Regional weather and climates of the British Isles – Part 2: South East England and East Anglia

Authors


Correspondence to: Julian Mayes

julian.mayes@tiscali.co.uk

This region contains the driest, calmest, sunniest, warmest (by day) and most climatically-continental places in the United Kingdom, as befits an area at the southeastward end of the northwest–southeast climatic gradient. The generally low-lying orography might be thought to determine a certain climatic unity and whilst this is perhaps true of much of East Anglia, this should not be confused with uniformity in weather conditions. There is sufficient variety of landscapes and soil types to produce notable frost hollows, and the varied configuration of the coastline helps to generate features such as convergence lines and sea breezes that often exert a major influence on local weather.

Two climatic sub-regions can be identified: the south of the region from Hampshire to Kent (including Surrey) is an area in which there is an autumn/winter rainfall maximum and a generally maritime character to the weather. Further north, rainfall is more evenly spread through the year as the prevailing southwesterly winds of autumn and winter have lost some of their moisture as they come overland, and a more continental character is often apparent with colder nights.

The legacy of past weather observations

Figure 1 shows the location of places mentioned in the text. The pre-eminent source of past weather observations in the region is Kew Observatory (southwest London) (Mayes, 1994) where instrumental records are available from 1842 to 1980, although for much of this period temperatures were recorded in a North Wall Screen. This uniqueness of the observing environment makes reconstruction of the weather observations for more recent years very difficult, which is a major disadvantage for studies of climatic change in the London area: there is a dearth of long-period observing sites in the capital. Since the closure of London Weather Centre in 2010, the fully-automatic climatological station at St James's Park is the sole observing site in central London: it has records from 1903, but there are occasional gaps. Also worthy of note is Hampstead Observatory where records began in 1911, whilst the longest running site in the region is the Radcliffe Observatory, Oxford, where observations have been carried out since 1815 (Smith, 1994). Rothamsted (Hertfordshire), where records commenced in 1872, is another significant site and is now used in the (Met Office) Central England Temperature series.

Figure 1.

Map showing location of places mentioned in this article.

The region has a wealth of privately-run weather stations contributing to the Climatological Observers Link (COL) Network, the large number in East Anglia possibly due to the importance of agriculture and horticulture in the local economy. In earlier times East Anglia also boasted the outstanding phenological record in the UK – the Marsham record from Norfolk – as well as voluntary bodies such as the Rainfall Organisation of Suffolk and Essex and the Norfolk Rainfall Organisation. Finally, the legacy of health resort observing sites, particularly along the south coast, is testimony to the dependence of tourism on the weather experienced at the main holiday centres: this is still evident at Eastbourne (www.eastbourne.co.uk/environment/weather) and Hastings, where records exist back to 1867 and 1875 respectively. Elsewhere, many long-standing resort sites have closed (examples include Worthing in 1993 and Bexhill in 2012).

Temperature

The warmth of southern Britain results partly from the longer duration of tropical air masses compared with areas further north. Other differences are more subtle, but still important. Prevailing southwesterly winds are ‘continentalised’ as they move inland across the region, diurnal temperature ranges increasing as cloud cover decreases. From Norfolk to northeast Kent, offshore west or southwesterly airflows obviously nullify any moderating influence to be found from the North Sea; by contrast, the cooling effect of onshore northeasterly or easterly winds in late spring and summer is a well-known feature of the local climate, especially when stratus persists through the day. In this synoptic situation, the highest temperatures in the region are often found around the Solent in southern Hampshire, following descent of the air after the northeasterly flow has crossed the South Downs.

Table 1 demonstrates how the overall climatology of the region is influenced by these effects: the contrast in maximum temperatures between Lowestoft and Cambridge is evident and reaches a peak of around 2 degC from April to August. The fairly urban nature of the environment around Heathrow is reflected in higher minimum temperatures than at Cambridge throughout the year (but only a small difference in maxima). The temperatures recorded at Reading are representative of much of the Thames Valley and are consistently lower than those of Heathrow, but not by a large margin. Due to its location on the relatively maritime Isle of Thanet, Manston has lower maximum temperatures than Heathrow in each month of the year. However, the mean minimum through the year is the same as Heathrow's. South Hampshire is mild in winter and quite warm in summer, especially at night, as shown by the data for Southsea (representative of Southampton and Portsmouth).

Table 1. Monthly and annual mean maximum and minimum temperatures (°C), 1981–2010
SiteJan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.Year
Lowestoft6.97.19.612.215.418.421.021.018.414.310.17.313.5
 2.01.83.35.08.210.913.313.411.38.54.92.67.1
Cambridge7.57.910.914.117.420.323.022.819.615.210.67.614.8
 1.51.23.04.47.310.312.412.29.97.24.01.76.3
Heathrow8.08.311.314.217.821.023.523.119.815.511.18.115.2
 2.52.33.95.58.711.713.913.711.48.55.13.07.5
Reading7.78.010.813.517.020.022.422.119.014.910.77.914.5
 1.91.73.54.77.710.512.712.510.37.64.42.26.7
Manston7.27.310.012.515.818.921.721.718.814.810.67.714.0
 2.52.13.65.38.411.213.513.811.88.85.33.27.5
Southsea8.38.310.613.116.419.221.621.619.115.511.78.914.5
 3.73.24.76.39.512.414.514.712.810.06.74.28.6

Although large international airports are sometimes criticised as being unrepresentative of their surroundings, temperatures at Heathrow are similar to locations within London's urban area, but are higher than at sites that are in large open spaces and frost hollows (Kew Gardens and Northolt) or at higher elevation (Hampstead).

The highest temperature recorded in the region (and the whole of the UK) is just over 38°C on 10 August 2003. The officially accepted value of 38.5°C observed at Faversham (Kent) has been disputed (Burt and Eden, 2004a; 2004b): the next highest reading is 38.1°C, recorded at Kew Gardens on the same day. The region can also lay claim to having recorded the warmest night in the British Isles: on 3 August 1990 Brighton recorded a minimum temperature of 23.7°C (Burt, 1992). The warmest night of the 2003 heatwave brought a minimum of 23.2°C at Ventnor (Isle of Wight) (Burt and Eden, 2004c).

In addition to the record-breaking days, mention should also be made of the consistency of warmth in the region in the summer half-year, particularly in westerly and southwesterly airflows when the east coast tends to be warmer than in anticyclonic conditions. Dry ground conditions often enhance the warmth by minimising latent heat losses.

Frost

The frequency of air frost varies from less than 25 days a year in central London and on the coast to more than 70 days over low-lying inland areas such as the Breckland of southwest Norfolk and in frost hollows elsewhere (Table 2). The frequency and severity of frost in the region is governed not only by distance from the coast and by urban effects but also by geology. The chalk of the North and South Downs, the Chilterns and the East Anglian Heights affords poor nocturnal heat retention and severe frost hollows are found in the dry valleys here (see Box 1). The sandy heaths of Norfolk's Breckland region and in west Surrey provide more widespread frosty locations and allow ground frost to be recorded in any month of the year.

Table 2. Monthly and annual averages of air frost (days), 1981–2010
SiteJan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.Year
Lowestoft6.47.34.01.0000000.22.46.327.6
Cambridge9.910.46.43.20.500001.95.610.448.3
Reading9.19.44.82.20.200001.14.49.841.0
Eastbourne5.14.81.60.20000000.94.116.7
Heathrow7.48.23.81.3000000.43.37.431.8

The lowest temperature recorded in the region is −21.6°C, recorded at Grendon Underwood (Buckinghamshire), situated in the Vale of Aylesbury below the scarp face of the Chilterns, in January 1982. South East England's lowest temperatures tend to be associated with polar continental air masses that arrive on eastsoutheasterly surface winds in winter, after a very short sea track across the Straits of Dover; the coast of East Anglia will be less cold on these occasions due to the longer sea track here. The presence of a deep, fresh snow cover is a further prerequisite as it increases the surface albedo. This combination gave rise to the lowest day maximum temperature of −9°C at Warlingham (Surrey) on 12 January 1987, remarkable for anywhere in lowland Britain, let alone immediately adjacent to London's suburbia (although this lay downwind on this occasion). A record deep cold pool covered southern England on this occasion (497dam at Hemsby: Pike and Webb, 2011).

Precipitation

The region can be divided into two rainfall sub-regions: East Anglia and the Thames Valley, where there is a fairly uniform distribution of rainfall through the year, and South East England (from the North Downs southwards) where there is a clearer maximum in autumn and winter (Table 3). In the former region, this lack of seasonality has meant that the wettest months in the year have varied over time. In the late nineteenth and early twentieth centuries, July tended to be the wettest month but in the 1971–2000 averages it was the driest. In the London area, the average rainfall for July was 62mm in 1916–1950 (Kew) and 39mm in 1971–2000 (Heathrow). Table 3 shows that there has been a recovery in July and August rainfall in the 1981–2010 statistics, which perhaps makes the latter more representative of the long term.

Table 3. Monthly and annual averages of precipitation (mm), 1981–2010
SiteJan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.Year
Lowestoft49.838.943.743.948.750.451.158.553.463.063.054.6619.0
Cambridge46.633.938.639.148.252.450.354.655.359.655.744.2578.5
Heathrow55.340.941.743.749.445.044.649.549.168.559.054.9601.6
Manston47.635.336.740.648.445.944.246.146.370.562.455.8579.8
Southsea74.548.853.243.338.638.638.147.553.384.572.576.2669.1
Eastbourne80.953.958.848.749.047.248.951.564.1104.995.990.8794.7

Eastbourne and Southsea show the extent to which rainfall increases towards the end of the year in southern coastal counties. This is indicative of the moisture-bearing potential of prevailing southwesterly airflows at this time of year, when they are energised by a warm sea in contrast to the dryness of the stable marine coastal air in the summer half-year. The southward shift of mean depression tracks in the winter half-year and the slight backing of surface winds that accompanies this also play a part. In summer, this area lies closest to ridges of high pressure associated with the Azores high and the mean wind direction veers slightly to a more westerly direction, reducing the frequency of onshore winds onto southern coasts. However, a cyclonic southerly airflow can generate high rainfall totals at any time of year: this may be in the form of thunderstorms in summer but southerlies in autumn can also bring heavy rain due to the influence of high sea-surface temperatures.

Slow-moving cyclonic weather systems may produce heavy, persistent rain at any time of the year. These events can be quite localised: examples include the ‘Norfolk storm’ of 25–26 August 1912 (Brooks, 2012), and the rains of 14–16 September 1968 in Essex, Surrey and Kent (Bleasdale, 1970; Jackson, 1977) when around 200mm fell in the wettest areas. Jackson (1977) suggested that the 1968 rains may have been influenced by local airflow convergence and warm water around the Thames Estuary and a similar mechanism may have enhanced the frontal rainfall of the 1912 storm as it approached the east coast. Mesoscale convergence lines – often derived from sea-breeze fronts – are also implicated in the development of thunderstorms in the region: the most notable example remains the Hampstead storm of 14 August 1975, when 169mm fell in 2.5 hours (Keers and Wescott, 1976).

Showers

The synoptic origin of precipitation is quite strongly influenced by the susceptibility of locations to showers as opposed to frontal precipitation. Whilst eastward or northeastward moving fronts tend to give less precipitation towards east Kent and East Anglia, these areas are exposed to showers brought by the subsequent northwesterly or northerly airflows of unstable polar or Arctic maritime air masses. It is therefore, perhaps, fitting that the following weather lore originates from Cambridgeshire:

When mountains and cliffs in the sky appear, some sudden and violent showers are near (quoted in Glenn (1987))

The distribution of showers is influenced by seasonal changes in the distribution of instability. In autumn and winter, counties close to the south coast have more convectional rainfall than they do in summer when the lower sea-surface temperatures reduce the instability of onshore winds. Conversely, in spring and summer, land-based convection induces convergence over the southwest peninsula as sea breezes move inland from both sides of the peninsula; this convergence zone may be further fed by sea breezes moving north across Hampshire, so that in southwesterly airflows the resulting shower line often tracks across the region from north Hampshire and over the Chilterns to East Anglia. Hand (2005) identified this as a feature of spring afternoons with a peak frequency near the coast of Norfolk and Suffolk; the pattern was less clear in summer although a low shower frequency was apparent on the south coast. Likewise, showers forming over Pembrokeshire and the Brecon Beacons may extend to Cambridgeshire and Norfolk on occasions.

There is a tendency for the region to experience lengthy (>1 year) periods of below-average rainfall that lead to water stress in agriculture and depleted riverflows and groundwater levels. Examples include 1975–1976, 1988–1992, 1995–1997 and 2011 (Dent, 2012). Several of these periods have been quite wet in northwestern parts of the UK (Mayes and Wheeler, 2013). The importance of groundwater as opposed to surface water for sustaining water supply and rivers means that the region may be relatively unaffected by short, intense dry spells but it may take many months to recover from longer dry periods.

Thunder and hail

The frequency of thunder (Figure 2) peaks in the summer half-year in response to increasing instability over the warmer land, although locations fairly close to the English Channel coast have quite a high frequency of thunder in the winter half-year due to thundery showers advected from a relatively warm sea surface: the resulting thunder is, though, often quite limited. The risk of significant thunderstorms increases everywhere in summer – either ‘home-grown’ or imported from the near-Continent, the latter sometimes in ‘Spanish plume’ synoptic situations.

Figure 2.

The average number of days of thunder heard at Ely (Cambs) and Horsham (West Sussex), 1981–2010.

Hail is observed in the heavier showers in very unstable air; as a result the frequency tends to peak in the spring when polar maritime air can be particularly unstable – a good example was ‘the Bracknell storm’ of 7 May 2000 when prolonged hail fell from a slow-moving storm that formed at the intersection of convergence lines associated with outflow from a previous storm and the sea-breeze front (Pedgley, 2003). Much of the region averages five to eight days a year with hail and only about one day with hailstones larger than 5mm diameter, though the exact frequencies at individual sites may reflect the vigilance of observers as much as climatological differences.

Snowfall

Sudden or widespread snowfalls in this region are particularly disruptive due to the high density of transport networks and the infrequency (leading to unpreparedness) with which they occur. This relative infrequency is a result of the low incidence of Arctic air masses, though this is tempered by the tendency of Arctic maritime – and to a lesser degree polar continental – air to bring snow showers to eastern coastal counties. Figure 3(a) shows that even low-lying parts of East Anglia have a higher frequency of falling snow than Hampstead, one of the highest parts of London. The eastern counties from Norfolk to Kent are susceptible to both frontal snow on southward-moving cold fronts and snow showers in the northerly flows that often follow. Not surprisingly, the frequency of snow is lower close to the south coast as the northerly or northeasterly airflows dry out as they cross the land.

Figure 3.

The average number of days of (a) sleet or snow observed to fall and (b) snow lying at Ely (Cambs), Hampstead (London) and Chichester (Birdham COL site, West Sussex), 1981–2010.

The average monthly frequency of lying snow (Figure 3(b)) shows much lower totals, highlighting the reluctance of snow to persist on most occasions, unlike in some northern uplands of the UK. A notable contributor to Figure 3 is the occurrence of snow at Hampstead on 28 October 2008.

Widespread snow tends to be associated with two synoptic types. Polar continental and Arctic maritime airflows (northeasterly and northerly airflows, respectively) can become destabilised over the relatively warm waters of the North Sea, especially as they approach the coast of East Anglia and the Thames Estuary, so that the resulting snow showers merge to bring longer periods of snow: in January 1987 over 40cm of level snow lay quite widely in coastal parts of East Anglia and Kent. The North Downs in east Kent are particularly vulnerable to this source of snow. Convergence lines can be an effective means of concentrating these snow showers into bands of more continuous snow and the processes involved are discussed in Box 2.

The second synoptic origin of snow is from the west when an approaching depression battles against a block of cold Continental air, either as a northeastward moving warm front or on the northern flank of a depression tracking east along the English Channel. These features tend to give the heaviest snow in Hampshire, the Isle of Wight and Sussex – that is, in the areas that often escape snow in the showery airflows mentioned above. After the blizzard of 18–21 January 1881, snow depths exceeded 50cm in south Hampshire.

Although the winters of 2009/2010 and 2010/2011 were cold, the region did not suffer from the depths of snow noted in the historical examples above; it was also fortunate not to experience widespread freezing rain events. The most disruptive recent snowfall was that of 1–2 February 2009 (see Box 2). However, on 6–7 January 2010 a southward-moving cold front brought an Arctic maritime air mass that moved south into central southern England. As a depression formed on the front, snow became heavy in Berkshire, Hampshire and West Sussex; level depths approached 30cm in these counties and 1000 motorists became stranded on the A3 in Hampshire. The snowfall intensity may have been enhanced by instability over the English Channel .

Figure 4.

Rainfall radar image for 0300 utc on 2 February 2009 showing multiple snowbands enhanced by coastal convergence moving westwards across Essex, London and Surrey. Greatest snow depths occurred under the southernmost of these snowbands.

Sunshine and visibility

This is the sunniest region of the UK: the sunniest part of the mainland on average is the East Sussex coast from Eastbourne to Hastings and both towns have the distinction of having recorded the largest duration of sunshine in a single month, 383.9 hours in July 1911. The Isle of Wight is slightly sunnier on average, as shown by the data for Shanklin in Table 4. The ‘advantage’ afforded by a coastal location is greatest in summer when convective cloud development is more pronounced inland as sea breezes clear such cloud from coastal margins: by July, Eastbourne has 30% more sunshine than either Luton or Bury St. Edmunds. By contrast, a stable northeasterly or easterly flow blowing off the cold North Sea in spring and summer may result in stratus hugging eastern coastal counties while areas further west or southwest are sunny (Figure 5); on some occasions the stratus flows inland to reach the Thames Valley overnight but often evaporates in the warmth of day over inland areas.

Table 4. Monthly and annual averages of bright sunshine (hours), 1981–2010
SiteJan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.Year
Bury St Edmunds (Broom's Barn)617711316420219320619414411870521593
Luton587510915418418619518713610466511505
London Wx C637811216520219821221514911774521637
Eastbourne719012819823324025323717212584591888
Shanklin (Isle of Wight)689013320124124826224117312283611923
Figure 5.

MODIS satellite image showing an extensive layer of stratus over the southern North Sea and clear skies inland on 25 March 2012. The role of the North Downs in Kent as a topographic barrier can be seen clearly. (

Source: NASA/GSFC

.)

Fog

Fog is defined climatologically as visibility of less than 1000m, although the definition of ‘thick fog’ (visibility below 200m) might accord more closely with the public's perception of fog. This means that published averages of days with fog include some of those overcast, misty winter days when stratus and drizzle obscures visibility, in addition to days with radiation, advection or hill fog. Average monthly fog frequency reaches a peak of around five days in December in the foggier parts of the region such as the Fens (e.g. Ely). It should be noted that, unlike other elements, fog frequency is based upon the visibility at the 0900 utc observation alone.

Radiation fog forms under clear skies in very light winds when there is a fairly moist boundary layer in the presence of a marked temperature inversion. Lowest temperatures are found in the valleys: the damp clay Vale of Oxford, Vale of Aylesbury and also the Fens are ‘favoured’ areas, often on the margins of fog over the Midlands. The more rural parts of the Thames Valley are also fog-prone, this sometimes forming first close to the reservoirs just southwest of London. Satellite imagery sometimes shows the Chiltern Hills and the North Downs standing proud of the fog above the inversion: Figure 6 shows this effect at Box Hill on the North Downs (see also Figure 6 in Galvin (2004)).

Figure 6.

A view over the top of radiation fog in the Mole valley looking southwest from Box Hill (Surrey). (

© Brendan Jones

.)

Advection fog most commonly affects the east coast in northeasterly or easterly flows in spring or early summer (Figure 5) but can occasionally occur on the English Channel coast as well.

Winds

Statistically, the region has the least windy climate in the British Isles, lying as it does further from the average position of North Atlantic depressions than any other region. In the lower Thames Valley, including London, there is an average of less than one day with a gale in a year in contrast to over 15 on the south coast (Mayes, 1997). However, sensitivity to strong winds is high, particularly when combined with North Sea storm surges. In westerly or northwesterly surface winds, the sea current will veer to a northerly or northeasterly due to the influence of the Ekman spiral, sweeping water onto south-eastern coasts. The east coast flood disaster of 31 January to 1 February 1953 (Prichard, 2013) remains the country's severest natural disaster with a death-toll of around 350, with many of the fatalities on the low-lying Canvey Island. Central London was last seriously flooded in 1928. Sea walls and embankments have been strengthened since those times, and London is protected by the Thames Barrier from sea flooding (and the risk of river flooding can be reduced by closing it when river levels are high to prevent a tidal inflow).

When vigorous depressions move east across England and Wales, damage can occur throughout the region, not so much due to the mean wind speed but to the high gust ratio over the high-friction surfaces of the region's many towns and cities. When a sting jet develops close to a deep depression (Baker, 2009), gusty winds can propagate down to the surface giving widespread high gusts, but mean speeds may not be exceptional. The highest gust in the Great Storm of 1987 was 100kn at Shoreham (West Sussex). Although the damage from this event was unprecedented in modern times, there was a gust of 91kn at Wittering (Cambs) on 3 January 1976, whilst little more than two years after the Great Storm of 1987, on 25 January 1990, there was a gust of 71kn at Kew Gardens – this time from the west, rather than the south as in 1987.

Sea breezes

With such a varied orientation in its coastline, a complex pattern of sea breezes and convergence zones can develop on fine days with light winds between mid-spring and early autumn. In very light gradient winds, sea breezes from the north and east converge over the centre of East Anglia, whilst as Kent is a peninsula sea breezes can converge from the north and south coasts. In both situations, a sea-breeze front may form with lines of cumulus cloud on convergence zones. A well-developed sea breeze from the English Channel is capable of reaching London and the Thames Valley by late afternoon. Indeed, Manley (1952) noted the welcome, if subtle, relief from the heat of warm days.

London's changing climate

If London did not exist, the lower Thames Valley would still be one of the warmest, driest and least windy areas of the UK but parts would also be sheltered frost hollows. The imposition of a densely-populated urban fabric warmed the atmosphere and the coming of the industrial revolution provided a rich source of smoke that resulted in a late nineteenth century peak in fog (Brimblecombe, 1987; Brimblecombe and Bentham, 1997). Atmospheric pollution remained a serious hazard until the mid twentieth century: the smog disaster of December 1952 claimed around 4000 extra deaths. The Clean Air Act of 1956, the development of smokeless zones and the shift away from coal as a fuel have changed the nature of London's air pollution, switching the main problems to traffic contributing nitrogen oxides and particulates to the atmosphere.

By the twentieth century, detailed analyses of London's urban heat island (such as Chandler, 1965) revealed an average daytime warming of around +0.9 degC and an average nocturnal warming of +1.9 degC. However, much depends on the observing sites used (in Chandler's studies it was St. James's Park, not a rooftop site). It remains to be seen whether recent increases in population density in the capital will outweigh improvements in energy efficiency and thus strengthen the heat island still further, though more anticyclonic summers would strengthen it (Wilby et al., 2011). The contrast in temperature is, of course, greatest on clear, calm ‘radiation’ nights following sunny days, when the urban fabric acts as a slow-release mechanism for solar energy. These are also the times when frost hollows are at their coldest, and it is not unusual for central London to have a minimum temperature over 10 degC higher than such places adjacent to London – though much of this difference is due to the frost hollow rather than the urban heat island.

Since the advent of the Clean Air Act, central London's winter sunshine has improved steadily (Figure 7). Bilham (1938) noted that, for 1881–1885, central London's sunshine was reduced by no less than 80% when compared with that of Kew Observatory. By contrast, since the 1970s a ‘doughnut’ effect is occasionally noted as fog clears from inner London well before it does from the rest of South East England.

Figure 7.

Monthly sunshine in central London, 1911–2005, expressed as mean daily sunshine hours. Based on Westminster, Kingsway and London Weather Centre (LWC) records. The apparent step-change in the December totals around 1960 could be a combination of the effect of the 1956 Clean Air Act and the change in the location of the sunshine recorder from Kingsway to LWC, High Holborn (which occurred in 1961). The high sunshine of recent Decembers is in contrast to the relative consistency of June sunshine, the dip in the 1980s being due to synoptic pattern changes.

Conclusions

South East England and East Anglia lie at the benign end of the northwest to southeast climatic gradient across the British Isles. The low rainfall, high sunshine and high summer temperatures are emblematic of the region's climate. However, a distinction can be made between this and the sensitivity to adverse weather. The region's crowded transport network and densely-populated terrain increase the impact of those occasional heavy snowfalls, floods, drought and even the very rare severe gale. Such weather events remind Londoners that they cannot always be oblivious to the weather.

Acknowledgements

The contribution of Geoff Sutton as co-author of the Eastern England chapter of Wheeler and Mayes (1997) is gratefully acknowledged. As in other articles in this series, the climatic data are the product of the efforts of both the original observers and the work of the Climatological Observers Link (particularly Burt and Brugge, 2011). The Met Office's National Climate Information Centre are thanked for allowing use of the 1981–2010 averages for the official observing network; particular thanks are due to John Prior. Finally, I am indebted to my colleague at MeteoGroup UK, Brendan Jones, for capturing the foggy scene at Box Hill (Figure 6).

Box 1.

Frost hollows

Remarkable frost hollows are numerous in both South East England and East Anglia, evidence of the relative importance of soil type and geology on temperature. The sandy heathland of the Breckland area of southwest Norfolk and its inland location create good such conditions: the observing site at Santon Downham lies at a clearing in the local forest and this identifies a crucial factor in the strongest frost hollows – an environment that allows cold katabatic airflows to stagnate and hence to inhibit mixing of air. Air can also be forced to stagnate by embankments, accentuating the frost hollows at Rickmansworth (Hertfordshire) and Chipstead Valley (Surrey). Hawke (1944) made the Rickmansworth frost hollow famous but subsequent urban development has moderated its intensity (Galvin, 2005). A remarkable example at Chesham (Buckinghamshire) lies close to the confluence of two valleys, another factor that favours stagnation of cold air. In the cold winter of 2009/2010, Chesham recorded 82 air frosts and Chipstead Valley 80 (source: COL bulletins). On 11 February 2012, when most places away from the frost hollows had minima around −10°C, the minimum temperature at Chesham was −18.3°C and Chipstead Valley was 0.1 degC colder (COL, February 2012): this was the lowest temperature recorded in the UK in 2012, the first occasion since 1947 when this has been observed in this region.

Box 2.

Convergence lines and snowbands

Many of the heaviest snowfalls in the region are associated with convergence lines enhancing snow showers into bands of more persistent snowfall. The main areas where convergence occurs are along the Thames Estuary into northwest Kent, over the English Channel and inland from the Wash. Norris et al. (2012) list three possible mechanisms for snowbands derived from convergence lines: thermally-driven land breezes, frictional contrasts between land and sea and deflection of air around topographic barriers. Frictional contrasts occur when winds blow parallel to a coast: the airflow over the land converges with that over the sea due to the effect of friction on velocity, hence weakening the Coriolis effect over the land.

Convergence lines develop most readily over the sea in cold spells in winter (due to thermal contrasts between the colder air and the still-warm sea), and these can create snowbands. A good example is shown in Figure 4 when these gave parts of Surrey its deepest snow for nearly 50 years with over 30cm of snow in places. Widespread disruption to transport ensued. Thames Estuary convergence lines are referred to informally by some weather forecasters as the ‘Thames Tickler’ (a cousin of the ‘Pembrokeshire Dangler’). In January 1987 snow depths reached 55cm at Southend and 65cm locally in north Kent.

Ancillary