PRELIMINARY INVESTIGATION OF A NOTTINGHAMSHIRE FROST·HOLLOWBy I. T. LYALL

Newark, Notls.

It has recently come to the notice of the author that for several years a meteorolo­gical station has been run in Southwell. This station is sited in what was considered tobe a potential frost-hollow, and a preliminary examination of the records of the sitehas been carried out. For 'control' purposes, the records of the College of Agricultureat Brackenhurst have been used.

STATION SITEFig. 1 shows the siting of the two stations. The Southwell station (A) is on the

edge of the main urban area and is situated near a stream, the 'Westhorpe Dumble'.This stream drains westward off a considerable area of relatively high ground above300 ft., and it is possible that some 'ponding' of the cold air may occur as the valleyflattens out and runs into the physical obstacles of the buildings of Southwell. Brack­enhurst College (B) is situated about one kilometre to the south in an open site andover 50 ft. above the valley level.

5 KM 1 KM

Fig. 1 Locations ofstations at Southwell (A), Brackenhurst (B)and Newark (C)

INCIDENCE OF AIR·FROSTDuring the years 1979-1980 a total of 174air-frosts was recorded at Southwell and

126 at Brackenhurst. Monthly totals are displayed as a histogram in Fig. 2. In none ofthe months were there more frosts at Brackenhurst than Southwell. The overall differ­ence in frost incidence is significant at the 5 per cent level by the Student's t-test. Therewere apparent differences in the mid-winter months: in January 1980 there were 21

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INCIDENCE OF AIR FROST

o Southwell

~ BrQckenhurst

Fig. 2 Incidence ofair frost at Southwell (total column heights) and Brack­enhurst (shaded)

frosts at Southwell, five more than at Brackenhurst, whilst in the cold February of 1979Brackenhurst recorded 23 frosts and Southwell 27. The Southwell site showed theusual susceptibility to spring and autumn frosts: in spring 1980, for example, therespective totals for Southwell and Brackenhurst were seven and one for April, andthree and zero for May.

This clearly suggests a frost-hollow effect; the Southwell station, it may be recal­led, is 50 ft. below Brackenhurst and on the edge of an urban area. At the author'sown station at Newark, 10 km to the east (see inset, Fig. 1) there were 118 frosts in1979-80, eight fewer than at Brackenhurst which is 100 ft. higher. Over the two yearsBrackenhurst had a mean minimum about 0·5 deg F lower; about what might beexpected considering the differences in altitude, and confirming that Southwell ratherthan Brackenhurst is climatically 'unusual'.

INDIVIDUAL NIGHTSDuring the two year period of this review, 14 nights were selected which had been

clear and with calm or light winds. Table 1 shews the relative minima at Southwell andBrackenhurst (and also Newark). On every occasion Southwell was colder than Brack­enhurst: the average difference was 5·4 deg F, with a variance of 2·1 deg. (TheFahrenheit scale is used since the Southwell records have been kept on that scale). Thedifference is, by the t-test, significant at 1 per cent probability. Comparison of theSouthwell with the Newark data shows a smaller difference; probably due to Newarkbeing on a light, sandy soil. The mean difference was 2·8 deg F with a variance of 1·6deg F, suggesting an acceptable accuracy in the Southwell record.

Individual nights willnow be considered when Southwell and Brackenhurst variedby 7 deg F or more:3 January 1979. The difference between the two sites of 9 degrees is the largest noted.

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TABLE 1 Air minima on individual nights ("F)

Date Southwell Brackenhurst Newark

19793 January 11 20 1628 January 10 13 105 April 29 31 2825 May 35 41 3928 August 37 44 3925 December 21 28 26198014 January 19 22 2019 January 23 27 251 February 23 27 2620 March 20 25 256 April 29 37 3310 May 28 34 3331 October 26 34 331 December 20 26 25

The ground was snow-covered at the time which would favour the formation of coldair in the upland drainage area. Remarkably, the wind was easterly, i.e. up-valley.28 August 1979. Whilst the minimum at Brackenhurst was 44°F that at Southwell was37°F. This suggests that a ground frost probably occurred at Southwell, although agrass minimum temperature is not available for confirmation. The night was virtuallycalm and clear.25 December 1979. Skies cleared late on the 24th with a light southerly breeze.Temperature fell to 21°F at Southwell and 28°F at Brackenhurst. There had been afairly long, quiet spell but no snow-cover.6 Apri/1980. Comparative values of 29°F and 37°F imply quite a sharp frost at South­well when Brackenhurst (and Newark, just) were frost free. The spell was anticyclonicin a polar air-mass; winds were north-easterly. .31 October 1980. A clear night occurred as an anticyclone began to retreat eastward;what little wind there was, was south-easterly. Southwell recorded a sharp frost (26°F):Brackenhurst and Newark were frost-free.

CONCLUSION

The conclusion would appear to be that the Southwell station is in a frost-hollow.Frost incidence was significantly higher than at a nearby station which showed noproclivity to be especially 'warm'. It showed several sharp early/late season frostswhich were not general; the most significant difference between the two stations waswhen there was a snow-cover.

ACKNOWLEDGMENTS

The author wishes to thank Mr C. F. Tudsbury and the Principal of BrackenhurstCollege of Agriculture for making data available for this study.Newark,Nottinghamshire

CORRECTIONWe apologise for two errors on p. 21 of the January issue in the article 'A

documented example of strong wind-shear' by D. A. Membery. Lines 11/12 shouldread'... prior to 0545 the wind had been a steady 290 deg/14kn. Thereafter it backedto 220 deg and at 0602 a gust of 28kn was recorded.'

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