SUM MARYThe paper attempts to clarify certain points raised during the discussion on orographic rain held by the Society on February 2 1 , 1945 (Q.J., 71 p. 41). It is shown that rail1 in the warm sectors of depressions plays a large and ofttn dominant part in large falls of orographic rain. A brief description is given of the outstanding orographic rains since rgog, with a note on the synoptic situation. Some recent cases with upper air information bring out the importance of a deep damp layer (almost or quite saturated) in the warm air, and it is considered that convergrncc is the main factor in producing this, though other factors also are taken into account It is shown that potential instability in the warm air is not a neccssary factor for heavy orographic rainfall. A general discussion of this subject is included.
A.-THE RAINFALL OF THE BRITISH ISLES.THE rainfall during 1921 in the British Isles, especially in the southeast of England, was so remarkable that it was considered worthy of a special investigation. This afforded an opportunity of considering the question of droughts in general. The objects of this section of the paper are therefore really twofold. In the first place, it is proposed to indicate some of the ways in which the rainfall of 1921 was unprecedented, and secondly, to discuss the comparison between 1921 and previous dry periods in such a manner that in the second half of the paper they can be considered in relation to the general circulation of the atmosphere.The great mass of statistical information bearing on the rainfall of the British Isles accumulated by the British Rainfall Organization affords an immense amount of material for an investigation of this nature, the very quantity of which makes it impossible, at such an early date, to discuss the whole exhaustively, and the comparison deals with the matter only in broad outline.The Year 19dl.-Considering the year as a whole, it is seen at once that 1 9 2 1 was, in certain areas, a year of unprecedentedly small rainfall. Sufficient data exist to construct maps showing the percentage of the average' rainfall occurring in the British Isles in each year back to about 1850. Sincc then there is no doubt that the only years which are in any way comparable with 1921 were 1854, 1864, 1870, and 1887. The last mentioned was by far the driest year hitherto known to have occurred in the British Isles. The year lS5S was also dry, especially in England and Wales, but not quite as dry as any of the other years mentioned. Space permits of only a brief account of the maps for these yews :-In 1854 data are wanting for Scotland and Ireland, but the rainfall deficiency seems to have been confined mainly to England and Wales. Isolated areas in the north-west of England had just above the average, and along a coastal strip of the west coast more than 80 per cent. of the average occurred. The deficiency-reached a maximum in the south-east, where it was more than 30 per cent. to the east of a line roughly from Bournemouth to Lincoln.In 1864 areas with excess were confined to the north-enst of Rngland and the centre of Scotland. Practically the whole of Ireland and the north-west of Scotland had between 100 and 85 per cent. of the average. Smaller percentages occurred only in England, where, south of a line from Carmarthen to Spurn Head, less than 80 per cent. was general. Over large areas along the east coast and in Devon and Herefordshire the deficiency exceeded 30 per cent.I n 1870 rainfall above the average occurred only in the North and East Ridings of Yorkshire. Deficiencies of more than 20 per cent. were recorded widely in the west and centre of Scotland, the south The average nsed is that for the period 1881-1915, unless Stated otherwise.
SUMMARYA detailed analysis of the rainfall of Adelaide, South Australia, has shown that throughout the gj years 18391933 there has been a definite oscillation, with a period and amplitude of approximately 23 years and 30 days respectively in the incidence and duration of the winter rains. The amplitude, though small, is nevertheless about 2 0 per cent of the length of the rainfall season. The total quantity precipitated has shown no statistically significant changes. I STRODUCTORYThe general characteristics of the Australian rainfall with respect to geographicaI and seasonal distribution and reliability have been the subject of numerous investigations by different workers. In dealing with the broad mass of data, only the simpler statistics are normally made use of, but it was considered desirable t o ascertain how far recent methods developed by Professor R. A.Fisher ( I ) could be applied to the specific case of the rainfall record of Adelaide, particularly as a clearer knowledge of its characteristics is of some considerable value from the point of view of agricultural production. DATAOn January Ist, 1839, Sir George Kingston established a daily rainfall record in Adelaide on a site approximately 500 yards from the present position of the Observatory. This record was continued until November, 1879. From May, 1860, to the present day, the readings hate been taken a t the Observatory so that over 19 years the two sets of observations were concurrent. During this interval the average annual difference between the gauges was 0.26 inch (Kingston's being the higher), and considering the proximity of the sites it may be taken that the two series in combination give a continuous and practically uniform record of the Adelaide rainfall.No definite statement could be found as to the diameter of the gauge employed by Kingston, nor, in fact, the size of the gauge used in the early days of the existence of the Observatory. It is fairly certain, however, that no radical departure could have been made from the standard 8 in. gauge which has been in use since I 870. SECULAR VARIATION OF THE RAINFALL AT ADELAIDEIt is noteworthy that this record is probably unique, both with respect to its continuity and length, and constitutes valuable material for statistical analysis.At this juncture a word might be added on the character of the climate. The most prominent feature is the marked winter incidence of the rains. Approximately 7 & per cent of the annual precipitation, the average of which is in the neighbourhood of 2 1 in., occurs within the period April to October, the summer months being characterised by hot dry atmospheric conditions, low rainfall and high evaporation. ANALYSISThe method employed in this analysis was devised by R. A. Fisher (Zoc. cit.) and in brief consists of the following steps.The rainfall of each year was divided into 61 six-day totals and to these was fitted a series of orthogonal polynomial functions of the fifth degree in time, thus furnishing six constants with which to express the quantity and distribut...
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