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which we have assumed here as ten thousand feet-there would be no gradient either way; and that still higher up the pressure would be actually greater over the cyclone, and the gradient inverted as compared to that on the surface. It is evident, therefore, that, though we may use the analogy of cyclones to whirlpools, we must not picture the former to ourselves as saucer-shaped depressions of the whole envelope of our atmosphere, like the little eddies that pit the surface of a flowing river. The important bearing of these vertical gradients on the problem of measuring heights by the barometer will be very obvious.

This brings us to a curious question, as to what gradients give direction to the upper winds. Some have maintained that the uprushing currents of a cyclone have so much momentum that they can override moderate gradients. Others, on the contrary, hold that it is only by a complete or partial inversion at high levels of the gradients which are found on the surface, that the observed phenomena of upper currents can be produced; but materials do not at present exist which can decide the question, and some of the published sketches of the higher isobars over a cyclone are more than problematical.

Another point on which considerable uncertainty exists is the relation of anticyclones to cyclones. There is no doubt of their partial dependence on one another, for cyclones always tend to travel round the anticyclone on whose edge they have been formed. But, on the other hand, we sometimes find cyclones which have detached themselves from their generating anticyclones, and this seems to prove that they can exist independently.

ANTITHESIS OF CYCLONIC AND ANTICYCLONIC WEATHER.

Perhaps the best method of showing the antithesis. between cyclone and anticyclone weather will be the method we have adopted in Figs. 21 and 22. We all know how much weather is affected by the time of day, as well as by the season of the year, and by local peculiarities. We have, therefore, selected two charts for the same day of different years, at the same hour of the morning, and for the same portion of Western Europe. Every diurnal, seasonal, or local influence is therefore identical in both cases, and the whole of the difference of wind and weather which we find between the two days is entirely due to cyclonic or anticyclonic influences.

In Fig. 21 we give the synoptic conditions of pressure, temperature, wind, and weather over Western Europe at 8 a.m., May 17, 1877. There we see a small oval cyclone of very moderate intensity lying over the south-west of England. Round this the wind circulates in the usual manner, but, as the gradients are not steep, the force nowhere exceeds a fresh breeze, as at Brest. Near the centre, and some distance in front, we find, by looking at the weather-symbols, nothing but rain reported; outside the rain, an overcast sky or detached clouds; and beyond them, blue sky in a few places. The path of the cyclone is marked by the letters a, a, so as to give the position of the front. Lastly, the isotherm of 60° Fahr. (16° C.) runs just north of the Pyrenees, while that of 50° Fahr. (10° C.) stretches from the north of Scotland to Denmark.

Now turn to Fig. 22, where we give the same data for May 17, 1874, at the same hour. Then an anticyclone

lay over the British Islands; the gradients were much less steep, and the wind, therefore, was everywhere light and variable. For this reason the general circulation is not so marked as in the preceding chart, but still it is very evident that on the whole the wind blew round and out in the direction of the watch-hands. Then the weathersymbols are very interesting. Almost every station which

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reported rain in the previous chart is now marked with b for blue sky, or with m for radiation mist in several places. Then, though, as at Fano, in Denmark, we find the same symbol, c, for detached cloud in both maps, we know that it does not refer to the same kind of cloud. The temperature also shows a marked contrast. The isotherm of 60° has not much altered its position, but

that of 50° Fahr. (10° C.) bends abruptly south from the north of Scotland, across England, and the west of France, while temperatures below 40° (5° C.) are reported from Hanover and the Netherlands. This is partly due to the prevailing set of the wind being from the north, whereas in the preceding chart it was from the south or south

west.

From these examples, we see that the whole of the difference of weather on the two days was produced by the difference of isobars, which we may put thus:

The weather was wet on May 17, 1877, over England, because then the atmosphere was eddying in that manner which we call cyclonic; while it was fine on May 17, 1874, because then the eddy-circulation took the form which we call anticyclonic, and drew down dry air from the upper regions of the atmosphere.

V-SHAPED DEPRESSIONS.

We must now describe a very interesting shape of isobars, to which the name of V-depressions is applied in England, but which the German writers call "tongueformed" depressions. In these the isobars are shaped like the letter V, and enclose an area of low pressure. In the northern hemisphere the point of the V is usually directed towards the south, as in Fig. 23. The wind follows the universal law of gradients; being from south to south-west in front, and from west to north-west in rear of the trough. This latter line is given at once by joining the southern points of each successive isobar, and in practice is nearly always curved, the convexity being

turned towards the east, as in the diagram. As the V is usually moving towards the east, this line marks out the position of all the places at which the barometer, having fallen more or less, has just turned to rise, and is called the "trough" of the V.

These features are common to all V's, but the position of rain divides these depressions into two distinct types.

Trough

29.7

S

Blue

29.8

Sky

or

Detached

Cumulus

Halo

Sky

Blue

FIG. 23.-Weather in V-depression.

In the first, and by far the commoner kind in Great Britain, a narrow strip of cloud precedes an area of rain, shaped like a portion of a crescent. This is shown in Fig. 23, where the single shading marks the position and shape of the cloud-area, and the double shading that of the rain. The rear of the rain-area is very sharply defined by the line of the trough, which also marks the position of a line of squalls. Beyond this we find detached clouds, and then blue sky.

The sequence of weather, as a V of this type drifts over an observer, is obviously from blue sky to halo, cloud, and, later on, rain, with a falling barometer and south-west wind; then a heavy squall, during which the wind jumps (does not veer) to north-west, and the sky rapidly clears as the barometer rises.

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