In these charts the state of the sea—whether “rough,”
“smooth,” “moderate,” “slight,” etc., is marked in capital
letters; and the state of the weather—as “clear,” “dull,”
“cloudy,” “showery,” etc., in small letters. The direction of
the wind is indicated by the arrows. Unlike the arrows of a
vane, these do not point towards the direction from which the
wind is coming, but are _flying_ arrows represented as moving
_with_ the wind, and consequently pointing to _where the wind
is going_. The force of the wind is represented in five degrees
of strength. 1st. A _calm_, by a horizontal line and zero—0
thus 0; 2nd. A _light wind_, by an arrow with one barb and no
feathers ______\; 3rd. A _fresh to strong breeze_, by an arrow
with two barbs and no feathers ——————>; 4th. A _gale_, by an
arrow with two feathers >——————>; and 5th. A _violent gale_, by
an arrow with four feathers >>——————>. The temperature—in the
shade—is marked in figures with a small circle to the right,
indicating degrees—as 60°. These figures stand in the places
where the observations are made. The other figures—usually
with decimals, and placed at the end of the dotted lines—give
the height of the barometer—the dotted line showing where
this particular height remained the same at the time of
observation. These dotted lines are called “isobars,” or _equal
weights_—the weight or over-head pressure of the atmosphere
being the same all along the line.
What must follow from this condition of the atmosphere? Clearly a great
flow or rush of air from the south towards the comparatively vacuous
regions of the north. The gases of our atmosphere, like the waters of
the ocean, are always struggling to find their level, and thereby the
winds are produced. The air flows from all sides towards the lowest
isobar. But what, then, must be the course of the wind? Will it be
in straight lines towards this point? If so, a strange conflict must
result when all these currents meet from opposite directions. What
will follow from this conflict? A skillful physicist can work out this
problem mathematically, but we are not all mathematicians, some of us
are not able to follow his formulæ, and, therefore, will do better
by resorting to simple observation of other analogous and familiar
phenomena. A funnel or any vessel with a hole in the bottom will answer
our purpose. Let us fill such a vessel with water, then open the hole,
and see what will be the course of the water when it is struggling to
flow from all sides to the one point of vacuity. It will very soon
establish a vortex or whirlpool, _i.e._, the water instead of flowing
directly by straight lines from the sides to the centre of the funnel,
will take a roundabout, spiral course, and thus screw its way down the
outlet of the funnel.
This is just what occurs when the air is rushing to fill a
comparatively vacuous atmospheric space. It moves in a spiral; and
in the Northern Hemisphere this spiral always turns in the same way,
viz., in the opposite direction to the hands of a clock when flowing
inwards, and _vice versâ_, or _with_ the clock hands, when the air is
overflowing from a centre of high pressure.
In the chart for October 5th both these cases are illustrated. North of
Dublin there is a curvature of isobars and an inrush of winds towards
a northward low pressure, or vacuous region; while south of Dublin the
isobar tends sharply round a high-pressure focus, and the overflowing
wind is correspondingly reversed in direction, as shown by the arrows.
The next chart, for October 6th, shows that the overflow has spread
northwards as far as Dublin, and the high-pressure focus has also
moved northwards. It follows from this that if you know the barometric
gradient, and stand with your left hand to the region of low barometer
and your right hand to that of the high barometer, the wind will
blow against your back, _i.e._, you will face the direction of the
wind, or of those flying arrows on the chart. This interesting and
important generalization is called “Buys Ballot’s Law.” In spite of the
proverbial fickleness of the winds this simple law is rarely infringed,
though it may require a slight modification of statement—inasmuch
as the wind does not move in _circles_ round the vacuous space, but
in spirals, and thus it blows not quite square to the back, but
rather obliquely, or a little on the right side. This is shown by the
arrows in the charts, and is most strikingly displayed in the chart
for October 6th, between the isobars of 30.3 and 30.5. To take, in
Ireland, the position required by Buys Ballot’s Law, one must have
stood facing the east, and accordingly, the westerly wind would then
blow upon one’s back. In Paris, at the same moment, the position would
be facing south-east, and the wind was curving round accordingly.
Further south—at Bordeaux or the Pyrenees—the position becomes almost
reversed, _i.e._, facing south-west, and the wind is reversed in equal
degree.
Here, then, on these days we had the chief conditions of wind and rain,
a steep and increasing barometric gradient, and a flow over our islands
of humid air from the south and west regions of the great Atlantic.
Strong winds and heavy rains did follow accordingly; and the prophetic
warnings of the Meteorological Office, which are conveyed by means of
signals displayed on prominent parts of the coast, were fulfilled.
Mr. Scott, the Director of the Meteorological Office, tells us that
“The degree of success that has attended our warnings in these islands,
on the average of the last two years, has been that over 45 per cent
have been followed by severe gales; and over 33 per cent in addition
have been followed by wind too strong for fishing-boats and yachts,
though in themselves not severe gales; this gives a total percentage of
success of nearly 80.”
In winter the movements of the air are more decided, and the changes
are often so rapid that the warning sometimes comes too late. With
increased means—_i.e._, more money to cover additional work, and more
stations—better results might be obtained. The United States expend
50,000_l._ a year in weather telegraphy, exclusive of salaries, while
the United Kingdom only devotes 3,000_l._ a year to the same purpose.
The difficulties on our side of the Atlantic are greater than on the
American coasts, on account of the greater changeableness of our
weather—mainly due to the more irregular distribution of land and
water on this side. This, however, instead of discouraging national
effort, should be regarded as a reason for increasing it. The greater
the changes, the greater is the need for warnings, and the greater
the difficulty the greater should be the effort. With our multitude
of coastguard stations and naval men without employment, we ought to
surpass all the world in such a work as this.
Those among our readers who are sufficiently interested in this subject
to devote a little time to it, may make a very interesting weather
scrap-book by cutting out the newspaper chart for each day, pasting it
in a suitable album, and appending their own remarks on the weather at
the date of publication, _i.e._ the day after the chart observations
are made. Such an album would be far more interesting than the postage
stamp and monogram albums that are so abundant.
Parents who desire their children to acquire habits of systematic
observation, and to cultivate an intelligent interest in natural
phenomena, will do well to supply such albums to their sons or
daughters, and to hand over to them the daily paper for this purpose.
The Meteorological Office supplies by post copies of “Daily Weather
Reports” to any subscriber who pays five shillings per quarter in
advance; such subscriptions payable to Robt. H. Scott, Esq., Director
Meteorological Office, 116 Victoria Street, Westminster, S.W.
These daily reports are printed on a large double sheet, on one half of
which are four charts, representing separately the four records which
are included in the one smaller newspaper chart—viz., those of the
barometer, the thermometer, the rain-gauge, and the anemometer. On the
other half of the sheet is a detailed separate tabular statement of the
results of observations made at the following stations:
Haparanda
Hernösand
Stockholm
Wisby
Christiansund
Skudesnaes
Oxö (Christiansund)
Skagen (The Skaw)
Fanö
Cuxhaven
Sumburgh Head
Stornoway
Thurso
Wick
Nairn
Aberdeen
Leith
Shields
York
Scarborough
Nottingham
Ardrossan
Greencastle
Donaghadee
Kingstown
Holyhead
Liverpool
Valencia
Roche’s Point
Pembroke
Portishead
Scilly
Plymouth
Hurst Castle
Dover
London
Oxford
Cambridge
Yarmouth
The Helder
Cape Griznez
Brest
L’Orient
Rochefort
Biarritz
Corunna
Brussels
Charleville
Paris
Lyons
Toulon
_On Winds and Currents, from the Admiralty Physical Atlas._
In the Northern Hemisphere the effect of the veering of the wind on the
barometer is according to the following law:
With East, South-east, and South winds, the barometer falls.
With South-west winds, the barometer ceases to fall and begins to rise.
With West, North-west, and North winds, the barometer rises.
With North-east winds, the barometer ceases to rise and begins to fall.
In the Northern Hemisphere the thermometer rises with East, South-east,
and South winds; with a South-west wind it ceases to rise and begins
to fall; it falls with West, North-west, and North winds; and with a
North-east wind it ceases to fall and begins to rise.