A SIMPLE SUMMER WEATHER INDEX

By TREVOR BAKER Southergt Independent Television, Southampton

INCE 1962, when Poulter produced his summer index for London, other S contributors to Weather have shown a continuing interest in methods of categorizing our summers, the latest being the Davis Optimum Index (1968). All the indices proposed use the three elements temperature, rainfall (or rain- days) and sunshine weighted in various ways, and all use only the three months June, July and August.

AN INDEX

I wonder whether it would be appropriate now to suggest yet another index but this time one which would perhaps appeal to the average man rather than to the statistically-minded meteorologist. It must be completely objective, simple to calculate and applicable equally to all parts of the country so that comparisons between different areas would be easy.

How do the men, women and children in the street, or on the beach, judge good summers? Almost certainly by their dryness and sunshine. When the sun is shining it is not raining (to all intents and purposes), so sunshine hours are also dry hours. Of course there can be cloudy but dry weather, but this is not generally welcomed by the public especially when on holiday or off-duty in the evenings and week-ends. To put it another way, a cloudy day does not impress itself as a good summery day, and three months of cloudy dry weather would not be considered a good summer by anyone-not even a professional meteorologist.

In the same context the shade temperature is, I feel, of little significance to the public. A cloudy day with temperature 70°F is not as pleasant - or as warm - as a sunny day with shade temperature 60°F. Whenever the sun is shining in summer it is acceptably warm unless, early in the season, you are, being attacked by a strong breeze from a cool sea and are stripped down to trunks or bikini. It would make the calculation of any index very complicated if wind were included, so following the example of my distinguished predecessors in index-producing I shall ignore it.

Night-time rainfall is of little or no account; most people would consider rain a t night as a bonus in an otherwise dry spell. Even in daytime the actual . anlount of rain does not mean much; 12 hours of rain yielding + inch is as frust- rating to a family at the seaside or to a cricket team as 2 inches in the same period.

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So, on balance, it would seem that total sunshine hours might well give a very good and acceptable, if not perfect, indication of the goodness or otherwise of a particular summer season. The use of total sunshine hours as an index would meet two of the criteria mentioned earlier-objectivity and ease of calculation. Comparison between different parts of the country would be slightly affected by the increase of the duration of daylight with increasing latitude and the consequent greater sunshine potential of the north, but this is a very minor reservation. Sunshine totals in the south are generally higher than those of the north in spite of the latitude effect.

SUMMER

Having accepted this kind of summer index it might be worthwhile con- sidering what summer is. Meteorologists for their own valid statistical purposes call it June, July and August, but nowadays many people take their holidays, play their ' summer ' sports, enjoy their gardens, in May and September - particularly September in the case of holidays, because school vacations extend well into that month. I should think that the 5 months May to September are szrinnzer to the general public.

The Simple Summer Weather Index is therefore the total sunshine hours from May to September. Table I shows this index for the years 1900 to 1968 for Southampton. If we take an index of 1100 or more (quite arbitrarily) as indicating a very good summer, 12 of the summers of this century come into this category and 9 of them occurred between 1900 and 1933 -we've been short of very good summers in more recent years and in the present decade there has been none so far - with only 1969 to come.

TABLE I. Southampton Simple Summer Indcx 1900-1968

Year I 2 3 4 5 6 7 8 9

1900 1102 1153 go7 1008 1063 1024 1123 938 1095 1004 I910 867 1289 763 873 rogo rozB 953 933 1138 1043 1920 838 1122 961 1020 974 rogz 984 922 1104 1153 '930 961 764 734 1121 1095 1023 861 934 790 990 '940 1182 1043 953 1069 981 896 831 956 891 1222 1950 1006 990 1025 1027 799 1079 930 1004 836 1269 1960 gro 1019 go8 874 972 810 987 981 845

TABLE 2. Dccadal annual averages of Index

Decade Average

Annual Index

1900-1909 1910-1g19

1930-1939 194-1949 1950-1959 1960-1968 (incompletc)

1920-1929

1oq1.G 997'5 1017.1 927'2 1002.4 996.5 923'0

TABLE 3. The best summers at Southampton, 1900-1968

Simple Poulter Year Index Year Index

1911 1289 1911 778 I959 1269 I949 768 I949 I222 ' I959 752 I940 I182 1947, 745 1901 1153 1933 744 1929 I153 1906 732 1918 1138 192 I 730 1906 1123 I955 729 1921 1122 I925 726 I933 1121 I940 719

TABLE 4. The worst summers a t Southampton, 1900-1968

Year Simple Index

I932 734 1912 763 1931 764 I938 790 I954 799 I965 810 1946 831 I958 836 1920 838 1968 845

The sunniest decade of the century (Table 2) was 1900 to 1909, and unless 1969 clears its skies to the extent of 966 hours, this decade will be the cloudiest.

The best 10 summers are given in Table 3 and for comparison the Poulter Index 10 best are included. The three best summers were, in order, 1911, 1959 and 1949, while the Poulter Index (for June, July and August only) makes these same years the best but in the order 1911, 1949 and 1959. 7 of the Poulter 10 best summers are in the other list. It is obvious, as experience of 1959 will remind us, that our exceptional summers extend through much of May and September as well as the middle conventional summer months. The ten worst summers are given in Table 4. It is interesting that 3 of the cloudiest summers (1920, 1932 and 1958) were immediately followed by very good summers.

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1300-

1200-

0.'

3 m

L 1 m -

10

I

800-

/! I

'?.

700' z 1503 10 20 30 40 50 69 68

Year

Southampton Simple Sutnn?er Indcx Fig. I .

To confirm what has been pointed out in many previous contributions, the odd years have the edge over the even years for good summers. Of the top ten only 3 were even, but of the top twelve 5 were even. Over the period 1900 to 1968 the average summer sunshine in the odd years was 1020 hours and in the even years 956 hours - a convincing win for the odds.

One might be tempted to draw a smoot11 curve tl~rough the recent peaks (1929, 1940, 1949 and 1959) in Fig. I and extrapolate it to an all-century high value in 1969 - or could it be 1970? Only time will tell, but why shouldn't we be optimistic?

ACKNOWLEDGMENT

I am indebted to Mr D. N. Love of the Southampton Weather Centre for letting me have access to the (unpublished) Poulter Summer Indices for Southampton.

RHFERHWCES

DAVIS. N. E. 1g0Y An optitnuin suninier weather index. Weather, 23 (S),

POULTBR, R. 11. 1902 The next few summers in London. hid., 17 (S), PP. 305-3'7

1'P. 253-7

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