thickness of the rotating floe and border ice increased. The ice-breaking capability of the floe was a result of its size and weight. The edges of the ice floe and the surrounding ice cover were rounded off by erosion by contact and by this time the ice floe and the hole had become circular.

The 1994 ice disc was probably formed in the same way (see Fig. 4). The ice disc started with a rather large irregular floe that in the course of time became more circular. The long lifetime of the ice discs must be explained by the erosion due to contact between the ice disc and border ice.

The observations in 1987 and 1994 imply that the direction of rotation is that of the whirlpool that is necessary for the phenomenon to occur. In both cases rapid waters were located upstream of the ice disc. Frazil ice may be important for the formation and rotation of the ice disc. The mechanisms that form and drive the rotating ice discs are not yet known. Occurring ice discs should be studied and future measurements and observations made. More detailed data of the ice discs on the Pite

River are available (Nordell and Westerstrom 1996).

Acknowledgements

We express our thanks to L. Fransson for calculating the bowl-shape of the ice disc and to A. Lundberg and H. Iversen for helping us with the field observations.

References EoS (1983) EOS Tram., American Geophysical Union,

64, 13 December 1983 La Hou& Blanche (1971) Remu internarionule de l'eau.

Revue de la Societe Hydrotechnique de France, No. 7/1971 (in French)

Nordell, B. (1987) Mystery of rotating ice disc solved. (Mysterium med roterande isflak uppklarat.) fl- lusmad Vemkap No. 7, Fogtdals Forlag, Malmo, Sweden (in Swedish)

Nordell, B. and Westersuiim, G. (1996) Large rotating ice disc on ice covered river - measurements. Div. Water. Res. Eng. Intern. Rep., 1996:01, Lule% Univer- sity of Technology

Scientific American (1995) SCi. Am., 1995

Air quality data summary, summer 1996

Andrew Deacon South East Institute of Public Health, Tunbridge Wells, Kent

This summary covers the months June, July and August 1996, and presents summary air quality data from six sites in five cities around the United Kingdom. Data are given for regional centre locations: Belfast, Birmingham, Cardiff, London (Bloomsbury and roadside data from Cromwell Road) and Edinburgh.

The pollutants reported here are carbon mon- oxide (CO), nitrogen dioxide (NO,), ozone (OJ, fine particulate matter (PM,,,) and sulphur dioxide (SO,). The measurements are all made routinely and hourly data are stored in the Na- tional Air Quality Archive. These pollutants are all of concern due to their effects both on human health and on other parts of the environment. During the summer months 0, is likely to reach

its highest levels, usually once the concentrations of precursor pollutants (hydrocarbons and nitro- gen oxides) have built up, and are transported towards the British Isles from the Continent on easterly winds. It is now also believed that sec- ondary particulate matter, formed by at- mospheric reactions involving sulphur and nitrogen, enhances PM,, levels during the sum- mer months. In this summary, hourly data are given to show the extreme values which may be reached during such episodes, whilst daily means help to identlfy extended periods of poor air quality, and monthly means give an idea of the overall pollution climate of the British Isles.

Table 1 shows that monthly mean CO levels were highest at the Cromwell Road site in London. This contrasts greatly with the situation in Edinburgh, and also Belfast, where traffic pollution often represents less of a problem. However, highest hourly CO levels were recorded at the LondodBloomsbury site (an urban back- ground site, designed to be representative of exposure to the general population), although these levels are still lower than the World Health

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Table 1 Air quality statist ics for summer 1996 CO (p.p.m.v.) NO? (P.P.b.v.) 0 3 (p.p.b.v.)

Hightst Hourly Monthly mran Highest Hourly Monthly mean Highest Hourly Monthly daily mas. daily max daily max. mean

B&ust June JUlY Aug. B2nllz?lg/rarrr June July Aug. Curdzj June

Aug. Lun&ol1/ Rlooiii~bq~ June July Aug. L.4llldOili Cmiiieodl Road June July Aug. Ediiiburxh June July Aug.

July

mean mean mran

0.8 1.3 0.3 27 52 15.0 41 72 21.4 0.6 1 . 1 0.4 38 63 13.3 36 60 18.7 0.8 2.0 0.5 32 58 15.3 36 61 19.5

1.5 2.h 0.9 39 69 19.2 44 76 26.0 1.5 2.1 0.8 35 89 17.4 58 90 23.1 0.8 1.6 0.4 33 67 20.0 51 72 19.7

0.7 2.8 0.4 34 69 20.0 44 95 23.2 1.3 2.0 0.6 28 63 17.2 45 86 18.3 0.7 1.6 0.4 37 61 18.7 53 75 21.1

3.9 6.5 0.0 59 101 32.0 39 90 17.8 0.7 1.4 0.4 61 120 32.4 30 63 10.7 0.8 1.8 0.5 58 85 34.5 35 76 12.3

1.8 3.2 1.1 94 157 48.1 1.6 3.0 1 .o 81 104 38.5 1.4 3.5 0.9 74 101 36.6

0.9 1.6 0.6 35 56 21.6 40 60 21.1 0.7 1.3 0.5 43 72 21.2 33 72 14.0 0.7 1.6 0.4 47 76 29.1 42 59 15.8

Pht,,, (m m-’1 SO, (p.p.b.v.) Highest Hourly Alonthly Highest Hourly .Ilonthly

dady max. mran daily mn. mean mran mran

Belfhst June July Aug. Bznllz?lg~~u~ll June July Aug. Curdlft June

Aug. Lotidoti! Bloorrisbrm

July

June July Aug. Lol l&l l / cro?llzuell Houd June July Aug. Edinburgh June July

49 103 31.3 16 74 183 32.2 15 71 174 34.6 27

42 54 51

51 54 52

54 64 60

27

132 22.1 10 77 20.4 13 81 22.4 14

36 1 23.0 9 347 22.1 11 140 23.3 10

128 30.0 27 152 28.4 18 135 29.3 23

13 15 18

52 16.1 9

45 6.2 50 6.4 80 7.0

33 4.2 44 5.1 43 4.8

43 3.5 66 3.7 32 2.9

62 6.3 55 6.6 54 10.2

57 6.7 23 8.1 72 6.0

36 3.8 34 63 15.5 6 40 2.4

Aug. 50 94 19.6 6 30 2.7

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Organization guideline of 10 parts per million by volume @.p.m.v.) as an 8-hour average.

NO, levels may also be elevated during sum- mer smog episodes, and again highest levels over this %month period were recorded in London. As for many of these pollutants (COY NO, and PM,,) motor vehicles are by far the dominant source of pollution, levels tend to be highest in the larger cities. High daily mean values are seen at the roadside, which again reflects the dominance of motor vehicles as a source of NO,. NO, is formed through the reaction of nimc oxide (NO) and 0,, which is rapidly scavenged at the road- side (thus it is not measured in roadside locations).

These reactions also mean that 0, levels are not highest in city centres, but increase downwind of cities in an ‘urban plume’. The weather also has a r6le to play, as sunlight is important in stimulating these photochemical reactions; this is why the south-east sees the greatest number of high 0, days in a typical year. 0, is generally at a maximum in late spring and early summer, and the highest values during this summer period were found in Birmingham, Cardiff and London/ Bloomsbury.

PM,, is currently a pollutant of great concern, as it is thought to cause approximately 10000 excess deaths in the UK every year, and recent

~~

Government-funded reports have highlighted a number of research needs in terms of understand- ing particulate matter sources and behaviour. Daily mean values are again high at Bloomsbury, although high levels are also recorded in Belfast (where coal and peat are still widely used for domestic purposes causing the formation of much particulate matter) and in Cardiff. The PM,, levels recorded in Cardiff represent extreme hourly values, and are likely to be caused by some form of local dust-forming activity (e.g. con- structions or demolition), which would have the effect of elevating levels in the vicinity.

SO, used to be a problem during, for example, the infamous London smogs of 1952. However, since that time levels have decreased greatly due to the switch to gas for domestic use, and the Clean Air Acts of 1956 and 1968. The levels reported here do not exceed World Health Or- ganization guidelines of 122 parts per billion by volume (p.p.b.v.) as an hourly average. SO, levels now tend to peak during plume grounding events, when industrial plumes touch down, bring un- diluted SO, emissions down to ground level. Such events are linked to meteorology, with turbulence and boundary-layer levels playing an important r6le in bringing the plume to the ground.

The severe winter frost with considerable snowfall was exceptional. It was the extreme minimum temperature of -21”C, recorded on 21 January 1940 in Am- bleside, Cumbria, that first led me to research this unusual month. Could a fresh snow-cover have

weather of January 1940 over Britain, with particular reference to north-west England

J. M. Burton Arnbleside, Curnbria

led to such an extreme temperature which is possibly the lowest recorded there since February 1895? This article concentrates on the very cold mid-month period of January 1940.

In January 1940 high pressure dominated high (polar) latitudes over the North Atlantic. From the 15th to the 17th an intense anticyclone cen- tred over Greenland extended a ridge of high pressure southwards to the west of the British Isles. The mean monthly pressure was 12mbar above average over Scotland, and 2mbar below

Rarely in our climate record does an event rank as average over the south-west approaches of extraordinary; January 1940, however, was one Britain. Easterlies were common over the south- such occasion. The intense cold and penetrating ern half of the country.

“They crawled over 15ft. drifts.”

Bkzckpool Evening Gazem, 12 February 1940

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