8 



NATURE 



[May 5, 1881 



may ultimately be possible to forecast British meteoro- 

 logical weather by means of magnetic weather some five 

 or six days previous to it. 



BAROMETRIC GRADIENT AND WIND 



I HAVE often felt surprised that the superiority in force 

 of northerly and easterly as compared with southerly 

 and westerly winds accompanying any given amount of 

 barometric gradient has. at least until recently/ excited 

 but httle attention, seeing that the superiority in question 

 is almost sufficient to suggest itself to any student of 

 daily weather-charts. The comparison of anemometric 

 records for the elucidation of this subject can only be im- 

 perfectly made, owing to the fact that there are very few 

 situations at which an instrument can be erected which 

 shall have a really equal exposure to winds from all points 

 of the compass ; neither is it possible, as I think, in com- 

 paring anemographic records from stations at our different 

 coasts to eliminate the various effects of local inequalities 

 of the earth's surface upon the force of the winds. There 

 are two methods which can be employed in the investiga- 

 tion of this question, which seem to yield reliable, though 

 necessarily somewhat rough and imperfect, results. One 

 of these is to examine separately the anemographs of our 

 imperfectly, but moderately well-exposed, inland stations, 

 in relation to various values of barometric gradient in 

 different directions. The other method is to discuss the 

 means of estimated wind forces in relation to amount and 

 direction of gradient for a large number of years and at 

 a large number of stations. I have hitherto but partially 

 and tentatively employed these two methods, but the 

 results arrived at may possibly be of interest to some 

 readers of Nature. The mean wind velocities at Stony- 

 hurst Observatory, obtained by me from the hourly 

 readings published by the Meteorological Committee for 

 the years 1874 to 1876 inclusive, for different moderate 

 amounts of atmospheric gradient are as follows : — 



The mean wind velocities at Kew Observatory for the 

 same period for similar gradients are as follows : — 



This shows that for any given (moderate) gradient 

 winds from north and east points are stronger than those 

 from south and west points at these stations. The second 

 method, in which the estimated wind-forces have been 

 employed, has been tried by me in the cases of twelve of 

 our English stations for periods varying from ten to three 

 years. The stations examined have been Shields, York, 

 Nottingham, Liverpool, Hurst, SciUy, Dover, London, 



' Sprung, "Sluiien uber den Wind und seine Beziehungen zum Lufi- 

 dkick," ii. p. 6. 



Oxford, Cambridge, Yarmouth, and Jersey. At all these 

 stations, excepting Liverpool and Jersey, with very low 

 gradients (viz. from 'ooi to -005 inch for fifteen miles), 

 mean estimated wind forces from points between north- 

 north-west and south-east, inclusive, have been higher 

 than those from points between south-south-east and 

 north-west inclusive. With the higher gradients we neces- 

 sarily find results opposed to this in the cases of stations 

 having a good exposure on the west or south and a bad 

 exposure on the north or east, just as, on the other hand, 

 we find the result above mentioned unduly heightened at 

 stations which have only a good east or north exposure. 

 If however we take stations whose exposure, though not 

 unexceptionable, seems tolerably fair, we find that with 

 somewhat steep as well as with low gradients, north and 

 east winds accompanying any given amount of gradient 

 have a higher estimation than south and west winds 

 accompanying the same. The following table shows 

 results at which I have aiTived from an examination of 

 the reports from three stations, viz. the two inland stations 

 of Nottingham and London and the one sea station of St. 

 Mary's, Scilly, which last, while very well exposed to all 

 winds, is perhaps most perfectly so to those from the 

 Atlantic. 



Gradient in 

 inches per fiftei 

 nautical mile: 



■001 to -005 

 •C05 to -010 

 -010 to -015 

 -015 to -020 

 •020 to -025 



•OOI to -005 

 ■005 to "OIO 

 •010 to -015 

 •015 to -020 

 -020 to "025 



•QOI to "005 



•005 to -OIO 

 -OIO to -015 



•015 to ^020 



s- « 



043 



1-88 



2-99 



3-64 

 4-41 



091 



I '45 



2'24 

 3-01 

 3-64 



2-42 



4-24 



5-45 

 6-40 



S-2 

 12-4 

 17-9 

 21-2 

 25-0 



7-5 

 10-2 

 14-2 

 18-0 

 21-2 



24-2 



3o'S 

 36-4 



I -06 

 2-26 

 4-06 

 4-61 

 5-35 



1-31 



2-00 

 2-93 

 4-18 



4-85 



2-49 

 4-86 

 5-68 



6-49 



a'g >^x. 



8-3 

 14-3 

 23 '3 

 26-0 



29"9 



9-S 

 13 o 

 17-6 

 23-9 

 27-2 



'S'4 

 27-3 

 317 

 36-9 



The suggestion which I off"er in explanation of this 

 difference of force in the two classes of winds is made 

 with some diffidence, since it involves a hydrodynamical 

 question, the solution of which is somewhat difficult. 

 Since the atmosphere is of greatest density near the 

 poles, while barometric pressure is less near the poles 

 than over the tropics, the pole-ward, and, under the 

 effects of the earth's rotation, eastward movements of the 

 atmosphere, at any given considerable altitude above the 

 earth' s surface, must necessarily greatly exceed the corre- 

 sponding movements at the surface of the earth. " The 

 planes of equal pressure receive," in short, "an ellipsoidal 

 form, the major axis of which is perpendicular to the axis 

 of the earth."' Thus the polar areas of Ipw pressure must 

 be far more permanent and far better marked in the 

 upper than in the lower regions of the atmosphere ; con- 

 sequently gradients for westerly winds when occurring at 

 the earth's surface must commonly extend into the higher 

 regions of the atmosphere ; while gradients for easterly 

 winds must, on the contrary, be usually accompanied by 

 gradients for westerly winds at no great distance above 

 them. Observations of the movements of the upper 

 clouds, and also of the winds experienced at the summits 



' H.mn, '-^eitschrift der oslerreich. Ges. liir Meteorologie," vol. .\iv. 



