36 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



Dust and Wind at Alford. 



In examining into the cause of the fluctuations in the number of particles at Alford 

 as given in this year's observations, it is at once seen that, here also, the direction of the 

 wind is the principal agent in producing the changes. With the exception of the 8th 

 September, on all days when the wind was W. or N.W. the number of particles was small 

 and the air clear, and when the wind went southerly the numbers became great. 



As the meteorological conditions remained very constant during most of the time 

 while these Alford observations were being taken, we shall describe somewhat fully the 

 conditions prevailing during the period. An examination of the weather charts shows 

 that when these observations began, the weather over our area was very much governed 

 by anticy clonic conditions. During the beginning of the month an anticyclone lay to 

 the S. of our area, and the centre of high pressure had moved in a north-easterly 

 direction, and lay off the S.E. coast of England on the evening of the 7th. The 

 winds on this day were southerly over most of our area, and the air at our station was 

 impure air from inhabited districts. On the 8th, the day when the testing began, the 

 centre of the anticyclone began to move westwards. This caused a change in the 

 direction of the winds, making them more westerly over most of our area ; but the wind 

 was light, and, as the figures in the table show, it had not yet cleared away the impure 

 southerly air, as the amount of dust was great. On the 9th the centre of the anti- 

 cyclone had moved still further W. , and the winds over our area were now all westerly ; 

 and, as will be seen from the table, the pure westerly air had displaced the southerly air, 

 the number of particles having fallen greatly. On the 10th and 11th the centre of 

 the anticyclone still lay to the S.W. of our islands. The wind, therefore, continued 

 to blow from a westerly direction, and the air remained pure. On the 12th the anti- 

 cyclone again approached our islands, and its centre was over the Irish Sea. The wind 

 was, however, westerly at our station, and, being from the direction of the Atlantic, it was 

 still pure. On the 13th, however, the centre of high pressure continued to move east- 

 wards, and now lay to the S.E. of our station, near where it was on the 7th, and a 

 corresponding change took place in the circulation. The air no longer came in from the 

 direction of the Atlantic, but from the inhabited parts of our islands ; the position of 

 the anticyclone giving rise to southerly winds at our station. During the rest of the 

 time the anticyclone remained fairly constant, moving about a little, generally in an 

 easterly direction ; but even so late as the 23rd its centre still lay over Western 

 Europe. 



On the 15th a depression approached our islands from the W. and moved N.W. 

 outside our area. As the area of high pressure was at this date situated to the E., 

 the isobars were all parallel and regular, but their direction was N. and S., i.e., 

 for southerly winds at our station. Under this distribution of pressure the polluted air of 

 the inhabited parts of England and Scotland was carried to our station, and the number 

 of dust particles observed was high while the cyclone passed. 



