SOME LOCAL TEMPERATURE VARIATIONS IN NORTH-EAST DERBY SHIRE

By I. T. LYALL Newark, Nottinghamshire

HIS article examines differences in monthly mean temperatures recorded T at stations within an area of about 100 kmz in the north-east of Derby- shire. Particular attention is paid to the effects of both altitude and urbanisa- tion. The observations used in this article were taken at four climatic stations maintained in and around Chesterfield in the period 1972-73, the locations of which are shown in Fig. 1 . Mr D. S. B. Fellowes, who has compiled monthly data for these stations, makes the following comments on station exposure:

Chesterfield: urban site; slightly restricted. Wingerworth: Meteorological Office climatological station; site in an

open field. Ashover: Meteorological Office climatological station; somewhat

restricted. Brampton Eastmoor: very open; moorland exposure.

As data were listed in degrees Fahrenheit that scale is used in this article.

Fig. 1 . Location of stations. Contours are at 200 f t intervals. Hatched area is Chesterfield urban area

Some attempt is made to relate the effects of altitude and urbanisation with the dominant climatic features in terms of wind strength and direction and cloud cover. The climatic assessment of a month was based on the six- hourly readings made at the Meteorological Office station at Watnall in Nottinghamshire, which are published in the Monthly Weather Report. Watnall is situated about 30 km south-east of the main area of investigation.

SOME FACTORS AFFECTING SCREEN TEMPERATURE The screen temperature is an estimate of the temperature of the free

atmosphere at a given location. In fact, the screen temperature is partly dependent upon the surface temperature. At a given site, the screen tem- perature depends upon the surface moisture content, heat capacity and conductivity. Generally, sites with a dry soil of low conductivity and low heat capacity will be warmer by day and colder by night than those with surfaces of different characteristics. In hilly country, the screen temperature

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will also depend on local winds, such as the well-recognised valley wind systems. These factors should be borne in mind in considering the data taken from screens. It will be seen that they bear upon the magnitude of effects measured and introduce difficulties in relating and separating various effects.

URBANISATION The Wingerworth station lies in an open site and is 380 ft above sea-

level. The Chesterfield station, approximately 5 km north of Wingerworth, is on the western edge of the town at 375 ft altitude. I t seems a fair assuaption that differences between the two sites will be determined very much by any effects of the urban heat-island. Fig. 2 suggests that Chesterfield does indeed have an urban heat-effect. The mean difference is 1.4 deg F with the effect being greater at night. Fig. 2 also shows marked month-to-month variations, especially in the minima where, in two months, the difference was 3 deg F.

I J F M A M J J A S O N D J F M A M J J A S O N D 1972 1973

'Fig. 2. Difference in monthly mean (maxima - - - - , minima ~ ). Chesterfield

Table 1 shows the effects of cloud, wind and season on the heat-island intensity. It will immediately be noted that the urban heat effect was greater in summer than in winter. This suggests that the extra urban heat depends only partly on domestic and industrial burning of fuel. Of equal or even more importance are the altered thermal characteristics of the urban surface, which comprises areas of quickly-heated and heat-retentive brick, concrete and tarmac. I t should be noted that nearly all the seasonal differences from the overall value are statistically significant.

Generally smaller differences exist between clear and cloudy months; and only the minima in cloudy winter months differ significantly. The same comment applies to the eEects of windy and calm months. The significant effect of calm weather is noted in summer minima. This is due to the reten- tion of day-time heat in the urban fabric and the subsequent slow release of heat at night resulting in the significantly higher minima.

In considering the effect of wind-direction it is again the minima that reflect the urban influences most strongly. With winds between south-west and west the urban area is relatively warm at night, but is relatively cool with winds between south and east. These results are rather unexpected in view of the usual tendency for urban heat-islands to be displaced down-wind. It may well be that we are seeing effects of wind-direction on Wingerworth minima when south-west to west winds will favour katabatic drainage of cold air and so make Wingerworth relatively cold rather than Chesterfield

minus Wingerworth (deg F)

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TABLE 1. Urban heat-island (deg F), Chesterfield minus Wingerworth, and effects of cloud and wind

Monthly means Max. Min.

Average Winter (Dec., Jan., Feb.) Summer (June, July, Aug.) Cloudy Summer (Apr. '72, June'72)

Clear

Windy Summer (May'72, May'73)

Calm

North-west-North-east wind South-east wind South-west-West wind

Winter (Jan., Feb., Dec. '72, Jan. '73) Summer (Aug. '72, June, July, Aug. '73) Winter (Mar., Nov. '73)

Winter (Jan. '72, Dec. '73) Summer (Sept. '72, July, Sept. '73) Winter (Jan., Oct. '73)

1 *1 1.6 0.8* 1.3* 1 -2 2.1** 0.9 1.7 0-7 1.1' 1.1 2.2'* 1 .o 1.7 1.3 1.6 0.7 1.6 1.1 2.1* 0.9 1.2 0.9 1.6 1-1 1.2* 1 *1 2.0*

*, ** Significant a t 5,O.l percent levels respectively

and may not average to the value for the whole year

Note In this and Table 2 values for 'Summer' and 'Winter' as seasons refer to three months each

relatively warm. This highlights the importance of a knowledge of local sites in studies such as this one.

In connection with the urban heat-island effect, it is interesting to take note of a recent study by Lee (1975) who concludes that there is a signifi- cantly positive correlation between the magnitude of a particular heat-island and the lapse of potential temperature at a rural site. If the magnitude of the difference between Wingerworth and Chesterfield is compared with that between Wingerworth and Brampton, a negative correlation coefficient is found ( r= -0*32), which is significant at the 5 per cent level. A stable, low- lapse atmosphere assists in retention of urban warmth at the surface (and so at screen level) with a resultant magnification of the urban heat-island.

ALTITUDE Harrison (1975) showed that: (a) the lapse with altitude of screen temperature was less than that of

(b) the lapse of screen temperature varies on a weekly time scale; (c) there is some uncertainty in the correction for height to be applied

Table 2 and Figs. 3 and 4 show differences between Wingerworth and Brampton Eastmoor (1100 ft) and Ashover (altitude 585 ft) and Brampton Eastmoor. Substantial differences in altitudinal effects existed from Winger- worth and Ashover (both equidistant) up to the higher station at Brampton. The mean lapse between Ashover and Brampton is equivalent to 4.3 deg F per 1000 f t and that between Wingerworth and Brampton to 2-8 deg F per 1000 ft. The smaller lapse from the latter site is mainly due to the drainage of cold air and immediately stresses the importance of site in evaluating the magnitude of the correction for altitude to be applied for an upland station. Fellowes and Proctor (1976) note that minima at Wingerworth are lower than in surrounding areas. Wingerworth and Ashover are in valleys in terrain which slopes generally to the east; but Wingerworth is nearer the bottom of a less steep-sided valley and is obviously the more favoured of the two for

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soil temperatures;

to upland screen temperatures.

TABLE 2. Effect of altitude (deg F)

Wingerworth minus Brampton Eastmoor

Monthly means Max. Min.

Ashover minus Brampton Eastmoor

Monthly means Max. Min.

Average Winter Summer Cloudy Summer

Clear Summer

Calm Summer

Windy Summer

Winter

Winter

Winter

Winter

3 .O 1.1 2.9 0.7* 2 9 1.5* 3 .O 1.9* 2.6* 1.5 4.1** 0,4* 3 .O 0.2* 2.9 0.6 3.1 1.9 2.5 1.3 2.9 1 . 1

2.3 2.1 2.3 2.2 2 .o 2.0 2.0 2.4 1.6* 1 *7 3$** 2.0 2.4 2.0 2 3 2.2 2.5 2.9 2.3 2.5 2.1 1.7

Cloudy, summer months etc. as in Table 1 *, ** Significant at 5 , O . l percent levels respectively

‘ponding’ of cold air which results in low minima. The only noted seasonal effect is that in winter the lapse between Wingerworth and Brampton be- comes even less at night (0.7 deg F). This may be related to the longer winter nights with more pronounced katabatic effects in calm weather.

Examination of Figs. 3 and 4 reveals marked month-to-month variation In the effects of altitude, by both day and night. In both Figs. 3 and 4 there is a marked correspondence in the month-to-month trend of maxima and minima differences with height.

In cloudy winter months both Wingerworth and Ashover show reduced differences of maxima compared with Brampton (Table 2). On the other hand, in cloudy summer months the difference in minima between Winger- worth and Brampton is significantly greater than on average. The latter effect is related to a reduction in radiative cooling so that Wingerworth remains relatively warm through the night. It is more difficult to ascribe a cause to the noted winter daytime effect.

Table 2 shows that in clear weather there are several differences from the average which are significant in the altitudinal effect between Wingerworth and Brampton, but less so between Ashover and Brampton. However in clear summer months the altitude effect on daytime maxima from both Winger- worth and Ashover is high (and significant at the 0.1 per cent level). The magnitude of these daytime lapses (4.1 deg F from Wingerworth to Bramp- ton, 3.8 deg F from Ashover to Brampton) exceeds the dry adiabatic lapse rate. These large effects must therefore be related to variations in surface heating between Brampton on the one hand and Ashover and Wingerworth on the other. The main cause is probably the wet moorland surface at Brampton which hinders daytime heating.

On clear nights in both seasons, there is a very small altitude difference from Wingerworth to Brampton; Fig. 3 shows that in some months, Bramp- ton was the warmer of the two at night. This again reflects the favoured siting at Wingerworth for ‘ponding’ of cold air.

Table 2 shows no significant difference in altitudinal effects which depend on the degree of windiness of a month from either Ashover or Wingerworth. Calm weather favours valley winds in general, but in fact many of the calmer months during 1972 and 1973 were also fairly cloudy. Valley winds, being dependent upon differential heating, are not well-developed in cloudy weather.

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8 5- 6 2 4-

2: 2- j s i E 0- 8

.c t- DIL 3-

0 1 -

Fig. 3 . Dilferences in monthly means, Wingerworth minus Brampton. (a) maximum; (b) minimum

5-

4-

3-

2-

1 -

0-

al C

0 E

I J F M A M J J A S O N D J F M A M J J A S O N D 1972 1973

Fig. 4. Differences in monthly means, Ashover minus &rampton. - - - - maximum; minimum

CONCLUSION Marked variations in temperature occur in the monthly averages for a

small area of north-east Derbyshire and effects of both altitude and urbanisa- tion have been noted. Station siting has a marked influence on the magnitude of the measured effects and introduces difficulties in isolating and quantifying any one factor.

ACKNOWLEDGMENT I am indebted to Mr D. S . B. Fellowes, who maintains the Wingerworth

station and compiles monthly summaries for the area, for permission to use his records.

REFERENCES FELLOWES,D.S.B. and 1976 The onset of the common cold in relation to

meteorological parameters. J . Met. , 1, pp. 177- 182

HARRISON, s. J. The elevation component of soil temperature varia- tion. Weather, 30, pp,. 397-408

LEE, D. 0. 1975 Rural atmospheric stability and the intensity of London’s heat-island. Ibid., 30, pp. 102-109

PROCTOR, I. R. D.

1975

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