Tropical and Extra-tropical Cyclones. 



is for this reason that a rise of pressure so often precedes both 

 tropical and extra-tropical cyclones. 



The typical rotation of the wind round a cyclone is undoubtedly 

 that of an in-going spiral, counter-clockwise in the northern, clock- 

 wise in the southern hemisphere. In the commonest British type, 

 when the general distribution of pressure surrounding the cyclone is 

 higher to the south than to the north, the greatest incurvature is in 

 the right or south-east front of the depression. It is important to 

 note this, for we shall find a different position for the most incurved 

 winds in the China Seas and the South Indian Ocean. 



The author has not yet found a satisfactory instance of the centre 

 of the wind rotation not coinciding with the minimum of the baro- 

 meter on a synoptic chart. 



But when the sequence of wind and barometer trace at any single 

 station is looked at, it is sometimes found that the sudden shift of the 

 wind belonging to the centre occurs either before or after the actual 

 minimum. The author has investigated the details of several examples, 

 and found that the apparent anomaly is to be explained on the sup- 

 position that the depth of the cyclone was either increasing or 

 decreasing rather rapidly. 



For instance, in one case he found that the minimum of the baro- 

 meter preceded the change of wind by three hours, because though 

 the mercury was falling at the rate of 0'03 inch per hour owing to the 

 passage of the cyclone, the depression as a whole was filling up at the 

 rate of 0*07 inch per hour. The ba]ance of rise was therefore 0'04< inch 

 per hour. Full details of this example are given in a work by the 

 author on "Weather," which is now in the press. 



The rotation of the upper winds may be briefly stated thus : — At 

 a level of about 4000 — 6000 feet the wind is nearly parallel to the 

 isobars, and above that height tends to blow more and more outwards ; 

 but the amount of out-curvature varies in different parts of a cyclone, 

 and need not be particularised here. 



As a consequence of this we get the following law of the vertical 

 succession of wind currents from the surface of the earth : — Stand 

 with your back to the wind, and the successive layers of cloud will 

 come continually more and more from your left hand. In the southern 

 hemisphere the succession is reversed ; that is to say, the upper 

 currents come more and more from the right. 



In fig. 1 the surface winds in a typical British cyclone are marked 

 by full-line arrows, while the direction of the highest cirrus is denoted 

 by dotted arrows. 



The distribution of rain, cloud, and weather generally in a British 

 cyclone may be described thus. An area of rain, surrounded by a 

 ring-shaped district of cloud, is associated with every cyclone. The 

 rain area is not exactly concentric with the isobars, as it usually 



b 2 



