August 27, 1885] 
NATURE’ 
393 
the author has been to give an account of the modern 
method of forecasting weather by means of synoptic 
charts; and although the general principles laid down 
hold all over the world, the details he gives refer only to 
Great Britain. The whole system of synoptic forecasting 
depends entirely on the observed association of different 
sets of phenomena, and is totally independent of any 
theory of atmospheric circulation. 
The synoptic charts prepared at the Meteorological 
Office are constructed in the following manner. Every 
day at 8 A.M. and 6 P.M. telegraphic reports are sent up 
to London from about fifty stations in the United King- 
dom, giving the readings of the barometer and thermo- 
meter, the direction and force of the wind, and the state 
of the weather. These readings are then plotted on a 
map, and the “isobars” and “isotherms” drawn, repre- 
senting lines of equal pressure and equal temperature. 
The isobars are the most important element in forecasting. 
The direction of the wind is shown by arrows which have 
a number of “fleches ” proportional to the force, while the 
weather is indicated by the letters of Beaufort’s notation. 
While the force of the wind depends on the closeness, 
and the direction on the trend, of the isobars, the weather 
is governed by the shape of the lines. Although the shape 
of the isobars is continually changing, several well-defined 
forms are always reproduced. Seven of these are de- 
scribed, to which the following names have been given :— 
1. Cyclone—an area of low pressure, bounded by circular 
or oval isobars ; 2. Secondary cyclone—a small, circular 
depression, subsidiary to the cyclone; 3. V-shaped de- 
pression—an area of low pressure bounded by V-shaped 
isobars, something like a secondary, but differing from it 
in many important particulars; 4. Anticyclone-—an area 
of high pressure bounded by circular or oval isobars ; 5. 
Wedge-shaped isobars—an area of high pressure bounded 
by isobars converging toa point like a wedge; 6. Straight 
isobars—a barometric slope, down which the isobars lie 
in straight lines; and 7. Co/ or neck of low pressure 
lying between two adjacent anticyclones. 
Cyclones.—A cyclone may be of any diameter, from 
100 to 3,000 miles. The most common are between 1,000 
and 2,000 miles ; and anything less than 1,200 miles across 
is a small one. The path of a cyclone is the path 
described by the centre. In this courtry 90 per cent. 
move towards some point of east, the most frequent 
direction being about east-north-east. The velocity is 
that of the centre; it may be anything from 70 miles 
an hour eastwards to 10 miles an hour westwards. 
About 20 miles is a common velocity, but sometimes 
a cyclone is stationary. The life of a cyclone is mea- 
sured by the number of days during which it can be 
traced on synoptic charts; the length of life may be 
anything from a few hours to 20 days. The details of 
wind, weather, &c., in the different portions of a cyclone 
may be briefly summarised as follows :—The temperature 
is always higher in the front than in the rear ; the warm 
air in the front having a peculiar close, muggy character, 
quite independent of the actual reading of the thermo- 
meter. The cold air in the rear, on the contrary, has a 
peculiarly exhilarating feeling, also quite independent of 
the thermometer. The front is always very damp, espe- 
cially the right-front, while the rear is dry to a marked 
degree. The wind blows around the centre in the direction 
contrary to the motion of the hands of a watch ; but as the 
direction is slightly inclined to the isobars,on the whole 
the circulation is an ingoing spiral. The amount of in- 
curvature is usually greatest in the right-front, and least in 
the rear ; so that sometimes the passage of the trough is 
marked by a sudden shift of wind. ‘The force of the wind 
depends almost entirely on the gradients ; in the centre it 
is a dead calm, and the steepest gradients are usually found 
at some distance from thecentre. The direction from the 
centre in which the strongest winds are found depends on 
the position of the surrounding areas of high pressure. 
There is no difference between ordinary weather and a 
storm, except in that property called zzéensity, and in this 
country a summer breeze and winter gale are equally the 
product of cyclones which differ only in intensity. Hence 
in forecasting storms it is necessary not only to foresee 
the arrival of a cyclone, but of one possessing sufficient 
intensity to cause a gale, and in tracking cyclones it by no 
means follows that the same one causes a storm during 
every day of its existence. Observation has also shown 
that a deepening cyclone is increasing in intensity, while 
one which is filling up is decreasing. When in watching 
the progress of cyclones by telegraph it is very important 
for forecasting to note changes in depth, as well as any 
other indications derived from the configurations of the 
isobars, or even from weather prognostics, which experi- 
ence has shown to be associated with intensity. 
Though the general chzracteristics of a cyclone are in- 
variably maintained, still, individual cases vary much in 
detail. The principal sources of variation which modify, 
but do not alter, the general characteristics are:—1, the 
type; 2, the intensity ; 3, the size; 4, local; 5, diurnal ; 
and 6, seasonal variation. 
Secondaries.—The secondary is a small cyclone formed 
on the side of a larger one which is called the “ primary.” 
Secondaries are almost invariably formed either along the 
prolongation of the trough of a cyclone, or else on that 
side of the primary which adjoins the highest adjacent 
pressure. The most important feature about them is the 
manner in which they deflect the isobars of the primary, 
so as to leave an area of slight gradients and light winds 
on the side of the secondary next the primary, and a line 
of steeper gradients and stronger winds on the side 
furthest from the primary. Their motionis usually parallel 
to that of the primary. The most striking difference 
between a secondary and a primary cyclone is the great 
amount of rain and cloud with absence of wind developed 
in the former, compared with the less rain and cloud but 
the stronger wind developed in the latter. In a second- 
ary when the rain comes on, it is usually very heavy and 
falls straight ; and in its general appearance and surround- 
ings is very different from the driving or drizzling rain, so 
characteristic of the front of a primary cyclone. In fore- 
casting, the principal indication of secondaries is rain, 
without much wind, and thunder-storms in summer ; and 
their sudden formation very often unexpectedly disturbs 
and vitiates former forecasts. Sometimes several second- 
aries are seen on the chart ; this is a sign of great inten- 
sity, and the indication in such cases is always for wild, 
broken weather, often with thunder-storms, but not for 
widespread gales. 
V-shaped Depressions. —These are generally formed 
along the southern prolongation of the trough of a cyclone, 
or in the col or furrow of low-pressure which lies be- 
tween two adjacent anticyclones. Their motion is gene- 
rally eastwards along with their associated cyclone, but 
they are very often short-lived. They are entirely non- 
cyclonic. The line of the trough, along which the baro- 
meter rises, marks out the line along which the weather 
changes very abruptly, and this change is very often 
accompanied by a violent squall. 
Anticyclones.—Anticyclonic isobars enclose an area of 
high pressure which is associated with fine weather and 
light winds circulating in the direction of the hands of a 
watch, but a little outcurved. The whole system is 
usually stationary for many days together. The general 
characteristics are a cold, dry air, and fine, or at least 
quiet, weather. The calms, or light winds, give free play to 
the radiation of the season, so that very hot days occur in 
summer and very cold nights and fog inwinter. In fore- 
casting, the indications are for settled fine weather, the 
details depending much on the season and local pecu- 
liarities. 
Wedge Isobars.—These consist of isobars converging 
almost to a point, but enclosing an area of high pressure, 
