Singularities and natural seasons

Popular weather lore expresses the belief that each season has its own weather (for example, in England, 'February fill-dyke' and 'April showers'). Ancient adages suggest that even the sequence of weather may be determined by the conditions established on a given date. For example, forty days of wet or fine weather are said to follow St Swithin's Day (15 July) in England; sunny conditions on 'Groundhog Day' (2 February) are claimed to portend six more weeks of winter in the United States. Some of these ideas are fallacious, but others contain more than a grain of truth if properly interpreted.

The tendency for a certain type of weather to recur with reasonable regularity around the same date is termed a singularity. Many calendars of singularities have been compiled, particularly in Europe. Early ones, which concentrated upon anomalies of temperature or rainfall, did not prove very reliable. Greater success has been achieved by studying singularities of circulation pattern; Flohn, and Hess and Brezowsky, have prepared catalogues for central Europe and Lamb for the British Isles. Lamb's results are based on calculations of the daily frequency of the airflow categories between 1898 and 1947, some examples of which are shown in Figure 10.7. A noticeable feature is the infrequency of the westerly type in spring, the driest season of the year in the British Isles and also in northern France, northern Germany and in the countries bordering the North Sea. The European catalogue is based on a classification of large-scale patterns of airflow in the lower troposphere (Grosswetterlage) over Central Europe. Some of the European singularities that occur most regularly are as follows:

1 A sharp increase in the frequency of westerly and northwesterly type over Britain takes place in about mid-June. These invasions of maritime air also affect central Europe, and this period marks the beginning of the European 'summer monsoon'.

Figure 10.7 The percentage frequency of anticyclonic, westerly and cyclonic conditions over Britain, 1898 to 1947.

Source: After Lamb (1950), by permission of the Royal Meteorological Society.

2 Around the second week in September, Europe and Britain are affected by a spell of anticyclonic weather. This may be interrupted by Atlantic depressions, giving stormy weather over Britain in late September, although anticyclonic conditions again affect central Europe at the end of the month and Britain during early October.

3 A marked period of wet weather often affects western Europe and also the western half of the Mediterranean at the end of October, whereas the weather in eastern Europe generally remains fine.

4 Anticyclonic conditions return to Britain and affect much of Europe in about mid-November, giving rise to fog and frost.

5 In early December, Atlantic depressions push eastward to give mild, wet weather over most of Europe.

In addition to these singularities, major seasonal trends are recognizable. For the British Isles, Lamb identified five natural seasons on the basis of spells of a particular type lasting for twenty-five days or more during the period 1898 to 1947 (Figure 10.8). These seasons are as follows:

1 Spring to early summer (the beginning of April to mid-June). This is a period of variable weather conditions during which long spells are least likely. Northerly spells in the first half of May are the most significant feature, although there is a marked tendency for anticyclones to occur in late May to early June.

2 High summer (mid-June to early September). Long spells of various types may occur in different years. Westerly and northwesterly types are the most common and they may be combined with either cyclonic or anticyclonic types. Persistent sequences of cyclonic type occur more frequently than anti-cyclonic ones.

3 Autumn (the second week in September to mid-November). Long spells are again present in most years. Anticyclonic types are mainly in the first half, cyclonic and other stormy ones generally in October to November.

4 Early winter (from about the third week in November to mid-January). Long spells are less frequent than in summer and autumn. They are usually of westerly type, giving mild, stormy weather.

5 Late winter and early spring (from about the third week in January to the end of March). The long spells

Figure 10.8 The frequency of long spells (twenty-five days or more) of a given airflow type over Britain, 1898 to 1947. The diagram showing all long spells also indicates a division of the year into 'natural seasons'.

Source: After Lamb (1950), by permission of the Royal Meteorological Society.

at this time of year can be of very different types, so that in some years it is midwinter weather, while in other years there is an early spring from about late February.

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