40 THE CLIMATE OF IRELAND. 



bre zes, experienced morning and evening under certain circumstances. 

 Anti-trade winds also, which set from the S.W. and W.S.W. in our lati- 

 tudes (Trade winds prevailing between g° N. and 30° N.) may, as was 

 held by Dr. Buchan, act a certain part in producing variability in wind 

 direction. The most influential factors, however, in causing change are the 

 winds eddying in cyclonic systems which reach Europe from tlie Atlantic, 

 sometimes in comparatively rapid succession. 



The origin of these storms is a subject of much debate upon which we need 

 not enter. They are by many believed to cross the Atlantic from shore to 

 shore. Prof. Loomis, an able authority, maintains that they undergo modi- 

 fication after leaving the American coast region, which of course interferes 

 somewhat with calculations as to the time and place at which they may reach 

 Europe.* The popular belief is that these points may be accurately fore- 

 told ; and it is xvorth noting that calculations in this respect are frequently 

 verified. 



It is, however, a fact generally accepted, that after traversing a great 

 distance across the Atlantic, these storms usually reach the European region 

 a little to the north of the British Isles, Ireland experiencing brushes of the 

 skirts of the vast aerial eddies in their easterly progress. Occasionally they 

 cross the British area, and cause a greater amount of meteorological dis- 

 turbance than usual. A peculiarity of these circular storms is that they 

 rotate, looked at downward, in a direction contrary to the hands of a watch, 

 north of the Equator. The centres are marked by reduced barometic read- 

 ings, the pressure of the atmosphere being there particularly low ; and the 

 pressure increases outward towards the margins of the eddies, where it is 

 approximately normal. As the storm passes over a locality therefore, the 

 barometer rapidly falls till the place is reached by the centre of the cyclone, 

 after which the barometer rises. And, as regard wind directions experi- 

 enced while the storm moves onward, if the centre passes to the north of 

 Ireland, in the way most usual, the direction of the wind will be perhaps 

 first S., then S.W., and then W. — the change, or veering as it is called, 

 being " with the sun." If it takes a more southerly course, and crosses Ire- 

 land, the winds will change " against the sun," backing, as it is called, with 

 a falling barometer, a condition which will be followed by a repetition of the 

 storm, and a rising barometer. 



By carefully mapping simultaneous barometric observations at many 

 places, and connecting the places of equal indicated atmospheric pressure 

 over large areas, the connecting lines {isobars as they are called), form 

 irregular concentric circles around the centres of the cyclones, and by com- 

 paring the positions of these circles from day to day, or more frequently, 

 the progress of the storm may be made apparent by means of maps. There 

 are also circles, obtainable in a similar way, surrounding points on these 

 weather charts where the barometer stands very high. These points are the 

 centres of " anticyclonic systems," around which winds circulate at rates not 

 dt all so rapid, and in an opposite direction to that uniformly observed in 

 cyclones ; the anticyclone circulates according to the hands of a watch. 



In summarising data for the preparation of a cyclone map from the 

 Weekly Records of the Meteorological Office, the present writer found that 



* The subject is discussed in a most interesting manner by M. Marie Davy in his Mefeorologic 

 Generale, pp. 223 to 234. Mr. R. H. Scott, F.R.S., treats the matter as one of doctors differing, 

 and perhaps wisely confines himself in his work on " Weather Charts and Storm Warnings" to 

 the simple questions of their existence, movements, effects, and characteristics. 



