FIFTY YEARS OF ENGLISH WEATHER By R. B. M. LEVICK

It is noteworthy that certain, types of weather, recognizable by their climatic features as well as the form of the isobars over and near the conntry, we inclined to persist for a considerable time-usually for a few days, and sometimes (especially the westerly type) for a few weeks.

Recently I have had the opportunity of examining the synoptic charts and weather reports of the past fifty years, and from these I have constructed a diagram covering the area of England and the period 1898-1947, which gives a complete picture of the occurrences of these types. It is, I believe, the general practice nowadays to describe weather in teims of types, although the number of types varies from very few to a great many, according to various systems of classification. I therefore decided to recognize six of them-the maximum number in which their characteristics appeared to me to be suffi- ciently distinct from one another-and called them the easterly, northerly- cyclonic, north-westerly , westerly , southerly and anticyclonic types, definitions of which are given below. I have experienced no difficulty in making .z definite classification on all daye except, possibly, four or five a year.

So far I have confined investigation to England, since it often happens that the weather in Scotland or Ireland is of a different type to that being experienced in England. For example, when a persistent anticyclone on the Chtinent extends over most of England, a depression usually exkta on the Atlantic, and troughs of low pressure moving north or north-east along our western seaboard cause continual cloud and drizzle in Scotland and Ireland, with strong winds between south and muth-weat-a distinctly southerly type of weather in these areas, but anticyclonic in England. In the same way it often happens that when an anticyclone is centred over Scandinavia, England is far enough away from it to be influenced by depressions to the south or eouth-west which are accompanied by an unsettled (easterly) type of weather, while conditions over Scotland are anticyclonic.

In making them investigations I have been attempting te provide an answer to the problem : does the f'requency of occurrence of the types vary in regular cyclee, either over a number of yearn or from month to month ? The answer is, apparently, that no regular cycle is in operation apart from an annual one which, however, is well-marked. It would appear from an examination of the two 25-year periods 1898-1922 and 1923-1947 separately, that moBt of the pronounced maxima and minima covering short periods of ten days or 80

would in the long run cancel ope another out-(though poeeibly not all of them : their positions in the two 25-year diagrams of the southerly.type show a distinct correapondence which may not be altogether due to chance). The annual cycle, therefore, is probably a gradual one. In addition, it is interesting to note that the frequency of the southerly type seems to have been gradually decreeeing throughout the fifty-yew period, and the sudden increase of the easterly type to about double the average in 1939,1940, 1941, 1940 and 1947 is striking.

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l'he six weat1u.r types are tlefinetl l~elow and the frequency of occurrence of these types for the period 1898-1947 is exhibited diagrammatically in Figures 1-3. In Figure 1, which gives a general picture of the weather during the 50 years, only two broad classifications have been employed, namely " anti- cyclonic " and " unsettled." Figure 2 shows the annual variation, arid Figure 3 tthe monthly variation of the six weather types.

Fig. 2. Frequency of weethx types, 1898-1948

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1. Anticyclonic Britain, or light winds.

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DEFINITIONS OF WEATHER TYPES

type. with a col between two anticyclones. Warm in summer ; misty in autumn ; very cold in winter.

Associated with an anticyclone centred over or near Mainly dry with

4 0 1

Fig. 3. Seannal frequency of weather typ3n

2. Eaeterly type. Associated with an anticyclone over Scandinevia or high pressure from Scandinavia to Iceland, and with a persietent low-preseure

m9

3.

4.

5.

6.

area to the SW. or S. of Britain. frequently move SE. from mid-Atlantic to the Bay of Biscay. cold from December to March with frequent snow. from June to August.

Northerly cyclonic type. Associated with an anticyclone to the west and north-west, producing a flow of polar air over the country. Depressions, often intense, move slowly southwards over Scotland and England or the Western European =aboard, or alternatively take the form of large stationary complex areas of low pressure while a belt of high pressure extend9 from the Greenland-Iceland area to the AzoreH. Cold and un- settled at all times of the year, with snow or sleet in winter.

North-westerly type. Occurs when the Azores anticyclone moves some- what north and east, with its centre between the Azores and Britain. Similar to the westerly type with its unsettled weather and changeable temperature, but colder on the whole, since the air of the warm sectors comes from higher latitudes.

Westerly type. Associated with the absence of anticyclones of any permanence near Britain, when a sequence of troughs, depressions and ridges moves from west to east across the country. Unsettled, with win& shifting rapidly between south and north-west, and occasionally eaat for a short time.

Southerly type. A m i a t e d with an anticyclone over Central Europe which prevents Atlantic depressions from moving eastward and tends to make them circulate in mid-Atlantic. Warm and thundery in summer ; very mild in winter with light to moderate rainfall and often strong winds from between south and south-west.

Depressions, often intense in winter, Intensely

Warm and thundery

Cool in summer ; mild in winter, with frequent gales.

CONCLUSIONS

There would appear to be no well-marked cycle with a period extencling over a number of years, but on the other hand there is a marked difference in the average frequency of the various types from month to month, with the result that the characteristic climate of a particular month is not always what one might expect. The occurrence of the various types during each month is summarized below.

Jmunry. Frequency of southerly and westerly types well above average ; that of anticyclonic type well below. Exceptionally unsettled, therefore, and generally mild for the time of year,

F e h n r y Increase in frequency of the easterly and northerly typee, with IL

corresponding decrease in the westerly and southerly type. Colt1 spells accordingly wre likely.

March Easterly type maximum ; northerly above average. Cold spells therefore even more likely than in February.

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April Northerly and easterly types still wcll above avcrage-also the north-weqterly. (:enerallg cool, therrforc, for the timc af the year, and unsettled.

J m e

July

Octoher

November

December

Marked increase in frequency of tlic anticyclonic type, at the expense of the wetrterly and north-wenterly types. (The weatarly type hns now reached a distinct minimum). The frequencies of the easterly and northerly types remain high, and there is a marked (temporary) incarease in the southerly type. During this month, therefore, the weather tends to go from one extreme to the other- exceptionally warm Hummer-like (lays (but often thundery) alter- nating with revernions to a wintry typc of weather.

Increaae in frequency of tlie westerly and north-westerly types iiiainly a t the expeme of tlie southcrly. Northerly typc xtill rthovr average. Unseasonably cool unsettled spells therefore tend to Iscome morc frequent, though warm Hcttled anticyclonic weather i N as frequent aR in May.

Marked decrease in frequency of the cnstcrly type, which i A OW

very rare indeed; the southerly type frequcncy is also below average, while that of the westerly type is increasing rapidly. Somewhat more cool and unsettled for the time of year than June, therefore, but with less likelihood of thunder.

Easterly type minimum ; northerly as wcll as southcrly type WCII below average, while frequencies of north-westerly and westerly typea are high. Mainly unsettled. therefore, and cool for thc time of year, though warm Bottled anticyclonic periods are not infrequent. Anticyclonic type maximum. There is also aomc incresec in frequency of tho southerly type a t the expense of the weatsrly. For the time of thc year, thcreforc, September tends to be somewhat warmer and more settled than August. Decrease in frequency of the anticyclonic type, while that of the westerly and easterly types is increasing. Becoming more unsettled. F'requency of the anticyclonic type continnee to decrease, while that of the unsettled types increases. The frequency of the southerly type, which ha# been increeeing gradually since August, has now reached a maximum ; while those of the northerly and a1.w the anticyclonic types are a t a minimum ; that of the emterly type is only average. Apart from very oc.ctwional cold spells, therefore, t h e month is unsettled and mild for the time of the year.

W e hope to piiblish similar artialm by R. B. M. Levick in due course on tlie weather of Sootland. Ireland and e~sewhere.-EDIToRs.

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