LONG-RANGE TRANSMISSION OF TREE POLLEN TO SHETLAND

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1 New PhytoL (1973) 72, LONG-RANGE TRANSMISSION OF TREE POLLEN TO SHETLAN III. FREQUENCIES OVER THE PAST HUNRE YEARS BY J. B. TYLESLEY Lerwick Observatory, Shetland {Received 13 November 1972) SUMMARY The catches of tree pollen at Lerwick in are related to Lamb's catalogue of daily weather types. This is done by examination of weather maps on particular days, and statistically by finding the effectiveness of certain weather types for pollen transport, and by examining the source areas of trajectories. It is found that in April-July, of all days when the surface track is from likely source areas, 50-80% bring measurable amounts of tree pollen to Shetland, depending on the weather type. The results are applied to statistics of the frequencies of the different weather types. It is estimated that the number of days per year of airborne tree pollen at Lerwick is , and that there has been no secular trend in this figure over the past hundred years. INTROUCTION Two previous papers (Tyldesley, 1973a, b) have reported catches of tree pollen at Lerwick, Shetland in 1970 and Results for 1972 are now presented, giving 3 years' sampling in all, but seasonal weather patterns vary a great deal from year to year and we wish to know how representative these three years are. The approach adopted is to identify the weather types over the British Isles on occasions of pollen catch, and to assess the effectiveness of each type in bringing tree pollen to Shetland. Statistics of effective and ineffective examples of each weather-type are assembled, and these are supported by case studies and statistics of trajectories on surface charts. The frequency of each weather type is known over the last hundred years, and this leads to estimates of the frequency of occurrence of tree pollen at Lerwick over the same period. RESULTS FOR 1972 In 1970 and 1971 the pollen season began in April, but in 1972 deciduous pollen was seen on a few days in the second half of March, and for the first time alder and not birch was sometimes the predominant species. Little was seen in April, but May produced substantial catches of deciduous pollen, with birch predominating. On 6 May a daily mean concentration of 80 birch grains m~^ was recorded, the previous highest value being 22 grains m"^ in May In June there were several substantial catches of pine pollen, up to 25 grains m"^. Smaller amounts were recorded at the end of May, and up to mid-july. Thus 1972 was akin to 1970 as regards pine pollen, while 1971 had been almost a complete blank. 691

2 692 J. B. TYLESLEY ate (1972) 18 March I May 4 May 6 May 7 May 9 May 12 May 17 May Table i. Birch pollen deposition at Lerwick 1972 It (Unots) I i-i V (cm second" ') C (grains m~^ aily deposition ) (grains cm~ Sum of daily totals gives total annual deposition of grains cm ^ The daily deposition of birch pollen has been calculated as before (Tyldesley, 1973b) and the results are given in Table i. The annual total of 18 grains em"^ is comparable with 1970 (16 grains cm"^) and 1971 (12 grains cm""^). However, in 1972 the annual total is dominated by one day, 6 May. WEATHER TYPES FOR POLLEN TRANSPORT Lamb (1950) has classified the predominant weather over the British Isles for each day from the present back to His twenty-seven categories specify the prevailing wind direction, and whether the curvature of the isobars was cyclonic or anticyclonic. E. B. Smith (1972) has used the classification to estimate the frequency of trajectories from the Midlands of England to various parts of the Continent, in connection with the effect of atmospheric sulphur dioxide on Swedish lakes. The present work is based on Smith's, but whereas the Midlands are roughly central in the area to which the classification applies, Shetland is on the northern boundary. This means that the weather type assigned to Britain as a whole, may not always be directly applicable to Shetland. Since this paper was prepared. Lamb (1972) has produced a definitive version of his catalogue, extending back to He points out (p. 9) that in these early years there was only very sparse data to the north of the British Isles and over the Atlantic. Less confidence attaches to the classification for these years, particularly in the area with which the present study is concerned. It was therefore decided not to extend the analysis to include the years before As afirststep all days in the 3 years' sampling with tree pollen concentrations of i grain m"^ or more were listed, together with Lamb's weather category for the day. There were 87 such days, and twenty-three of the twenty-seven weather types were represented. This result, discouraging for a relation between the Lamb types and pollen transport to Shetland, was tempered by the discovery that most of the weather types either occurred but rarely, or were associated with only small concentrations of tree pollen. Eive types, occurring between them on 60% of occasions, brought 90% of the tree pollen. They were:, easterly anticyclonic; E, easterly unspeeified; E, easterly eyclonic; Y, nondirectional anticyclonic; Z, non-directional cyclonic. The remainder of this paper will be concerned with these five types. POLLEN TRANSPORT FOR THE FIVE TYPES The months April, May and June were taken as the tree pollen season, the few oecur-

3 Pollen to Shetland. Ill 693 rences in March and July being ignored since the concentrations were small. For each of the sampling years , the number of days of each type bringing pollen, and also the total number of days of these types, were found (Table 2). These are totalled for the 3 years and the ratio of the totals for each type gives a measure of that type's effectiveness in bringing tree pollen to Shetland. Marked differences occur. Types E and F are effective but rather infrequent; types Y and Z contribute more days because although less effective they are common. Type is both infrequent and ineffective (in the sense of number of occasions) but is included in the list because it is associated with high concentrations. Table 2. Tree pollen days at Lerwick and weather type, April-June Totals ays bringing pollen as % of all days Weather type E F Y Z ays bringing pollen I 4 0 I I Total I 9 E II Weather type F Y All days 3 II 2 27 I Z Tot SOME TYPICAL TRACKS FOR THE FIVE TYPES In Tyldesley (1973a) it was shown that transport of tree pollen to Shetland in any quantity is always associated with an air track at low level from likely source areas in Britain or the Continent. Not every such track brings measurable pollen, because there may be low output at the source or washout or high dilution along the track. A suitable track is a prerequisite, however. Therefore days of the five types which did and did not bring pollen to Shetland have been examined so that this aspect may be understood. They are described as 'successful' and 'unsuccessful' respectively. Fig. I. shows typical weather maps for each Lamb type, successful and unsuccessful occasions being shown side by side. The following brief interpretations are offered. Type. Successful: a track from the Baltic area round an anticyclone over Scandinavia. Unsuccessful: the centre of the anticyclone is over the North Atlantic and the track to Shetland is from the north. Type E. Successful: high pressure to the north of Britain and a direct track from Scandinavia. Unsuccessful: centre of anticyclone just north of Shetland, and winds light and variable. Type F. Successful: depression over Spain; direct track from Low Countries. Unsuccessful: depression over Biscay, but track to Shetland is north-easterly and just misses the Norwegian coast. Type Y. Successful: anticyclone over southern North Sea and track to Shetland over Southern England, Ireland and Scotland. Unsuccessful: over England there is an anticyclone but fronts arc passing from the west around its northern f^ank and over Shetland. Type Z. Successful: low over the Channel; track from south Baltic to Shetland. Unsuccessful: low west of Ireland; track to Shetland from the north.

4 Weather type SUCCESSFUL UNSUCCESSFUL 5 June June May June 197(j 7 May April March May April June 19?! Fig. I. Some typical surface weather maps for five of the Ivamb weather categories. 'Successful' tree pollen caught at l.erwicu. 'Unsuccessful' no tree pollen caught. All charts are for hours GMT on dny shown.

5 Pollen to Shetland. Ill 695 STATISTICS OF TRAJECTORIES FOR THE FIVE TYPES It is clear that although certain Lamb weather types are favourable for pollen transport, none of the five types always produces a track to Shetland. This section investigates how often a blank sample is due to this, and how often to other causes. Trajectories to Shetland were prepared for all days of types, E, F, Y and Z in the months March-June , and some additional days were taken in to increase the sample of the less frequent types, E and F. The choice of years was governed by availability of aily Weather Reports of the Meteorological Office, on which the trajectories were constructed by the method described in Tyldesley (1973a). The first intersected land was described as the source area, and classified as Scandinavia, Low Countries or Britain. Other tracks were pursued until it was clear that they had been over the ocean for several days at least. Some interesting results emerged in detail. For instance, type produced no sources in the Low Countries, and type E none from Britain. However, the sample was not large enough in most cases to consider each source area separately, so the tracks were described finally either as from a source area or not. Table 3. Statistics of trajectory source and pollen catch (a) Number of days in trajectory sample (b) Number of days from source areas (c) % of days from.source areas = ioo (b)/(a) (d) "/ of days in wbich brought pollen (e) pollen days as %, of source days = 100 (d)/(c) Weather F E type Y Table 3 gives the results. The first line shows that at least 20 days of each weather type were used for trajectories, except for type F for which only 6 days could be found. The next line shows the number of days when the tracks came from source areas, and the third gives these figures as a percentage. The fourth line gives for comparison the percentage of days which brought pollen. These are less in each case than the percentages from source areas, which is reassuring as the samples are rather small and the periods considered not the same. This deficit is due to factors previously mentioned such as low production and loss in transit, and in the final line is shown the number of pollen days as a fraction of the source days, expressed as a percentage. This shows that of days with winds from suitable sources, between 50% and 80% bring pollen to Shetland. The highest values are for the easterly types E and F. These usually give fine weather in the north of Britain. The lower figure of 51% for the anticyclonic easterly type is at first surprising. However, as pointed out in the section on typical tracks, this type for Britain as a whole often means that disturbed weather is not far to the north of Shetland, even when the track is favourable. Types Y and Z both give about 50%, the anticyclonic type Y showing a rather higher figure, which is understandable since it is associated with fair weather and limited vertical spread of the pollen. We conclude that the figures derived in Table 2 for the percentage of days of each type which brought pollen to Shetland, make sense in terms of the associated tracks and weather, and that despite the smallness of the sample may be cautiously applied to other periods. THE LAST HUNRE YEARS If the percentage of days bringing pollen were applied to annual frequencies of the Z

6 696 J. B. TYLESLEY different weather types it would obviously lead to large errors since the sample of days of any type in any one pollen season may be only one or two. Instead the percentages have been applied to decade means. This does not prevent an estimate of year-to-year variability, as will be shown later. Table 4 shows means of days of each of the five weather types in the months April- July, and the estimates of pollen days per year obtained from them by the application of the percentages derived in Table 2. The contributions of each weather type to the estimated number of tree pollen days are displayed as a nomogram in Fig. 2. Although the Table 4. Annual means of frequencies of the five weather types in April-July, and derived frequencies of tree pollen days at Lerwick ~' I Means Weather type E F Y All days I.I I.O I z II Weather type :ve E F Y Z Total Estimate of tree pollen days I Fig. 2. The estimated contributions to the number of tree pollen days per year at Lerwick, of the five most significant weather types., easterly antieyelonic; E, easterly unspecified; F, easterly cyclonic; Y, non-directional antieyelonic; Z, non-directional cyclonic.

7 Pollen to Shetland. Ill 697 contributions of each type and particularly the easterly types, vary from decade to decade, the totals are not very variable and no secular trend is apparent. The mean is 15.0 days per year, with standard deviation of the mean (for the samples of 10) of 1.02 days per year. (For this purpose the period which is only seven years, has been treated on the same basis as the nine io-year periods which follow.) If 5 is the standard deviation of means over 10 years, and s the standard deviation for individual years, we have (assuming the values to be normally distributed) 10* or ^ = 1.02 X 10* = 3.3 days per year This estimate of 15 ±3 pollen days per year, when compared with the 18, 22 and 12 days found in 1970, 1971 and 1972 respectively, shows that the 3 years of detailed sampling were probably typical of the last hundred years. These conclusions relate only to the number of days with pollen catch above a certain threshold, and are not immediately applicable to the amount of pollen in the air. ISCUSSION Looking backwards, palynologists will note with interest the evidence that tree pollen days at Lerwick have shown no secular trend over the past hundred years, despite noticeable changes in climate over that time. It would be dangerous, although tempting, to extrapolate this result to more distant periods when the circulation of the atmosphere in these latitudes may have been quite different. Looking forward, a worker planning to use pollen arriving at Lerwick say as a subject for study of deposition after long-distance travel ean expect between 9 and 21 suitable days per year, with 95% confidence. ACKNOWLEGMENTS The author thanks a number of colleagues who have given help and advice, and particularly r F. B. Smith for making available his unpublished work. The paper is published with the permission of the irector-general of the Meteorological Office. REFERENCES LAMB, H. H. (1950). Types and spells of weather around the year in the British Isles. Q.JIR. Met. Soc, 86, LAMB, H. H. (1972). British Isles weather types and a register of the daily sequence of eirculation patterns Meterological Office Geophysical Memoirs, No H.M.S.O. London. SMITH, F. B. (1972). A contribution to the estimation of pollutant dosages arising from a U.K. source using a simplified trajectory method. Unpublished note. TYLESLRY, J. B. (1973a). Long-range transmission of tree pollen to Shetland. I. Sampling and trajectories. New Phytol., 72, 175. TYLRSLF.Y, J. B. (ig73b). Long-range transmission of tree pollen to Shetland. IL Calculation of deposition. New Phytol., 72, 183.

National Meteorological Library and Archive

National Meteorological Library and Archive Fact sheet No. 4 Climate of the United Kingdom Causes of the weather in the United Kingdom The United Kingdom lies in the latitude of predominately westerly