or a halo round it, are all signs of rain and possibly wind. So the
most superficial observer may with these few suggestions inform himself
of the chances of fine or wet weather.