British airflow patterns and their climatic characteristics

The daily weather maps for the British Isles sector (50 to 60°N, 2°E to 10°W) from 1873 to the present day have been classified by H. H. Lamb according to the airflow direction or isobaric pattern. He identified seven major categories: westerly (W), northwesterly (NW), northerly (N), easterly (E) and southerly (S) types

France Continentality France

Figure 10.2 Continentality in Europe. The indices of Gorczynski (dashes) and Berg (solid lines) are explained in the text.

Source: Partly after Bluthgen (1966).

Figure 10.2 Continentality in Europe. The indices of Gorczynski (dashes) and Berg (solid lines) are explained in the text.

Source: Partly after Bluthgen (1966).

Hythergraph
Figure 10.3 Hythergraphs for Valentia (Eire), Bergen and Berlin. Mean temperature and precipitation totals for each month are plotted.

- referring to the compass directions from which the airflow and weather systems are moving. Cyclonic (C) and anticyclonic (A) types denote when a low-pressure or high-pressure cell, respectively, dominates the weather map (Figure 10.4).

In principle, each category should produce a characteristic type of weather, depending on the season, and the term weather type is sometimes used to convey this idea. Statistical studies have been made of the actual weather conditions occurring in different localities with specific isobaric patterns - a field of study known as synoptic climatology. The general weather conditions and airmasses that are to be associated with the airflow types identified by Lamb over the British Isles are summarized in Table 10.1.

On an annual basis, the most frequent airflow type is westerly; including cyclonic and anticyclonic subtypes, it has a 35 per cent frequency in December to January and is almost as frequent in July to September (Figure 10.5). The minimum occurs in May (15 per cent), when northerly and easterly types reach their maxima (about 10 per cent each). Pure cyclonic patterns are most frequent (13 to 17 per cent) in July to August and anticyclonic patterns in June and September (20 per cent); cyclonic patterns have >10 per cent frequency in all months and anticyclonic patterns >13 per cent.

Climatic Types And Their Charateristics
Figure 10.4 Synoptic situations over the British Isles classified according to the primary airflow types of H.H. Lamb.

Source: Lamb; O'Hare and Sweeney (1993). From Geography 78(1). Copyright © The Geographical Association and G. O'Hare.

Figure 10.5 illustrates the mean daily temperature in central England and the mean daily precipitation over England and Wales for each type in the mid-season months for 1861 to 1979.

The monthly frequency of different airmass types over Great Britain was analysed by J. Belasco for 1938 to 1949. There is a clear predominance of northwesterly to westerly polar maritime (mP and mPw) air, which has a frequency of 30 per cent or more over southeast England in all months except March. The maximum frequency of mP air at Kew (London) is 33 per cent (with a further 10 per cent mPw) in July. The proportion is even greater in western coastal districts, with mP and mPw occurring in the Hebrides, for example, on at least 38 per cent of days throughout the year.

Airmass types can also be used to describe typical weather conditions. Northwesterly mP airstreams produce cool, showery weather at all seasons. The air is unstable, forming cumulus clouds, although inland in winter and at night the clouds disperse, giving low night temperatures. Over the sea, heating of the lower air continues by day and night in winter months, so showers and squalls can occur at any time, and these affect windward coastal areas. The average daily mean temperatures with mP air are within about ±1°C of the seasonal means in winter and summer, depending on the precise track of the air. More extreme conditions occur with mA air, the temperature departures at Kew being approximately -4°C in summer and winter. The visibility in mA air is usually very good. The contribution of mP and mA air masses to the mean annual rainfall over a five-year period at three stations in northern England and North Wales is given in Table 10.2, although it should be noted that both airmasses may also be involved in non-frontal polar lows. Over much of southern England, and in areas to the lee of high ground, northerly and northwesterly airstreams usually give clear sunny weather with few showers. This is illustrated in Table 10.2. At Rotherham, in the lee of the Pennines, the percentage of the rainfall occurring with mP air is much lower than on the West Coast (Squires Gate).

Maritime tropical air commonly forms the warm sector of depressions moving from between west and south towards Britain. The weather is unseasonably mild and damp with mT air in winter. There is usually a complete cover of stratus or stratocumulus cloud, and drizzle or light rain may occur, especially over high ground, where low cloud produces hill fog. The clearance of cloud on nights with light winds readily cools the moist air to its dew-point, forming mist and fog. Table 10.2 shows that a high proportion of the annual rainfall is associated with warm-front and warm-sector situations and therefore is largely attributable to convergence and frontal uplift within mT air. In summer the cloud cover with this airmass keeps temperatures closer to average than in winter; night temperatures tend to be high, but daytime maxima remain rather low.

Table 10.1 General weather characteristics and airmasses associated with Lamb's 'Airflow Types' over the British Isles.

Type Weather conditions

Westerly Unsettled weather with variable wind directions as depressions cross the country. Mild and stormy in winter, generally cool and cloudy in summer (mP, mPw, mT).

Northwesterly Cool, changeable conditions. Strong winds and showers affect windward coasts especially, but the southern part of Britain may have dry, bright weather (mP, mA).

Northerly Cold weather at all seasons, often associated with polar lows. Snow and sleet showers in winter, especially in the north and east (mA).

Easterly Cold in the winter half-year, sometimes very severe weather in the south and east with snow or sleet.

Warm in summer with dry weather in the west. Occasionally thundery (cA, cP).

Southerly Warm and thundery in summer. In winter it may be associated with a low in the Atlantic, giving mild damp weather especially in the southwest, or with a high over central Europe, in which case it is cold and dry (mT, or cT, summer; mT or cP, winter).

Cyclonic Rainy, unsettled conditions, often accompanied by gales and thunderstorms. This type may refer either to the rapid passage of depressions across the country or to the persistence of a deep depression (mP, mPw, mT).

Anticyclonic Warm and dry in summer, occasional thunderstorms (mT, cT). Cold and frosty in winter with fog, especially in autumn (cP).

Global Airflow Patterns

Figure 10.5 Average climatic conditions associated with Lamb's circulation types for January, April, July and September, 1861 to 1979. (A) Mean daily temperature (°C) in central England for the straight (S) airflow types; at the right side are the quintiles of mean monthly temperature (i.e. QI/Q2 = 20 per cent, Q4/Q5 = 80 per cent). (B) Mean daily rainfall (in millimetres) over England and Wales for the straight (S) and cyclonic (C) subdivisions of each type and terciles of the mean values (i.e. TI/T2 = 33 per cent, T2/T3 = 67 per cent). (C) Mean frequency (per cent) for each circulation type, including anticyclonic (A) and cyclonic (C).

Figure 10.5 Average climatic conditions associated with Lamb's circulation types for January, April, July and September, 1861 to 1979. (A) Mean daily temperature (°C) in central England for the straight (S) airflow types; at the right side are the quintiles of mean monthly temperature (i.e. QI/Q2 = 20 per cent, Q4/Q5 = 80 per cent). (B) Mean daily rainfall (in millimetres) over England and Wales for the straight (S) and cyclonic (C) subdivisions of each type and terciles of the mean values (i.e. TI/T2 = 33 per cent, T2/T3 = 67 per cent). (C) Mean frequency (per cent) for each circulation type, including anticyclonic (A) and cyclonic (C).

Source: After Storey (1982), reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

Table 10.2 Percentage of the annual rainfall (1956 to I960) occurring with different synoptic situations.

Station Synoptic categories

Table 10.2 Percentage of the annual rainfall (1956 to I960) occurring with different synoptic situations.

Station Synoptic categories

Warm

Warm

Cold

Occlusion

Polar

mP

cP

Arctic

Thunderstorm

front

sector

front

low

Cwm Dyli (99 m)*

18

30

13

I0

5

22

0.I

0.8

0.8

Squires Gate (10 m)t

23

16

14

15

7

22

0.2

0.7

3

Rotherham (21 m)t

26

9

II

20

I4

I5

I.5

I.I

tOn the Lancashire coast (Blackpool). tIn the Don Valley, Yorkshire. Source: After Shaw (1962), and R. P. Mathews (unpublished).

Continentality Britain
Figure 10.6 Distribution of thunderstorms over western Europe during the period 19 to 21 August 1992 (storms shown for the four-hour period preceding the times given). A small depression formed over the Bay of Biscay and moved eastward along the boundary of warm air, developing a strong squall line.

Source: Blackall and Taylor (1993). Reprinted from the Meteorological Magazine (Crown copyright ©) by permission of the Controller of Her Majesty's Stationery Office.

In summer, 'plumes' of warm, moist mT air may spread northward from the vicinity of Spain into western Europe. This air is very unstable, with a significant vertical wind shear and a wet-bulb potential temperature that may exceed 18°C. Instability may be increased if cooler Atlantic air is advected under the plume from the west. Thunderstorms tend to develop along the leading northern edge of the moist plume over Britain and northwest Europe. Occasionally, depressions develop on the front and move eastward, bringing widespread storms to the region (Figure 10.6). On average, two mesoscale convective systems affect southern Britain each summer, moving northward from France (see p. 201).

Continental polar air occasionally affects the British Isles between December and February. Mean daily temperatures are well below average and maxima rise to only a degree or so above freezing point. The air is basically very dry and stable (see easterly type in January, Figure 10.4) but a track over the central part of the North Sea supplies sufficient heat and moisture to cause showers, often in the form of snow, over eastern England and Scotland. In total this provides only a very small contribution to the annual precipitation, as Table 10.2 shows, and on the West Coast the weather is generally clear. A transitional cP-cT type of airmass reaches Britain from southeastern Europe in all seasons, although less frequently in summer. Such airstreams are dry and stable.

Continental tropical air occurs on average about one day per month in summer, which accounts for the rarity of summer heatwaves, since these south or southeast winds bring hot, settled weather. The lower layers are stable and the air is commonly hazy, but the upper layers tend to be unstable and occasionally surface heating may trigger off a thunderstorm (see southerly cyclonic type in July, Figure 10.4).

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