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Volume 31, 1898
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Art. LVI.—On the Interaction of Cyclones' upon one another.

[Read before the Wellington Philosophical Society, 20th September, 1898.]

Plates LII. and LIV.

Last year, in a paper read before this Society on the 14th July, I gave the history of two storms—-the one antarctic, the other tropical—which met in the region of Cook Strait on the 30th January, 1897, and which seemed to repel one another the antarctic storm being diverted southwards and the tropical storm northwards. This year—in June last—-a somewhat similar meeting between an antarctic and a tropical storm took place; but, instead of repelling one another, the antarctic storm blended with and absorbed the other, in the same way that one antarctic storm often blends with another which has been delayed, and which it overtakes.

The two storms to which I drew attention last year, were moving in nearly opposite directions when they met and repelled each other; and I suggested that probably there was a limiting angle between the directions of the tracks of two storms whose paths met inside of which angle they coalesce, and outside of which they are diverted. This supposition appears to be supported by the behaviour of the two storms which coalesced on this occasion, and the limiting angle seems to be about 120°.

The circumstances on this occasion were as follows: On the 23rd June a tropical cyclone of moderate intensity, and moving slowly from north-west to south-east, struck the northern extremity of New Zealand. The force of the wind was sufficient to interrupt telegraphic communication as far

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south as a line joining New Plymouth and Gisborne. The isobar 29.7 was recorded at Patea and Napier and 29.9 at Westport and Castlepoint at 9 a.m. on that day.

On the 24th the storm had progressed so far south that the isobar 29.9 had reached the Bluff with east wind, and the barometric reading was as low as 29.3 on the line from New Plymouth to Russell, with wind from the north and east. The centre of the disturbance was to the immediate west of this line, and was probably about a hundred and fifty miles wide from east to west and four hundred miles long from north to south. The whole area of the storm within the isobar 29.9 extended from New Caledonia, or a little to the west of it, to about the same distance to the west of Chatham Island, or nearly one thousand four hundred miles, and perhaps nine hundred miles across in its widest part.

On the 25th the centre of the storm, included within the isobar 29.3, had reached the line Castle Point to Wanganui with easterly wind, and thence round to Gisborne with westerly wind. The eastern boundary of the central area was probably two hundred miles eastward, of Napier. The total diameter of the storm within the isobar 29.9 was about one thousand two hundred miles, and it was approximately circular, the original pear-shape having been modified by the resistance met with in passing over New Zealand.

But now an easterly moving antarctic storm is approaching from the west, its centre being somewhere south-west of Tasmania, and its eastern edge, with north wind, close to the western edge of the tropical cyclone, with its south wind; the divide between the two circulations being about two hundred miles to the west of Milford Sound. (See Plate LIII.)

On the 26th the tropical storm has advanced to the southeast so far that the south-eastern edge of the centre isobar 29.3 has reached Chatham Island with its north-easterly wind. The general shape of the outline of the storm has been changed from the original pear-shape, with its long diameter lying north-west to south-east, into an oval, with its long diameter lying north-east to south-west, and the north-west isobar 29.6 about, one to two hundred miles to the east of New Zealand from the East Cape to Dunedin. In the meantime the antarctic storm has advanced eastwards, and has partially blended with the tropical storm, so that the isobars 29.9, 29.8, 29.7 include both storms; but there is a divide between the antarctic isobar 29.6 with north-east wind and the tropical isobar of 29.6 with south-west wind, a little to the east of the east coast of the South Island. The advancing edge of isobar 29.3 of the antarctic storm has just reached Milford Sound. Over the North Island lies a “high,” with anticyclonic circulation, included within the isobar 29.7, the

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highest pressure recorded being at Rotorua, 30.5; a divide with opposite circulations running round the west, north, and east of the northern part of the North Island.

This small anticyclonic “high” seems to have been formed during the blending of the outer parts of the two storms by the rapid onward movement of the antarctic storm, and probably the peculiar bend in the circulation of the antarctic isobar 29.6 at New Plymouth gives an illustration of the way in which the more rapidly progressing storm blends with the other which is moving more slowly. (See Plate LIV., fig. 1.)

On the 27th June the tropical storm has disappeared entirely; so also has the small anticyclonic “high” which was formed over the North Island. They seem to have neutralised one another, while the antarctic cyclonic circulation has closed in on both sides, and the eastern edge of its valley has advanced eastwards about half-way between Christchurch and Chatham Island, stretching north also into the open sea to the east of Napier. (See Plate LIV., fig. 2.)

The onward rate of progress of the tropical storm moving from north-west to south-east was about two hundred and fifty miles in twenty-four hours, while that of the Antarctic storm moving eastward was about five hundred miles in twenty-four hours, both being retarded by the land which their tracks crossed. These tracks crossed each other at an angle of about 45°; both were moving eastwards, but as the tropical storm moved also to the south its eastward velocity was not more than one-fourth of that of the antarctic storm, which therefore overtook it, and blended with it.

In the case last year, when two such storms repelled one another, they were moving, one from the north and the other from the south, without much easterly tendency in either case.

It is abundantly evident from my observations of a balanced wind-vane during the last year that the circulation in cyclones is chiefly upwards in combination with the horizontal circulation, and in anticyclones it is downwards; and the upward motion in the former is more apparent near the earth's surface than the downward motion in anticyclones, which is quite what might be expected. As yet I have been unable to detect any distinct indication of the upward motion being greater or less in the front part or rear part or on either side of a moving cyclonic circulation, and the motion is so complicated that no one has yet been able to follow it accurately. The numerous observations that are now being made in Europe and America with kites and balloons will, I hope, soon throw light on the subject; and may enable us to understand the interaction of cyclones upon one another.