THE ANTI-TEADES OE PASSAGE WINDS. ISST' 



"The setting in of a Southerly wind with a decreasing pressure imme- 

 diately after a North-Westerly wind on the outward passage is illustrated 

 by the diagrams,"^ which explain the nautical saying that the wind backs to' 

 blow. Experience shows that the common habit of the wind in these 

 latitudes is to back from N.W. to South, the exception being when it com- 

 pletes the circuit and goes to North and East." 



(116.) Captain Toynbee, in his pamphlet on " Weather Forecasting," 

 remarks : — Systems of low barometric pressure change their positions and 

 sometimes move very quickly, generally having a high barometer to their 

 right and a low barometer to their left as they proceed along their tracks. 

 Considering the general disposition of barometric pressure in the Northern 

 part of the Atlantic, which has been alluded to in (26), we have the reason 

 why the Atlantic storms generally move along an Easterly track, and 

 often diverge to the North-Eastward as they approach our islands ; in fact, 

 they have a tendency to move as subsidiary systems round the South and 

 East sides of a prevailing area of low barometric pressure which has its 

 centre somewhere to the Southward of Iceland. 



(117.) Cyclones and Anti-Cyclones. — Before proceeding farther with this 

 subject, it is necessary that some description should be given of the two 

 classes of atmospheric disturbance, to which several allusions have been 

 made in the preceding pages, especially in Section 2, on " The Motions 

 and Pressure of the Atmosphere," pages 108 — 124. Farther on, when we 

 describe the Winds of the British Isles, we shall give further particulars 

 respecting them, illustrated by diagrams showing the distribution of 

 Clouds and Weather. 



(118.) Cyclones. — The term "Cyclone," says Mr. Ley, "was originally 

 only applied to the Hurricanes of the lower latitudes, but since the laws 

 which connect the wind with the distribution of pressure have been shown 

 to be universal, it is obvious that around any area of reduced pressure the 

 wind must circulate in the same general way as it circulates in a Tropical 

 Hurricane. Now the areas of reduced pressure, though subject to great 

 variety of figure, approximate, generally speaking, to circular or oval forms, 

 as we should, from our general knowledge of the motion of fluids, expect 

 to be the case. All such systems of reduced pressure, and their co-existing 

 systems of currents, are therefore spoken of as " Cyclonic"! 



In a Cyclonic system, in the Northern Hemisphere, the wind blows 



round a central area of low barometric pressure, in 

 a direction contrary to the hands of a watch, draw- 

 ing slightly imoards towards the centre, as shown 

 in the accompanying diagram. From this the in- 

 ference is drawn that within a relatively low baro- 

 metric area the air is ascending, consequent on the 

 condensation of aqueous vapour over a large area, 

 while the surrounding air is forced in by the higher 

 pressure upon it. 



* " Report on the Meteorology of the Norbh Atlantic, between lat. 40° and 50° N.," 

 by Captain H. Toynbee. 

 t " Aids to the Study and Forecast of Weather," by W. Clement Ley, M.A. 



